**1. Introduction**

#### **1.1. Importance of fertilizer application**

With an ever-increasing world population, modern agriculture faces the challenge to satisfy the increase in food demands. Fertilizers are indispensable for higher yields in agricultural food production [1]. They supply plants with primary nutrients like nitrogen (N), phosphorus (P), and potassium (K) with the focus of this chapter lying on N. Even though around 80% of the atmosphere consists of molecular N, it cannot directly be used by most plants due to it being highly stable and chemically inert [2]. Only a limited number of plants (legume plants) can fix atmospheric N through a symbiosis with N-fixing rhizobia bacteria called biological N fixation. The plant-microbe interaction results in new plant organs called root nodules, where the bacteria convert atmospheric N into available N for the plant [2, 3]. Thus, a regular replenishment of nutrients, e.g., in the form of manure, soil amendments, or organic or synthetic fertilizers, is necessary to keep the harvest quantity and quality of nonleguminous crops high [4]. The establishment of the Haber-Bosch process and with it the production of chemical N fertilizers markedly increased agricultural yields [1]. However, agricultural intensification comes with a price. To satisfy the large increases in food production, N production through the Haber-Bosch process increased by 20% from 100 in 1995 to 121 Tg N year−<sup>1</sup> in 2005 and will continue to rise with estimates of about 145 Tg N year−<sup>1</sup> in 2050 with N fertilizer production exceeding N fertilizer consumption by about 7% annually [5, 6]. This poses a huge threat to environmental systems because not all of the fertilizer N produced will actually reach the plants, be it through an overproduction of N or through direct losses of N to the environment after N fertilizer application.

**1.3. Enhanced efficiency fertilizers (EEFs): possible solutions for fertilizer** 

Countermeasures against the environmental impact of N from fertilizer N include the use of enhanced efficiency fertilizers (EEFs). There are several types of enhanced efficiency fertilizers (EEFs). Some contain nitrification or urease inhibitors (stabilized fertilizers), while others slowly release N components (slow-release fertilizers) or release N at more predictable rates

**Figure 1.** Losses of nitrogen (red boxes) after urea application through different processes. For simplification reasons some processes of N turnover have been omitted (e.g., steps in the denitrification process, aerobic, and chemo-denitrification).

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75

The use of EEFs is primarily to improve the N-use efficiency of crops. Choosing the right type of EEFs can synchronize the demands of N by the crops and the release of N by the fertilizers [12]. As an alternative approach, farmers may apply normal and/or enhanced efficiency fertilizers several times during the crop-growing season based on the requirements of the crop which is called split application [13]. However, the split application is often labor-demanding and not cost-efficient. We note that farmers normally decide to use the EEFs based on their cost performance unless they are required to use EEFs due to some environmental regulations. Thus, the mitigation of N-related environmental impacts is a secondary benefit for the farmers. There are different types of EEFs available on the market. In the next part of this

Fertilizers containing N stabilizers extend the time that the N added to the soil stays in its original form. Stabilized fertilizers like nitrification and urease inhibitors directly slow down microbial processes. For example, nitrification inhibitors (NIs) contain substances that inhibit ammonia

**application**

(controlled-release fertilizers) [8–11].

chapter, we list their characteristics.

*1.3.1. Stabilized fertilizers*

#### **1.2. Problems of fertilizer application**

Only around half of the conventional N fertilizers applied are utilized by the plants, while the rest is lost to the environment, with around 3, 20, and 25% of N lost through accumulation in the soil, leaching to aquatic systems, and emissions to the atmosphere, respectively [6, 7]. Plants assimilate N from the soil solution as nitrate (NO<sup>3</sup> <sup>−</sup>) or ammonium (NH<sup>4</sup> + ) ions. However, NH<sup>4</sup> + can be adsorbed to the soil and is used in nitrification processes leading to a competition for N with the soil biome, whereas NO3 <sup>−</sup> is negatively charged and easily lost from the soil (also negatively charged) through leaching [8]. The application rates of conventional fertilizers often exceed plant demands, and about 40–70% N is lost to the environment (**Figure 1**) [5, 6, 8]. Especially, N fertilizers are susceptible to loss because of processes such as leaching, mineralization, volatilization, and other gaseous emissions (e.g., ammonium (NH<sup>3</sup> ) and nitrous oxide (N2 O) emissions). As a consequence, N fertilizer application can lead to heavy environmental pollution due to excessive N not taken up by the plants. Two major processes of N pollution include NO3 − leaching to the groundwater leading to eutrophication and N2 O emission to the atmosphere via nitrification and denitrification processes of soil microbes [4].

Mitigation of Nitrous Oxide Emissions during Nitrification and Denitrification Processes… http://dx.doi.org/10.5772/intechopen.81548 75

**Figure 1.** Losses of nitrogen (red boxes) after urea application through different processes. For simplification reasons some processes of N turnover have been omitted (e.g., steps in the denitrification process, aerobic, and chemo-denitrification).

#### **1.3. Enhanced efficiency fertilizers (EEFs): possible solutions for fertilizer application**

Countermeasures against the environmental impact of N from fertilizer N include the use of enhanced efficiency fertilizers (EEFs). There are several types of enhanced efficiency fertilizers (EEFs). Some contain nitrification or urease inhibitors (stabilized fertilizers), while others slowly release N components (slow-release fertilizers) or release N at more predictable rates (controlled-release fertilizers) [8–11].

The use of EEFs is primarily to improve the N-use efficiency of crops. Choosing the right type of EEFs can synchronize the demands of N by the crops and the release of N by the fertilizers [12]. As an alternative approach, farmers may apply normal and/or enhanced efficiency fertilizers several times during the crop-growing season based on the requirements of the crop which is called split application [13]. However, the split application is often labor-demanding and not cost-efficient. We note that farmers normally decide to use the EEFs based on their cost performance unless they are required to use EEFs due to some environmental regulations. Thus, the mitigation of N-related environmental impacts is a secondary benefit for the farmers. There are different types of EEFs available on the market. In the next part of this chapter, we list their characteristics.

#### *1.3.1. Stabilized fertilizers*

**1. Introduction**

**1.1. Importance of fertilizer application**

74 Soil Contamination and Alternatives for Sustainable Development

20% from 100 in 1995 to 121 Tg N year−<sup>1</sup>

**1.2. Problems of fertilizer application**

processes of N pollution include NO3

+

[6, 7]. Plants assimilate N from the soil solution as nitrate (NO<sup>3</sup>

a competition for N with the soil biome, whereas NO3

of about 145 Tg N year−<sup>1</sup>

application.

However, NH<sup>4</sup>

and nitrous oxide (N2

tion and N2

microbes [4].

With an ever-increasing world population, modern agriculture faces the challenge to satisfy the increase in food demands. Fertilizers are indispensable for higher yields in agricultural food production [1]. They supply plants with primary nutrients like nitrogen (N), phosphorus (P), and potassium (K) with the focus of this chapter lying on N. Even though around 80% of the atmosphere consists of molecular N, it cannot directly be used by most plants due to it being highly stable and chemically inert [2]. Only a limited number of plants (legume plants) can fix atmospheric N through a symbiosis with N-fixing rhizobia bacteria called biological N fixation. The plant-microbe interaction results in new plant organs called root nodules, where the bacteria convert atmospheric N into available N for the plant [2, 3]. Thus, a regular replenishment of nutrients, e.g., in the form of manure, soil amendments, or organic or synthetic fertilizers, is necessary to keep the harvest quantity and quality of nonleguminous crops high [4]. The establishment of the Haber-Bosch process and with it the production of chemical N fertilizers markedly increased agricultural yields [1]. However, agricultural intensification comes with a price. To satisfy the large increases in food production, N production through the Haber-Bosch process increased by

sumption by about 7% annually [5, 6]. This poses a huge threat to environmental systems because not all of the fertilizer N produced will actually reach the plants, be it through an overproduction of N or through direct losses of N to the environment after N fertilizer

Only around half of the conventional N fertilizers applied are utilized by the plants, while the rest is lost to the environment, with around 3, 20, and 25% of N lost through accumulation in the soil, leaching to aquatic systems, and emissions to the atmosphere, respectively

from the soil (also negatively charged) through leaching [8]. The application rates of conventional fertilizers often exceed plant demands, and about 40–70% N is lost to the environment (**Figure 1**) [5, 6, 8]. Especially, N fertilizers are susceptible to loss because of processes such as leaching, mineralization, volatilization, and other gaseous emissions (e.g., ammonium (NH<sup>3</sup>

heavy environmental pollution due to excessive N not taken up by the plants. Two major

O emission to the atmosphere via nitrification and denitrification processes of soil

can be adsorbed to the soil and is used in nitrification processes leading to

O) emissions). As a consequence, N fertilizer application can lead to

− leaching to the groundwater leading to eutrophica-

in 2005 and will continue to rise with estimates

<sup>−</sup>) or ammonium (NH<sup>4</sup>

<sup>−</sup> is negatively charged and easily lost

+ ) ions.

)

in 2050 with N fertilizer production exceeding N fertilizer con-

Fertilizers containing N stabilizers extend the time that the N added to the soil stays in its original form. Stabilized fertilizers like nitrification and urease inhibitors directly slow down microbial processes. For example, nitrification inhibitors (NIs) contain substances that inhibit ammonia monooxygenase—the first enzyme involved in the oxidation process of nitrification—and thus delay the NH<sup>4</sup> + oxidation to NH<sup>2</sup> <sup>−</sup> by nitrifiers [8, 14, 15]. By slowing down the nitrification rate, plants have a higher chance of assimilating NO<sup>3</sup> <sup>−</sup> over a longer period which increases the N-use efficiency of the fertilizer. With reduced NO<sup>3</sup> <sup>−</sup> concentrations in the soil, NO3 <sup>−</sup> leaching and N2 O loss via denitrification processes (which require NO<sup>3</sup> <sup>−</sup> as a substrate) are minimized [8]. In summary, NIs slow down the two main processes of N<sup>2</sup> O production in soils with the indirect inhibition of denitrification being the most important effect since denitrification is the main contributor to N2 O emissions from agricultural soils [15]. NIs can be applied with chemical as well as organic fertilizers. Three commonly used NIs are N-(n-butyl) thiophosphoric triamide (NBPT), dicyandiamide (DCD), and 3,4-dimethylpyrazole phosphate (DMPP). NBPT is the most widely used NI due to it having a similar solubility and diffusivity as urea and thus a high effectiveness. However, under high temperature and humidity, e.g., under tropical conditions, its effectiveness might be reduced since it is rapidly degraded [8, 16]. Urease inhibitors suppress the hydrolytic action of the enzyme urease and therefore slow down the rate at which urea is hydrolyzed to NH<sup>3</sup> which reduces volatilization losses of NH<sup>3</sup> to the air [9]. In this chapter, the focus will lie on NIs since they are more promising in mitigating N<sup>2</sup> O emissions than UIs [14].

On the other hand, they are quite expensive, and their role in mitigating N<sup>2</sup>

environmental impacts like N<sup>2</sup>

*1.3.3. Organic amendments*

not yet fully understood with contrasting results [4, 9, 10, 17]. In conclusion, maximizing the N-use efficiency should be the goal when applying N fertilizers and EEFs in order to reduce

Even though organic fertilizers—also called organic amendments (OAs)—like plant residues, composts, or animal manure are not EEFs, they are often said to be more environmental-friendly than conventional fertilizers. The advantages of using OAs include improvement of soil C and N content, yield, and microbial biomass and activity. In addition, using OAs can recycle already fixed N [19]. Thus, animal manure and plant residue applications have been the traditional way of fertilizing agricultural fields for centuries, but recently—with the increasing demand of food—chemical fertilizers became more and more common. Still, due to the many negative effects for the environment of chemical fertilizers as well as their relatively high cost compared to OAs and unavailability in many local areas of the world, the use of OAs is rapidly increasing as well [20]. Negative environmental impacts of OAs include nutrient leaching and greenhouse gas (GHG) emissions, but their impact is much less severe than that of chemical fertilizers.

Animal manures, e.g., from livestock (cattle, pigs, or sheep) and poultry, are one of the most commonly used OAs that are rapidly increasing in quantity due to the increase in worldwide meat production. Animal manure has a substantial amount of nutrients like N, P, and K and can help to improve soil fertility and health [20]. Composts (e.g., made from crop residues or manure) are a good C source and help to enhance soil aggregate stability and prevent erosion, but most composts contain little amounts of nutrients, e.g., only 1–2% N (urea contains 46% N) which leads to the necessity of either additional addition of other fertilizers or high application rates. Also, during composting, high C and N losses occur through GHG emissions [19, 20]. Plant residues include stalks, stems, leaves, and seed pots and can function as a soil N source. Most plant residues are from cereals or legumes (e.g., red clover, winter vetch, and ryegrass). Their application method—e.g., surface application or incorporation into soil—as well as soil moisture and residue quality can have different impacts on GHG emissions with potential increases in denitrification after incorporation. Residues with a high N content and

low C/N ratios like legumes might release N faster which can result in higher N<sup>2</sup>

while high C/N ratios might decrease N<sup>2</sup>

**O emissions**

**1.4. N2**

*1.4.1. N2*

Nitrous oxide (N2

than that of CO2

radation which stimulates microbial immobilization [20–23].

time of 114 years—even low concentrations of atmospheric N<sup>2</sup>

*O as a threat for climate and ozone layer*

regarding climate change. Global emissions of N<sup>2</sup>

O emissions and at the same time increase the yield.

Mitigation of Nitrous Oxide Emissions during Nitrification and Denitrification Processes…

O emissions is

77

http://dx.doi.org/10.5772/intechopen.81548

O emissions,

O have strong negative impacts

O occur mostly through biological processes.

O emissions through released N during residue deg-

O) is a major GHG with a global warming potential around 298 times greater

over a 100 year period [24, 25]. Due to the high global warming potential—

which depends on its high potential of absorbing infrared light and a long atmospheric life-

#### *1.3.2. Controlled-release fertilizers (CRFs)*

Slow- and controlled-release fertilizer notations can be used interchangeably in most cases. One possible difference is that the nutrient release pattern of slow-release fertilizers is unpredictable due to the dependence on soil and climatic conditions, while it can roughly be predicted for controlled-release fertilizers [8]. Controlled-release fertilizers (CRFs) retard or control the release rate of nutrients to the soil which can lead to higher yields and a reduction of environmental pollution from nutrient loss through chemical modifications or coating. Coatings can be based on, e.g., sulfur, polymers, or superabsorbent/water retention materials with new and more accurate and efficient CRFs being developed on a constant basis [4, 8, 10, 11, 17, 18]. Coatings can only be applied to chemical fertilizers. Coated N fertilizers release N through a physical process like diffusion through the semipermeable coating, while slow-release fertilizers depend on chemical or biochemical processes [9]. One possible release mechanism of coated fertilizers is called multistage diffusion model where water penetrates the coating and condenses on the solid fertilizer core which dissolves the nutrients slowly. The osmotic pressure rises within the core, and the granules start to swell which leads to a slow diffusional release of the N due to a concentration and/or pressure gradient [10]. Since urea is one of the most widely used N fertilizers in the world, coated urea has been one of the most researched CRFs. Coated urea is a granule CRF which releases more N with increasing temperature and is designed to release N as close to the plant's N demand as possible. The N release can be controlled by altering the coating properties (e.g., the thickness) and thus changing the N diffusion rates. Coated urea reduces N<sup>2</sup> O emissions by limiting the available N for microbial processes like nitrification and denitrification in the soil [9, 10, 17].

Advantages of EEFs over conventional fertilizers include higher N-use efficiency (nutrients taken up by plants compared with the amount applied) and N availability over the whole crop season. The resulting environmental benefits are decreases in NO<sup>3</sup> <sup>−</sup>-N loss through leaching, NH<sup>3</sup> loss through volatilization, and N<sup>2</sup> O emissions through microbial processes. On the other hand, they are quite expensive, and their role in mitigating N<sup>2</sup> O emissions is not yet fully understood with contrasting results [4, 9, 10, 17]. In conclusion, maximizing the N-use efficiency should be the goal when applying N fertilizers and EEFs in order to reduce environmental impacts like N<sup>2</sup> O emissions and at the same time increase the yield.

#### *1.3.3. Organic amendments*

monooxygenase—the first enzyme involved in the oxidation process of nitrification—and thus

indirect inhibition of denitrification being the most important effect since denitrification is the

as well as organic fertilizers. Three commonly used NIs are N-(n-butyl) thiophosphoric triamide (NBPT), dicyandiamide (DCD), and 3,4-dimethylpyrazole phosphate (DMPP). NBPT is the most widely used NI due to it having a similar solubility and diffusivity as urea and thus a high effectiveness. However, under high temperature and humidity, e.g., under tropical conditions, its effectiveness might be reduced since it is rapidly degraded [8, 16]. Urease inhibitors suppress the hydrolytic action of the enzyme urease and therefore slow down the rate at which urea is

Slow- and controlled-release fertilizer notations can be used interchangeably in most cases. One possible difference is that the nutrient release pattern of slow-release fertilizers is unpredictable due to the dependence on soil and climatic conditions, while it can roughly be predicted for controlled-release fertilizers [8]. Controlled-release fertilizers (CRFs) retard or control the release rate of nutrients to the soil which can lead to higher yields and a reduction of environmental pollution from nutrient loss through chemical modifications or coating. Coatings can be based on, e.g., sulfur, polymers, or superabsorbent/water retention materials with new and more accurate and efficient CRFs being developed on a constant basis [4, 8, 10, 11, 17, 18]. Coatings can only be applied to chemical fertilizers. Coated N fertilizers release N through a physical process like diffusion through the semipermeable coating, while slow-release fertilizers depend on chemical or biochemical processes [9]. One possible release mechanism of coated fertilizers is called multistage diffusion model where water penetrates the coating and condenses on the solid fertilizer core which dissolves the nutrients slowly. The osmotic pressure rises within the core, and the granules start to swell which leads to a slow diffusional release of the N due to a concentration and/or pressure gradient [10]. Since urea is one of the most widely used N fertilizers in the world, coated urea has been one of the most researched CRFs. Coated urea is a granule CRF which releases more N with increasing temperature and is designed to release N as close to the plant's N demand as possible. The N release can be controlled by altering the coating properties (e.g., the thickness) and thus

which reduces volatilization losses of NH<sup>3</sup>

focus will lie on NIs since they are more promising in mitigating N<sup>2</sup>

changing the N diffusion rates. Coated urea reduces N<sup>2</sup>

N for microbial processes like nitrification and denitrification in the soil [9, 10, 17].

crop season. The resulting environmental benefits are decreases in NO<sup>3</sup>

loss through volatilization, and N<sup>2</sup>

Advantages of EEFs over conventional fertilizers include higher N-use efficiency (nutrients taken up by plants compared with the amount applied) and N availability over the whole

<sup>−</sup> by nitrifiers [8, 14, 15]. By slowing down the nitrification

O emissions from agricultural soils [15]. NIs can be applied with chemical

<sup>−</sup> concentrations in the soil, NO3

<sup>−</sup> over a longer period which increases the

<sup>−</sup> as a substrate) are minimized

O production in soils with the

to the air [9]. In this chapter, the

O emissions by limiting the available

O emissions through microbial processes.

<sup>−</sup>-N loss through

O emissions than UIs [14].

<sup>−</sup> leaching

delay the NH<sup>4</sup>

main contributor to N2

hydrolyzed to NH<sup>3</sup>

leaching, NH<sup>3</sup>

*1.3.2. Controlled-release fertilizers (CRFs)*

and N2

+

oxidation to NH<sup>2</sup>

76 Soil Contamination and Alternatives for Sustainable Development

rate, plants have a higher chance of assimilating NO<sup>3</sup>

N-use efficiency of the fertilizer. With reduced NO<sup>3</sup>

O loss via denitrification processes (which require NO<sup>3</sup>

[8]. In summary, NIs slow down the two main processes of N<sup>2</sup>

Even though organic fertilizers—also called organic amendments (OAs)—like plant residues, composts, or animal manure are not EEFs, they are often said to be more environmental-friendly than conventional fertilizers. The advantages of using OAs include improvement of soil C and N content, yield, and microbial biomass and activity. In addition, using OAs can recycle already fixed N [19]. Thus, animal manure and plant residue applications have been the traditional way of fertilizing agricultural fields for centuries, but recently—with the increasing demand of food—chemical fertilizers became more and more common. Still, due to the many negative effects for the environment of chemical fertilizers as well as their relatively high cost compared to OAs and unavailability in many local areas of the world, the use of OAs is rapidly increasing as well [20]. Negative environmental impacts of OAs include nutrient leaching and greenhouse gas (GHG) emissions, but their impact is much less severe than that of chemical fertilizers.

Animal manures, e.g., from livestock (cattle, pigs, or sheep) and poultry, are one of the most commonly used OAs that are rapidly increasing in quantity due to the increase in worldwide meat production. Animal manure has a substantial amount of nutrients like N, P, and K and can help to improve soil fertility and health [20]. Composts (e.g., made from crop residues or manure) are a good C source and help to enhance soil aggregate stability and prevent erosion, but most composts contain little amounts of nutrients, e.g., only 1–2% N (urea contains 46% N) which leads to the necessity of either additional addition of other fertilizers or high application rates. Also, during composting, high C and N losses occur through GHG emissions [19, 20]. Plant residues include stalks, stems, leaves, and seed pots and can function as a soil N source. Most plant residues are from cereals or legumes (e.g., red clover, winter vetch, and ryegrass). Their application method—e.g., surface application or incorporation into soil—as well as soil moisture and residue quality can have different impacts on GHG emissions with potential increases in denitrification after incorporation. Residues with a high N content and low C/N ratios like legumes might release N faster which can result in higher N<sup>2</sup> O emissions, while high C/N ratios might decrease N<sup>2</sup> O emissions through released N during residue degradation which stimulates microbial immobilization [20–23].

#### **1.4. N2 O emissions**

#### *1.4.1. N2 O as a threat for climate and ozone layer*

Nitrous oxide (N2 O) is a major GHG with a global warming potential around 298 times greater than that of CO2 over a 100 year period [24, 25]. Due to the high global warming potential which depends on its high potential of absorbing infrared light and a long atmospheric lifetime of 114 years—even low concentrations of atmospheric N<sup>2</sup> O have strong negative impacts regarding climate change. Global emissions of N<sup>2</sup> O occur mostly through biological processes. Thus, factors affecting N<sup>2</sup> O emissions from soils include oxygen (O2 ) concentration, temperature, moisture content, soil texture and type, and soil-NH<sup>4</sup> + and soil-NO3 <sup>−</sup> concentrations that are readily available for nitrification and denitrification [26, 27]. In addition, soil organic C content and soil drainage as well as microbial community structure, abundance, and activity influence the magnitude of N<sup>2</sup> O emissions. Agricultural management-related factors controlling N2 O emissions include fertilizer and crop type, tillage system, as well as N application rate, timing, and technique [26, 27]. It is estimated that—due to human activities—N<sup>2</sup> O emissions have increased by more than 40% compared to preindustrial levels [27]. For example, [25] estimated that global emissions have increased from 12 in 1500 to 19 Tg N year−<sup>1</sup> in 2006, with natural emissions accounting for about 55% and anthropogenic emissions accounting for about 45% in 2006.

Next to its global warming potential, N2 O also has a huge effect on the ozone layer. It is said to be the main ozone-depleting substance today and will remain the largest threat to the ozone layer throughout the twenty-first century if the current emission trend continues. Nitrous oxides (NOx ) catalytically destroy the ozone layer, and their main source is surface N2 O emissions. N<sup>2</sup> O already outweighs the chlorofluorocarbons—the historically dominant ozone-depleting substances—because of its abundance and continued anthropogenic emissions as well as a high ozone depletion factor of 0.017 [28]. Thus, N<sup>2</sup> O emissions affect both the ozone layer and our climate. We need to be aware that when fighting climate change through the mitigation of CO2 emissions, N2 O emission could unintendedly rise, e.g., as a consequence of enhanced crop growth for biofuel production [28]. To avoid exchanging one GHG with another, we need to mitigate anthropogenic N<sup>2</sup> O emissions which mainly arise from N fertilization.

Denitrification is the anaerobic process of the reduction of NO<sup>3</sup>

and spatially variable than nitrification and is influenced by NO<sup>3</sup>


offer a unique opportunity to gain insights into processes contributing to N<sup>2</sup>

denitrification which, under some circumstances, can even outweigh denitrification. It is the

O emissions from agriculture between 1990 and 2030 (data obtained from [35]).

entirely emanate from nitrifier denitrification. Favorable environments for this process are

tilization and it is thus a major concern in terms of atmospheric GHG increases [34]. The N<sup>2</sup>

emission factor (EF) which is the "percentage of fertilizer N applied that is transformed into fertilizer-induced emissions" is estimated as 1% which means that from 100 kg of applied

O emissions, we first need to quantify the N<sup>2</sup>

recently laser spectroscopy approaches have highly increased our knowledge on the function-

By using isotopic ratio mass spectrometry, it is possible to estimate (e.g., through natural abundance measurements) and quantify (e.g., through enrichment experiments) the (micro-

O budget [36].

low in oxygen and high in pH and have fluctuating aerobic-anaerobic conditions [33].

26, 27]. A very important, yet often overlooked, process of N<sup>2</sup>

It is estimated that around half of the global anthropogenic N<sup>2</sup>

N2

O with high losses of N2

<sup>−</sup> by NH<sup>3</sup>

N, 1 kg is lost to the environment as N<sup>2</sup>

ing and the controls regarding the N2

bial) processes associated with N2

processes that contribute to N2

In order to mitigate N<sup>2</sup>

ficult to assign N<sup>2</sup>

relationship between N application and N2

addition, it is predicted that agricultural N2

tial curve which would indicate that global N<sup>2</sup>

stand the mechanisms and processes linked with N<sup>2</sup>

reduction of NO2

**Figure 2.** Global N2

<sup>−</sup> to N2

<sup>−</sup>, C, and O2

+

O emissions [34]. However, a 1% EF assumes a linear

O emissions while [34] rather suggest an exponen-

O emissions will increase by 20% until 2030 [35].

O production based on 15N and 18O isotopes [37]. It is dif-

O production in different microsites of the soil. To attribute N<sup>2</sup>

O production to different processes mainly due to many simultaneous

O emissions are underestimated in general. In

O production. Stable isotope techniques

O to the atmosphere. Denitrification is much more temporally

Mitigation of Nitrous Oxide Emissions during Nitrification and Denitrification Processes…

O emissions from NH<sup>4</sup>

via NO<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.81548

O production is called nitrifier

O emissions arise from N fer-

<sup>−</sup>, NO, and

79

O

O

availability [8,

might in some cases

O budget and under-

O emissions, and

#### *1.4.2. N fertilization as a source for microbially produced N2 O*

The agricultural sector (including livestock production systems) is responsible for about 42% of the global anthropogenic N2 O emissions [15, 25, 29]. Agricultural soils are the main contributor to anthropogenic N2 O emissions, and it is estimated that emissions will continue to rise in the future (**Figure 2**) mainly due to increases in N fertilizer use and manure application [30]. The most commonly used N fertilizer is urea (CO(NH<sup>2</sup> ) 2 ). When applied to the soil in the presence of water, an enzyme called urease converts urea into NH<sup>4</sup> + , hydroxyl ions (OH−), and bicarbonate (HCO<sup>3</sup> <sup>−</sup>) [10, 17, 27]. The N<sup>2</sup> O emissions from N fertilization (primarily the addition of chemical fertilizers and organic fertilizers like animal manure) mainly arise from microbial processes called nitrification (aerobic) and denitrification (anaerobic). Nitrification is the aerobic process of catabolic oxidation where NH<sup>4</sup> + is firstly transformed to nitrite (NO<sup>2</sup> <sup>−</sup>) by *Nitrosomonas* sp. and then to NO3 − by *Nitrobacter* and *Nitrosolobus* sp. bacteria. It is mainly influenced by the availability of NH<sup>4</sup> + and O2 [26, 27]. Also, many archaea species oxidize NH<sup>4</sup> + , and a recent study discovered that there are *Nitrospira* species that can perform the complete nitrification process (the two steps mentioned above) on their own [31]. Nitrification is a very important microbial process in the soil—especially for plant productivity and environmental quality—as it determines the form of N present. Nitrification converts the relatively immobile NH<sup>4</sup> + to the highly mobile NO3 <sup>−</sup>. NO<sup>3</sup> <sup>−</sup> is often the major compound utilized by plants due to it being easily available, but at the same time, it can also be easily lost through leaching and denitrification [8, 27, 32].

Mitigation of Nitrous Oxide Emissions during Nitrification and Denitrification Processes… http://dx.doi.org/10.5772/intechopen.81548 79

**Figure 2.** Global N2 O emissions from agriculture between 1990 and 2030 (data obtained from [35]).

Thus, factors affecting N<sup>2</sup>

influence the magnitude of N<sup>2</sup>

Next to its global warming potential, N2

for about 45% in 2006.

Nitrous oxides (NOx

O emissions. N<sup>2</sup>

from N fertilization.

to anthropogenic N2

(HCO<sup>3</sup>

NO3

then to NO3

ability of NH<sup>4</sup>

<sup>−</sup>. NO<sup>3</sup>

through the mitigation of CO2

the global anthropogenic N2

<sup>−</sup>) [10, 17, 27]. The N<sup>2</sup>

catabolic oxidation where NH<sup>4</sup>

+ and O2

ling N2

N2

ture, moisture content, soil texture and type, and soil-NH<sup>4</sup>

78 Soil Contamination and Alternatives for Sustainable Development

O emissions from soils include oxygen (O2

are readily available for nitrification and denitrification [26, 27]. In addition, soil organic C content and soil drainage as well as microbial community structure, abundance, and activity

sions have increased by more than 40% compared to preindustrial levels [27]. For example,

with natural emissions accounting for about 55% and anthropogenic emissions accounting

said to be the main ozone-depleting substance today and will remain the largest threat to the ozone layer throughout the twenty-first century if the current emission trend continues.

ozone-depleting substances—because of its abundance and continued anthropogenic emis-

the ozone layer and our climate. We need to be aware that when fighting climate change

consequence of enhanced crop growth for biofuel production [28]. To avoid exchanging one

The agricultural sector (including livestock production systems) is responsible for about 42% of

future (**Figure 2**) mainly due to increases in N fertilizer use and manure application [30]. The

cal fertilizers and organic fertilizers like animal manure) mainly arise from microbial processes called nitrification (aerobic) and denitrification (anaerobic). Nitrification is the aerobic process of

[26, 27]. Also, many archaea species oxidize NH<sup>4</sup>

at the same time, it can also be easily lost through leaching and denitrification [8, 27, 32].

covered that there are *Nitrospira* species that can perform the complete nitrification process (the two steps mentioned above) on their own [31]. Nitrification is a very important microbial process in the soil—especially for plant productivity and environmental quality—as it determines

) 2

is firstly transformed to nitrite (NO<sup>2</sup>

− by *Nitrobacter* and *Nitrosolobus* sp. bacteria. It is mainly influenced by the avail-

<sup>−</sup> is often the major compound utilized by plants due to it being easily available, but

sions as well as a high ozone depletion factor of 0.017 [28]. Thus, N<sup>2</sup>

GHG with another, we need to mitigate anthropogenic N<sup>2</sup>

*1.4.2. N fertilization as a source for microbially produced N2*

most commonly used N fertilizer is urea (CO(NH<sup>2</sup>

water, an enzyme called urease converts urea into NH<sup>4</sup>

+

the form of N present. Nitrification converts the relatively immobile NH<sup>4</sup>

emissions, N2

rate, timing, and technique [26, 27]. It is estimated that—due to human activities—N<sup>2</sup>

[25] estimated that global emissions have increased from 12 in 1500 to 19 Tg N year−<sup>1</sup>

O emissions include fertilizer and crop type, tillage system, as well as N application

+

) catalytically destroy the ozone layer, and their main source is surface

O already outweighs the chlorofluorocarbons—the historically dominant

*O*

O emissions, and it is estimated that emissions will continue to rise in the

+

O emissions [15, 25, 29]. Agricultural soils are the main contributor

O emissions from N fertilization (primarily the addition of chemi-

O emissions. Agricultural management-related factors control-

and soil-NO3

O also has a huge effect on the ozone layer. It is

O emission could unintendedly rise, e.g., as a

). When applied to the soil in the presence of

+

+

, hydroxyl ions (OH−), and bicarbonate

<sup>−</sup>) by *Nitrosomonas* sp. and

, and a recent study dis-

to the highly mobile

) concentration, tempera-

<sup>−</sup> concentrations that

O emissions affect both

O emissions which mainly arise

O emis-

in 2006,

Denitrification is the anaerobic process of the reduction of NO<sup>3</sup> <sup>−</sup> to N2 via NO<sup>2</sup> <sup>−</sup>, NO, and N2 O with high losses of N2 O to the atmosphere. Denitrification is much more temporally and spatially variable than nitrification and is influenced by NO<sup>3</sup> <sup>−</sup>, C, and O2 availability [8, 26, 27]. A very important, yet often overlooked, process of N<sup>2</sup> O production is called nitrifier denitrification which, under some circumstances, can even outweigh denitrification. It is the reduction of NO2 <sup>−</sup> by NH<sup>3</sup> -oxidizing bacteria. N<sup>2</sup> O emissions from NH<sup>4</sup> + might in some cases entirely emanate from nitrifier denitrification. Favorable environments for this process are low in oxygen and high in pH and have fluctuating aerobic-anaerobic conditions [33].

It is estimated that around half of the global anthropogenic N<sup>2</sup> O emissions arise from N fertilization and it is thus a major concern in terms of atmospheric GHG increases [34]. The N<sup>2</sup> O emission factor (EF) which is the "percentage of fertilizer N applied that is transformed into fertilizer-induced emissions" is estimated as 1% which means that from 100 kg of applied N, 1 kg is lost to the environment as N<sup>2</sup> O emissions [34]. However, a 1% EF assumes a linear relationship between N application and N2 O emissions while [34] rather suggest an exponential curve which would indicate that global N<sup>2</sup> O emissions are underestimated in general. In addition, it is predicted that agricultural N2 O emissions will increase by 20% until 2030 [35].

In order to mitigate N<sup>2</sup> O emissions, we first need to quantify the N<sup>2</sup> O budget and understand the mechanisms and processes linked with N<sup>2</sup> O production. Stable isotope techniques offer a unique opportunity to gain insights into processes contributing to N<sup>2</sup> O emissions, and recently laser spectroscopy approaches have highly increased our knowledge on the functioning and the controls regarding the N2 O budget [36].

By using isotopic ratio mass spectrometry, it is possible to estimate (e.g., through natural abundance measurements) and quantify (e.g., through enrichment experiments) the (microbial) processes associated with N2 O production based on 15N and 18O isotopes [37]. It is difficult to assign N<sup>2</sup> O production to different processes mainly due to many simultaneous processes that contribute to N2 O production in different microsites of the soil. To attribute N<sup>2</sup> O emissions to denitrification, acetylene (C<sup>2</sup> H2 ) is added which inhibits nitrification. However, this method can underestimate denitrification because no additional NO<sup>3</sup> <sup>−</sup> is supplied via nitrification. In addition C<sup>2</sup> H2 can be used as a substrate if C in the soil is limited which can lead to biased results [37]. Natural abundance approaches can be used as an estimate of the N2 O sources in soil, but the results should be handled with caution due to a fractionation in favor of <sup>14</sup>N relative to <sup>15</sup>N and higher fractionation during nitrification than denitrification. Thus, even though natural abundance approaches have the advantage of being noninvasive, they are not suitable for exact N2 O source partitioning and thus best be coupled with genetic and/or molecular approaches [37].

Many studies have assessed the effectiveness of stabilized fertilizers, but the results vary. The

in various studies [38–40]. For example, [38] found that DCD and DMPP, two NI fertilizers in combination with urea, decreased cumulative emissions by 35 and 38%, respectively, in a 1 year experiment on a wheat-maize rotation field when compared to only urea application. In addition, yield, aboveground biomass, and nitrogen uptake increased significantly when using NIs, possibly through an increase in soil inorganic nitrogen and a shift from NO<sup>3</sup>

forms. Other studies that assessed the effectiveness of DCD and/or DMPP concluded

tion, even though urea + nitrapyrin did mitigate them in a 2 year study on an intensively used

volatilization after NI application are considered [42]. A study by [16], who quantified N<sup>2</sup>

emissions in a 1 month field trial with maize plants on a tropical acrisol after urea + NI fertilizer (nitrapyrin and NBPT) application, assessed this problem. They found that the urea + nitra-

nitrapyrin was added after urea application during maize sowing, NBPT was added during the V5 stage of maize growth which increased maize yield significantly. The authors concluded that the combination of timing, placement, as well as the specific use of fertilizers are vital for

volatilization as well as for higher N-use efficiency and yields. Recent studies also suggest

might have little economic benefits since they are relatively expensive and might not increase biomass and yield as expected. This is due to the usage of suboptimal N rates in many studies

O emissions after NI application [43]. Thus, the effectiveness of stabilized fertilizers depends on many factors. For example, in sys-

moderately fertilized and that are adapted to the N demand of plants [15]. Ref. [15] also sug-

the N fertilizer, e.g., when the NI is sprayed on mineral-N fertilizer granules or is thoroughly mixed with liquid fertilizers. Thus, on pasture soils with high spatial N concentration varia-

In general, when long-term experiments and meta-analyses are conducted, NIs usually show

effectiveness of NIs because with short-term experiments and low-frequency measurements

O emissions increase strongly if N application rates are higher than the N uptake.

O emissions as well as NO3

Mitigation of Nitrous Oxide Emissions during Nitrification and Denitrification Processes…

O emissions

<sup>−</sup> leaching, e.g., from

O emissions from increased

volatilization. In addition, while

O emissions after urea + DCD applica-

http://dx.doi.org/10.5772/intechopen.81548

O emissions by 49%, while NBPT reduced the NH<sup>3</sup>

O emissions directly and indirectly through a reduction of

O mitigation can simply be achieved by optimizing the N input

O reduction occurs under conditions where the NI remains close to

O emissions in agricultural systems that are

O mitigation strongly varies depending on N<sup>2</sup>

O emissions. Thus, high-frequency measurements over

O emissions and assess the overall

<sup>−</sup> to

81

O emis-

O emissions;

O emissions—they

O

O

application of DCD and/or DMPP has shown positive results in mitigating N<sup>2</sup>

cow urine in grazed pastures or in intensive vegetable production systems [39, 40].

vegetable field. Recently, it is suggested that the positive effects of NIs on reducing N<sup>2</sup>

volatilization by 35% compared to the urea treatment. NBPT tended to increase N<sup>2</sup>

that—even though NIs like DCD and DMPP are a good option to reduce N<sup>2</sup>

O emissions indirectly by reducing NH<sup>3</sup>

NH<sup>4</sup> +

NH<sup>3</sup>

NH<sup>3</sup>

because N2

that they have a great potential to reduce N<sup>2</sup>

pyrin treatment reduced cumulative N<sup>2</sup>

the effectiveness of NIs to reduce N<sup>2</sup>

that focus on the effects of N<sup>2</sup>

tems with a high N surplus, N2

gest that the optimal N2

But EEFs can be an effective way to mitigate N<sup>2</sup>

tions due to urine and manure patches, N2

a long time period are important to accurately quantify N<sup>2</sup>

hotspots dispatched across the soil.

a positive effect on the mitigation of N<sup>2</sup>

however, it reduced N<sup>2</sup>

However, e.g., [41] found no significant reduction of N<sup>2</sup>

sion might be overestimated or even reversed when the indirect N<sup>2</sup>

#### **2. Enhanced efficiency of N fertilizers and their role in mitigating N2 O emissions: comparing conventional, organic, and enhanced efficiency fertilizers**

One option to reduce N loss to the environment as N<sup>2</sup> O gas is to use EEFs. As previously described, EEFs match the N release from fertilizers and N demands by crops. Thus, the excess accumulation of fertilizer-derived N in soils can be avoided. This fact is very important to minimize the N2 O emissions from soils because N2 O emissions are related to the microbial availability of N in soils. Even though environmental issues are important, the main reason for applying EEFs in recent years was to increase the N-use efficiency through optimized N management practices, which, e.g., can reduce the number of split applications a farmer has to do [13, 34]. The main goal for farmers is thus to decrease the amount of work and at the same time increase the yield due to an optimized N fertilizer utilization rate. The main goal for humanity however should be to combine high agricultural yields with a minimum environmental pollution [15].

#### **2.1. Stabilized fertilizers**

In a meta-analysis by [14]—who evaluated the overall effectiveness of EEFs by comparing them with conventional fertilizers using datasets from field experiment data—they concluded that nitrification inhibitors (NIs) have the highest potential in mitigating N<sup>2</sup> O emissions with a mean reduction of 38% (when compared to conventional fertilizers) and a relative constant effectiveness for different soil and land use types. The effectiveness of NIs to reduce N<sup>2</sup> O emissions varied with land use type, e.g., mitigation in grasslands (−54%) was much higher than paddy fields (−30%). This indicates that the mitigation effectiveness of NIs depends on the mean N2 O emissions of the land use. The higher the mean N<sup>2</sup> O emissions, the more effective are NIs in reducing them. However, in their meta-analysis, urease inhibitors (UIs) seemed to be ineffective in reducing N<sup>2</sup> O emissions. In another meta-analysis by [32]—who assessed how NIs affect both hydrologic and gaseous N losses and N-use efficiency—NI application increased NH<sup>3</sup> emissions by a mean of 20% but reduced N leaching, N2 O emissions, and NO emissions by 48, 44, and 24%, respectively, which led to a total net reduction of 16.5% of N released to the environment. Thus, overall, NIs seem to be a good option in reducing N<sup>2</sup> O emissions, but their effectiveness may vary strongly on a local scale.

Many studies have assessed the effectiveness of stabilized fertilizers, but the results vary. The application of DCD and/or DMPP has shown positive results in mitigating N<sup>2</sup> O emissions in various studies [38–40]. For example, [38] found that DCD and DMPP, two NI fertilizers in combination with urea, decreased cumulative emissions by 35 and 38%, respectively, in a 1 year experiment on a wheat-maize rotation field when compared to only urea application. In addition, yield, aboveground biomass, and nitrogen uptake increased significantly when using NIs, possibly through an increase in soil inorganic nitrogen and a shift from NO<sup>3</sup> <sup>−</sup> to NH<sup>4</sup> + forms. Other studies that assessed the effectiveness of DCD and/or DMPP concluded that they have a great potential to reduce N<sup>2</sup> O emissions as well as NO3 <sup>−</sup> leaching, e.g., from cow urine in grazed pastures or in intensive vegetable production systems [39, 40].

emissions to denitrification, acetylene (C<sup>2</sup>

80 Soil Contamination and Alternatives for Sustainable Development

H2

One option to reduce N loss to the environment as N<sup>2</sup>

O emissions from soils because N2

that nitrification inhibitors (NIs) have the highest potential in mitigating N<sup>2</sup>

O emissions of the land use. The higher the mean N<sup>2</sup>

emissions, but their effectiveness may vary strongly on a local scale.

nitrification. In addition C<sup>2</sup>

they are not suitable for exact N2

**efficiency fertilizers**

environmental pollution [15].

to be ineffective in reducing N<sup>2</sup>

**2.1. Stabilized fertilizers**

the mean N2

increased NH<sup>3</sup>

and/or molecular approaches [37].

N2

**N2**

to minimize the N2

H2

**2. Enhanced efficiency of N fertilizers and their role in mitigating**

**O emissions: comparing conventional, organic, and enhanced** 

described, EEFs match the N release from fertilizers and N demands by crops. Thus, the excess accumulation of fertilizer-derived N in soils can be avoided. This fact is very important

availability of N in soils. Even though environmental issues are important, the main reason for applying EEFs in recent years was to increase the N-use efficiency through optimized N management practices, which, e.g., can reduce the number of split applications a farmer has to do [13, 34]. The main goal for farmers is thus to decrease the amount of work and at the same time increase the yield due to an optimized N fertilizer utilization rate. The main goal for humanity however should be to combine high agricultural yields with a minimum

In a meta-analysis by [14]—who evaluated the overall effectiveness of EEFs by comparing them with conventional fertilizers using datasets from field experiment data—they concluded

a mean reduction of 38% (when compared to conventional fertilizers) and a relative constant effectiveness for different soil and land use types. The effectiveness of NIs to reduce N<sup>2</sup>

emissions varied with land use type, e.g., mitigation in grasslands (−54%) was much higher than paddy fields (−30%). This indicates that the mitigation effectiveness of NIs depends on

tive are NIs in reducing them. However, in their meta-analysis, urease inhibitors (UIs) seemed

how NIs affect both hydrologic and gaseous N losses and N-use efficiency—NI application

emissions by 48, 44, and 24%, respectively, which led to a total net reduction of 16.5% of N released to the environment. Thus, overall, NIs seem to be a good option in reducing N<sup>2</sup>

emissions by a mean of 20% but reduced N leaching, N2

O emissions. In another meta-analysis by [32]—who assessed

lead to biased results [37]. Natural abundance approaches can be used as an estimate of the

O sources in soil, but the results should be handled with caution due to a fractionation in favor of <sup>14</sup>N relative to <sup>15</sup>N and higher fractionation during nitrification than denitrification. Thus, even though natural abundance approaches have the advantage of being noninvasive,

this method can underestimate denitrification because no additional NO<sup>3</sup>

) is added which inhibits nitrification. However,

O gas is to use EEFs. As previously

O emissions are related to the microbial

can be used as a substrate if C in the soil is limited which can

O source partitioning and thus best be coupled with genetic

<sup>−</sup> is supplied via

O emissions with

O emissions, the more effec-

O emissions, and NO

O

O

However, e.g., [41] found no significant reduction of N<sup>2</sup> O emissions after urea + DCD application, even though urea + nitrapyrin did mitigate them in a 2 year study on an intensively used vegetable field. Recently, it is suggested that the positive effects of NIs on reducing N<sup>2</sup> O emission might be overestimated or even reversed when the indirect N<sup>2</sup> O emissions from increased NH<sup>3</sup> volatilization after NI application are considered [42]. A study by [16], who quantified N<sup>2</sup> O emissions in a 1 month field trial with maize plants on a tropical acrisol after urea + NI fertilizer (nitrapyrin and NBPT) application, assessed this problem. They found that the urea + nitrapyrin treatment reduced cumulative N<sup>2</sup> O emissions by 49%, while NBPT reduced the NH<sup>3</sup> volatilization by 35% compared to the urea treatment. NBPT tended to increase N<sup>2</sup> O emissions; however, it reduced N<sup>2</sup> O emissions indirectly by reducing NH<sup>3</sup> volatilization. In addition, while nitrapyrin was added after urea application during maize sowing, NBPT was added during the V5 stage of maize growth which increased maize yield significantly. The authors concluded that the combination of timing, placement, as well as the specific use of fertilizers are vital for the effectiveness of NIs to reduce N<sup>2</sup> O emissions directly and indirectly through a reduction of NH<sup>3</sup> volatilization as well as for higher N-use efficiency and yields. Recent studies also suggest that—even though NIs like DCD and DMPP are a good option to reduce N<sup>2</sup> O emissions—they might have little economic benefits since they are relatively expensive and might not increase biomass and yield as expected. This is due to the usage of suboptimal N rates in many studies that focus on the effects of N<sup>2</sup> O emissions after NI application [43].

Thus, the effectiveness of stabilized fertilizers depends on many factors. For example, in systems with a high N surplus, N2 O mitigation can simply be achieved by optimizing the N input because N2 O emissions increase strongly if N application rates are higher than the N uptake. But EEFs can be an effective way to mitigate N<sup>2</sup> O emissions in agricultural systems that are moderately fertilized and that are adapted to the N demand of plants [15]. Ref. [15] also suggest that the optimal N2 O reduction occurs under conditions where the NI remains close to the N fertilizer, e.g., when the NI is sprayed on mineral-N fertilizer granules or is thoroughly mixed with liquid fertilizers. Thus, on pasture soils with high spatial N concentration variations due to urine and manure patches, N2 O mitigation strongly varies depending on N<sup>2</sup> O hotspots dispatched across the soil.

In general, when long-term experiments and meta-analyses are conducted, NIs usually show a positive effect on the mitigation of N<sup>2</sup> O emissions. Thus, high-frequency measurements over a long time period are important to accurately quantify N<sup>2</sup> O emissions and assess the overall effectiveness of NIs because with short-term experiments and low-frequency measurements effects may be over- or underestimated. This is due to the regulation of N<sup>2</sup> O emissions via soil temperature, moisture, and inorganic N contents which leads to strong seasonal and even daily fluctuations [38].

After 7 years of applying both fertilizers, they found (14 months after the last application) that denitrification rates were higher in the pig compost treatment probably due to higher organic

even though denitrifier activity increased under pig compost, it is still a vital option to reduce

rates. The decreases in soil pH and the subsequent enhanced microbial respiration lead to a decrease in the redox potential which provides good conditions for denitrifiers [20, 21]. Other factors favoring denitrification are increased soil moisture (which leads to decreas-

sions mainly derived from denitrification and were stimulated after residue application in all treatments. However, the level of emissions strongly depended on soil moisture and residue quality. The incorporation of legumes, which are characterized by high N contents and low C/N ratios, induced faster and greater emissions than ryegrass and control soil. Thus, higher soil moisture and residues with a low C/N ratio like lettuce or clover can lead to higher N<sup>2</sup>

Especially when applied to soils with low C content, animal manure and slurry tend to

ing synthetic N fertilizer application, residue type, and application method, as well as land

applied together with synthetic fertilizers but increased by 42% if only residues were applied.

through decreased dissolved organic carbon (DOC) in the soil, thus decreasing denitrifica-

paddy fields but increased by 93 and 24% in fallows and uplands, respectively. For upland soil, the application of residues led to favorable (increased) water and soil temperature condi-

emissions. For example, mulching of crop residues, which seems to enhance microbial activity, increased emissions by 63%, while ditch mulching, burying, and burning all decreased

and 138%, respectively, probably due to their low C/N ratios of 7.5 and 12, respectively [22].

of plant residues with or without urea also found that lower C/N ratios led to higher DOC

O emissions highly depended on land use type and were decreased by 27% in

The combination of chemical fertilizer with crop residues seems to inhibit N2

emissions by around 27%. Also, lettuce and bean residues enhanced N<sup>2</sup>

increased, while residues + urea decreased DOC which affected N<sup>2</sup>

newly available C leads to a sudden rise in microbial activity and O<sup>2</sup>

ates anaerobic conditions favorable for denitrification [20].

A meta-study that analyzed crop residue effects on N<sup>2</sup>

use type. For example, compared to controls, N<sup>2</sup>

An incubation study that assessed the effects on N<sup>2</sup>

O emissions because of their large readily available C and N concentrations. The

O emissions when averaged across all studies [22]. They concluded

O emissions. The application method can also influence N<sup>2</sup>

O emissions. They also found that the residue treatments

O emissions from OAs like residues depends on many factors includ-

into loamy sand soil with different soil moisture levels. The authors found that N<sup>2</sup>

 concentrations) and using residues with a high N content and low C/N ratios. In a 28 day study by [21], different residues (leguminous species and ryegrass) were incorporated

O emissions were lower, possibly due to high

O emissions through increased denitrification

production.

Mitigation of Nitrous Oxide Emissions during Nitrification and Denitrification Processes…

production. The authors concluded that

http://dx.doi.org/10.5772/intechopen.81548

O emis-

consumption which cre-

O emissions

O emissions by 123

O

O emissions found no statistically

O emissions were decreased by 12% when

O emissions of a wide range of C/N ratios

O emissions [47].

O

83

C contents and microbial biomass. However, N<sup>2</sup>

However, OA application can also increase N<sup>2</sup>

N2

ing O2

emissions [21, 23].

significant effect on N<sup>2</sup>

that the quantity of N<sup>2</sup>

tions for microbes related to N2

concentrations which increased N2

increase N2

tion rates. N<sup>2</sup>

organic C contents and higher pH which favors N<sup>2</sup>

O emissions if the conditions are more favorable for N<sup>2</sup>

In conclusion, N<sup>2</sup> O mitigation using NIs is only advisable when "initial" N<sup>2</sup> O emissions from conventional chemical fertilizers are critically contributing to the annual N<sup>2</sup> O emissions. Thus, careful assessment is needed before NIs are introduced to the system especially when economic and ecologic results are considered.

## **2.2. Controlled-release fertilizers (CRFs)**

Numerous studies have shown the effectiveness of CRFs like coated urea to mitigate N<sup>2</sup> O emissions compared to conventional fertilizers [9, 44]. Ref. [9], for example, recorded a reduction of N2 O emissions of 42% with polymer-coated urea when compared with urea in no-till and strip-till environments for irrigated corn in Colorado. It was shown in a global metaanalysis that—while N2 O emissions from urea were high—coated urea had one of the lowest emission factors and manure was somewhat in the middle among all investigated fertilizers [34]. Ref. [14] found a reduction of N2 O emissions by 35% compared with conventional fertilizers when CRFs were applied. However, the effectiveness varied depending on the soil and land use type. CRFs were very effective for imperfectly drained Gleysol grassland (77% N<sup>2</sup> O reduction) but ineffective for well-drained Andosol upland field. Land use type seems to be a strong factor controlling N2 O emissions after CRF application [14, 44–46]. Ref. [46] evaluated the effects of soil and fertilizer types on oil palm plantations using urea and coated urea and sandy loam, sandy, and peat soil. They found high variations in the N<sup>2</sup> O emissions dependent on the treatments with sandy soil emitting the lowest and peat soil the highest emissions overall. Coated urea reduced N<sup>2</sup> O emissions by around 40% in the sandy loam soil compared to the conventional fertilizer, but emissions increased in the other soils during dry season. In a study that tested an NI (DCD) and coated urea on two contrasting soils, Andosol and Fluvisol, the NI only significantly reduced N<sup>2</sup> O emissions (compared to urea) in Andosol, and nitrification was the dominant process compared to Fluvisol. For coated urea, N<sup>2</sup> O emissions even increased in the Andosol and were ineffective in the Fluvisol. The authors concluded that varying activities in the main N<sup>2</sup> O production processes, nitrification and denitrification, in the different soils were responsible for the contrasting results of the EEFs [45].

#### **2.3. Organic amendments**

The role of OAs in altering N<sup>2</sup> O emissions is problematic [15, 20]. Organic amendments directly contribute to N2 O emissions through N transformation processes. OAs also indirectly influence N<sup>2</sup> O emissions, e.g., through altering the soil pH or through their C:N ratio. A high C:N ratio increases soil organic matter; however, a low C:N ratio may increase N<sup>2</sup> O emissions from soils [20–22, 47].

The results concerning N2 O emissions after OA application are contradictory and also depend on which OA is applied. Some studies report reduced or similar N<sup>2</sup> O emissions after OA application when compared to conventional fertilizers. Ref. [19] tested the effects of composted pig manure compared to a conventional fertilizer (ammonium nitrate) on N<sup>2</sup> O emissions. After 7 years of applying both fertilizers, they found (14 months after the last application) that denitrification rates were higher in the pig compost treatment probably due to higher organic C contents and microbial biomass. However, N<sup>2</sup> O emissions were lower, possibly due to high organic C contents and higher pH which favors N<sup>2</sup> production. The authors concluded that even though denitrifier activity increased under pig compost, it is still a vital option to reduce N2 O emissions if the conditions are more favorable for N<sup>2</sup> production.

effects may be over- or underestimated. This is due to the regulation of N<sup>2</sup>

conventional chemical fertilizers are critically contributing to the annual N<sup>2</sup>

daily fluctuations [38].

economic and ecologic results are considered.

82 Soil Contamination and Alternatives for Sustainable Development

**2.2. Controlled-release fertilizers (CRFs)**

[34]. Ref. [14] found a reduction of N2

In conclusion, N<sup>2</sup>

tion of N2

analysis that—while N2

strong factor controlling N2

overall. Coated urea reduced N<sup>2</sup>

Fluvisol, the NI only significantly reduced N<sup>2</sup>

that varying activities in the main N<sup>2</sup>

**2.3. Organic amendments**

directly contribute to N2

from soils [20–22, 47].

The results concerning N2

influence N<sup>2</sup>

The role of OAs in altering N<sup>2</sup>

temperature, moisture, and inorganic N contents which leads to strong seasonal and even

O mitigation using NIs is only advisable when "initial" N<sup>2</sup>

Thus, careful assessment is needed before NIs are introduced to the system especially when

Numerous studies have shown the effectiveness of CRFs like coated urea to mitigate N<sup>2</sup>

emissions compared to conventional fertilizers [9, 44]. Ref. [9], for example, recorded a reduc-

and strip-till environments for irrigated corn in Colorado. It was shown in a global meta-

emission factors and manure was somewhat in the middle among all investigated fertilizers

izers when CRFs were applied. However, the effectiveness varied depending on the soil and land use type. CRFs were very effective for imperfectly drained Gleysol grassland (77% N<sup>2</sup>

reduction) but ineffective for well-drained Andosol upland field. Land use type seems to be a

the effects of soil and fertilizer types on oil palm plantations using urea and coated urea and

on the treatments with sandy soil emitting the lowest and peat soil the highest emissions

to the conventional fertilizer, but emissions increased in the other soils during dry season. In a study that tested an NI (DCD) and coated urea on two contrasting soils, Andosol and

even increased in the Andosol and were ineffective in the Fluvisol. The authors concluded

application when compared to conventional fertilizers. Ref. [19] tested the effects of compos-

nitrification was the dominant process compared to Fluvisol. For coated urea, N<sup>2</sup>

in the different soils were responsible for the contrasting results of the EEFs [45].

C:N ratio increases soil organic matter; however, a low C:N ratio may increase N<sup>2</sup>

ted pig manure compared to a conventional fertilizer (ammonium nitrate) on N<sup>2</sup>

on which OA is applied. Some studies report reduced or similar N<sup>2</sup>

sandy loam, sandy, and peat soil. They found high variations in the N<sup>2</sup>

O emissions of 42% with polymer-coated urea when compared with urea in no-till

O emissions from urea were high—coated urea had one of the lowest

O emissions after CRF application [14, 44–46]. Ref. [46] evaluated

O emissions by around 40% in the sandy loam soil compared

O emissions (compared to urea) in Andosol, and

O production processes, nitrification and denitrification,

O emissions is problematic [15, 20]. Organic amendments

O emissions through N transformation processes. OAs also indirectly

O emissions after OA application are contradictory and also depend

O emissions, e.g., through altering the soil pH or through their C:N ratio. A high

O emissions by 35% compared with conventional fertil-

O emissions via soil

O emissions from

O emissions.

O

O

O emissions dependent

O emissions

O emissions

O emissions.

O emissions after OA

However, OA application can also increase N<sup>2</sup> O emissions through increased denitrification rates. The decreases in soil pH and the subsequent enhanced microbial respiration lead to a decrease in the redox potential which provides good conditions for denitrifiers [20, 21]. Other factors favoring denitrification are increased soil moisture (which leads to decreasing O2 concentrations) and using residues with a high N content and low C/N ratios. In a 28 day study by [21], different residues (leguminous species and ryegrass) were incorporated into loamy sand soil with different soil moisture levels. The authors found that N<sup>2</sup> O emissions mainly derived from denitrification and were stimulated after residue application in all treatments. However, the level of emissions strongly depended on soil moisture and residue quality. The incorporation of legumes, which are characterized by high N contents and low C/N ratios, induced faster and greater emissions than ryegrass and control soil. Thus, higher soil moisture and residues with a low C/N ratio like lettuce or clover can lead to higher N<sup>2</sup> O emissions [21, 23].

Especially when applied to soils with low C content, animal manure and slurry tend to increase N2 O emissions because of their large readily available C and N concentrations. The newly available C leads to a sudden rise in microbial activity and O<sup>2</sup> consumption which creates anaerobic conditions favorable for denitrification [20].

A meta-study that analyzed crop residue effects on N<sup>2</sup> O emissions found no statistically significant effect on N<sup>2</sup> O emissions when averaged across all studies [22]. They concluded that the quantity of N<sup>2</sup> O emissions from OAs like residues depends on many factors including synthetic N fertilizer application, residue type, and application method, as well as land use type. For example, compared to controls, N<sup>2</sup> O emissions were decreased by 12% when applied together with synthetic fertilizers but increased by 42% if only residues were applied. The combination of chemical fertilizer with crop residues seems to inhibit N2 O emissions through decreased dissolved organic carbon (DOC) in the soil, thus decreasing denitrification rates. N<sup>2</sup> O emissions highly depended on land use type and were decreased by 27% in paddy fields but increased by 93 and 24% in fallows and uplands, respectively. For upland soil, the application of residues led to favorable (increased) water and soil temperature conditions for microbes related to N2 O emissions. The application method can also influence N<sup>2</sup> O emissions. For example, mulching of crop residues, which seems to enhance microbial activity, increased emissions by 63%, while ditch mulching, burying, and burning all decreased emissions by around 27%. Also, lettuce and bean residues enhanced N<sup>2</sup> O emissions by 123 and 138%, respectively, probably due to their low C/N ratios of 7.5 and 12, respectively [22]. An incubation study that assessed the effects on N<sup>2</sup> O emissions of a wide range of C/N ratios of plant residues with or without urea also found that lower C/N ratios led to higher DOC concentrations which increased N2 O emissions. They also found that the residue treatments increased, while residues + urea decreased DOC which affected N<sup>2</sup> O emissions [47].

More studies are necessary to evaluate the effectiveness of OAs in reducing N<sup>2</sup> O emissions. OA application mainly affects soil parameters like moisture, pH, O<sup>2</sup> concentration, and substrate availability; however, these factors are the main influencers of nitrification and denitrification processes. Possibilities to influence and mitigate N<sup>2</sup> O emissions after OA application mainly arise from changing the management practices, e.g., cultivation practices and application rates. To avoid NH<sup>3</sup> volatilization, OAs should be incorporated into the soil to avoid contact with the atmosphere; however, this might increase N<sup>2</sup> O emissions due to higher N availability in the soil [20].

equity interests; and expert testimony or patent-licensing arrangements) or nonfinancial interest (such as personal or professional relationships, affiliations, knowledge, or beliefs) in

Mitigation of Nitrous Oxide Emissions during Nitrification and Denitrification Processes…

http://dx.doi.org/10.5772/intechopen.81548

85

[1] Erisman JW, Sutton MA, Galloway J, et al. How a century of ammonia synthesis changed

[2] Allito B, Nana E-M, Alemneh A. Rhizobia strain and legume genome interaction effects on nitrogen fixation and yield of grain legume: A review. Molecular Soil Biology.

[3] Ferguson BJ, Mens C, Hastwell AH, et al. Legume nodulation: The host controls the

[4] Timilsena YP, Adhikari R, Casey P, et al. Enhanced efficiency fertilisers: A review of formulation and nutrient release patterns. Journal of the Science of Food and Agriculture.

[5] Galloway JN, Townsend AR, Erisman JW, et al. Transformation of the nitrogen cycle.

[6] Galloway JN, Dentener FJ, Capone DG, et al. Nitrogen Cycles: Past, Present, and Future.

[7] Inselsbacher E, Hinko-Najera Umana N, Stange FC, et al. Short-term competition between crop plants and soil microbes for inorganic N fertilizer. Soil Biology and Biochemistry.

[8] Trenkel ME. Slow and Controlled-Release and Stabilized Fertilizers: An Option for Enhancing Nutrient Use Efficiency in Agriculture. Paris: International Fertilizer Industry Association (IFA); 2010. DOI: 10.1017/CBO9781107415324.004. [Epub ahead of print] [9] Halvorson AD, Snyder CS, Blaylock AD, et al. Enhanced-efficiency nitrogen fertilizers: Potential role in nitrous oxide emission mitigation. Agronomy Journal. 2014;**106**:715-722

[10] Azeem B, Kushaari K, Man ZB, et al. Review on materials & methods to produce controlled release coated urea fertilizer. Journal of Controlled Release. 2014;**181**:11-21

Biogeochemistry. 2004;**70**(2):153-226. DOI: 10.1007/s10533-004-0370-0

the subject matter or materials discussed in this manuscript.

\*Address all correspondence to: uchiday@chem.agr.hokudai.ac.jp

the world. Nature Geoscience. 2008;**1**:636-639

party. Plant, Cell & Environment. 2018:1-11

Research Faculty of Agriculture, Hokkaido University, Sapporo, Japan

**Author details**

**References**

2015;**6**:1-12

2015;**95**:1131-1142

2010;**42**:360-372

Science. 2008;**320**:889-892

Yoshitaka Uchida\* and Isabell von Rein

A possible option to mitigate N<sup>2</sup> O emissions from OAs is to combine them with EEFs. In combination with NIs, N<sup>2</sup> O emissions from liquid manure like cattle slurry or pig manure can be greatly reduced [15, 48–50]. [48] tested the effect of the NI DMPP in reducing N<sup>2</sup> O emissions from grassland after cattle slurry application. They found that DMPP reduced cumulative N<sup>2</sup> O emissions by 69 and 48% in autumn and spring, respectively. In another study using pig manure on irrigated rice in a Mediterranean environment, DMPP also reduced N<sup>2</sup> O emissions [50]. But so far, studies on the combination of OAs with NIs are scarce compared to the single treatments.

In summary, OAs have the potential to be environmental-friendly if soil parameters like moisture and pH are carefully evaluated, and the best management practices, e.g., no tillage or split application, are applied. In addition, careful assessment of optimal OA rates, improved timing of OA application, and crop N demand or using a combination of OAs and stabilized fertilizers are some of the options to decrease N2 O emissions after OA application [20].

## **3. Conclusion and recommendations**

Overall, the N budget has to be understood first to estimate the potential benefit of EEF on N<sup>2</sup> O emissions. N<sup>2</sup> O emission peaks and the annual contribution of different periods (e.g., sowing, after harvest, rainy season) have to be assessed to determine the use of EEF effectively. Also, the contribution of nitrification, denitrification, and other processes concerning N turnover needs to be assessed in more detail. Especially, microbes, the drivers of these processes, need to be addressed in terms of changes in their activity, community structure, and abundance after EEF application and the subsequent changes in N turnover processes. In summary, more studies are necessary to assess the overall effectiveness of EEFs since their power in mitigating N2 O emissions varies with many factors. An individual fertilizer plan and type which is optimized for the prevalent climate conditions, soil type, management practice, etc. would be optimal in order to effectively reduce N<sup>2</sup> O emissions. The use of EEFs has to be combined with the assessment of an N balance approach to fully exploit their potential.

## **Conflict of interest**

The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interests; and expert testimony or patent-licensing arrangements) or nonfinancial interest (such as personal or professional relationships, affiliations, knowledge, or beliefs) in the subject matter or materials discussed in this manuscript.
