**Table 1.**

*Terrestrial carbon stock.*

**Figure 1.** *Pictorial view of terrestrial carbon stock.*

outflows. However, a disturbance of severe magnitude can disturb this balance. For instance, disturbance caused by human activities like too much fossil fuel burning or deforestation has caused an imbalance leading to high levels of carbon dioxide in atmosphere and consequent global warming/climate change. Though current concentration of CO2 is a small figure of 0.04% (corresponding to about 410 ppm), the greenhouse effect caused by it is severe enough to result in global warming/ climate change. As rise of CO2 levels in atmosphere is on account of anthropogenic emissions, onus lies on humans to take the remedial measures.

#### *2.1.1 Nitrogen cycle*

Like carbon cycle, nitrogen cycle is also a sub cycle of biogeochemical cycles that moves nitrogen. But nitrogen is huge 78% of air as against 0.04% carbon dioxide, though it is inert and not usable. The nitrogen cycle moves and converts the inert atmospheric nitrogen gas into other active forms through processes of nitrogen fixation, nitrification, and denitrification. Organic nitrogen existing in tissues of

#### *Leveraging Soil Microbes with Good Farming Practices for Higher Soil Carbon Sequestration… DOI: http://dx.doi.org/10.5772/intechopen.105201*

organisms moves on consumption of food from food to the food consumer. The atmospheric nitrogen is inorganic which is made available to plants by the process of nitrogen fixation (NF) converting the inert nitrogen into reactive forms like ammonia (NH3). Nitrogen fixation occurs naturally by lightning. Another natural process, called biological nitrogen fixation (BNF) mediated by the symbiotic bacteria converts atmospheric nitrogen into ammonia (NH3) and later into ammonium (NH4). The symbiotic bacteria carrying out BNF are known as diazotrophs. The Azotobacter and Rhizobium are well known examples of diazotrophs. Lastly, the nitrogen fixation is also done by humans as industrial production of nitrogen fertilizers. Under nitrogen cycle, nitrogen fixation is followed by the process of nitrification which converts ammonia/ammonium into nitrites and nitrates. Nitrification, mediated by bacteria in soil makes nitrogen nutrients available in soil for feeding the plants and completes transfer of nitrogen from atmosphere into plants. It is analogous to photosynthesis in carbon cycle which transfers carbon from atmosphere to plants. On consumption of plant-produces, atmospheric nitrogen enters bodies of animals/humans who consumed the plant-produces. When plants/ animals die, the decomposed dead bodies release organic nitrogen back to soil as ammonium which is nitrified to nitrates to feed plants. In nitrogen cycle, nitrification is followed by denitrification process mediated by a set of bacteria that convert nitrates into gaseous nitrogen. Denitrification completes the nitrogen cycle.

Nature controls nitrification and denitrification processes to maintain balance between the two types of nitrogen to sustain life on the planet. But, like carbon cycle, the nitrogen cycle has also been disturbed by human activities like combustion of fuels and use of synthetic nitrogen fertilizers. These activities increase proportion of reactive nitrogen as compared to inert nitrogen unbalancing the cycle. Increasing use of synthetic nitrogen fertilizers deliver reactive nitrogen directly to the soil ecosystem without natural nitrification processes. Hence the cumulative amount of reactive nitrogen in the form of NH3 and NOx is unduly increased which, in turn, increases deposits on land that impacts radiation balance of the earth. In addition, the very process of manufacturing nitrogen fertilizers impacts GHG emissions which is compounded by their application. Hence caution is necessary in this matter.

#### *2.1.2 Carbon and nitrogen linkage*

The biogeochemical cycles of carbon and nitrogen are tightly coupled with each other due to metabolic needs of the organisms for these two elements. In other words, ratio of carbon and nitrogen is fixed in an organism though different organisms may have different C:N ratio. Thus C:N ratio is the inviolable parameter that links carbon with nitrogen for the organisms. So is the case with inorganic substances like fertilizers as well as with different soil ecosystems. So, the C:N ratio characterizes an organism or substance or soil. The carbon-nitrogen (C:N) ratio plays an important role in evaluating suitability of a fertilizer/manure for a particular soil and crop. As nutrient exchange in rhizosphere is mostly through soil microbes, it is important that any soil amendment or fertilizer to be used should be compatible with C:N ratio of the soil microbes. Generally C:N ratio of soil microorganisms is about 8:1, the C:N ration of fertilizer should be good enough to meet this metabolic need along with energy need. As energy need is met from carbon and it is double of metabolic need. Thus, the fertilizer should have a C:N ratio of 24:1 out of which 16:1 will be for energy needs and 8:1 will be for metabolic needs. Foods or fertilizers with less than 24:1 ratio fall short of microbe's carbon needs and cause release of nitrogen from the fertilizer in soil raising the C:N ratio to around 24:1. Similarly, with foods/fertilizers with higher C:N ratio, microbes feeling short

of nitrogen draw nitrogen from soil causing "N" deficit in soil called immobilization, which is made up on death of some microbes, called mineralization. It can also cause release carbon from soil bring down the C:N ratio to about 24:1. Synthetic fertilizers have high C:N ratio and, therefore, are of low quality while composts/ manures having low C:N ratio are of high quality. The low C:N ratio food/residue is favorite of microbes who decompose it fast. The C:N ratio of crop plants is considered while deciding crop rotation. Thus, legume cover crop of low C:N ratio can be followed by wheat crop of high C:N ratio. The C:N ratio also plays a vital role in carbon sequestration in humus having C:N ratio of 10:1 as carbon cannot be sequestered unless adequate nitrogen is available in the carbonic substance being sequestered. In fact, performance of microbial function is also gauged from microbial carbon use efficiency (CUE) which is the ratio of carbon assimilated relative to the carbon lost as carbon dioxide.

#### **2.2 Agriculture as an ecosystem**

As agriculture has biotic and abiotic components interacting within themselves to sustain life, it is an ecosystem. However, it is not a natural ecosystem as farm produces and residues are not allowed to be recycled but removed from the farm. Thus, agriculture is a mixed ecosystem where both nature and farmer operate simultaneously. Since growth of crop plants results in depletion of nutrients in the soil, farmer must replenish or recoup the nutrients. While biogeochemical cycles follow the laws of the nature, there is no law governing the farming activities. Farmers may or may not recognize existence of the natural ecosystem and treat soil as a natural resource to be preserved or treat as nutrient mine to be mined until all reserves are exhausted. Overexploitation or abuse of the natural resource of the soil is counterproductive and self-destructive. Orienting farmers to have an in-depth understanding of the natural ecosystem including soil ecosystem is, therefore, imperative. The cost involved for such orientation/training should be financed by the state as it is more in the interest of the community.

#### **2.3 Soil as ecosystem**

As reservoir of nutrients, soil is the natural resource for the terrestrial ecosystem sustaining life. Soil is an ecosystem also as it has both biotic and abiotic components interacting among themselves and with adjacent ecosystems of atmosphere, oceans, and biosphere (plants/animals) to sustain life. While inorganic/organic nutrients, air and water are major abiotic components, microorganisms with other creatures like worms are biotic components of the soil ecosystem that live on organic matter.

Soil comprising organic and inorganic matter is formed from rocks fragmented by streams, rain, wind, animals, microorganisms, and chemical actions over a long period of time. Though the organic matter content is a small fraction (within 10%) of the soil, it plays the main role in vegetation growth. In fact, soil without organic matter is lifeless dirt unable to support any vegetation. Soil organic matter (SOM), however, is not a homogeneous mass but a combination of live and dead plants/animals under different states of decomposition. As SOM contains 50–60% soil organic carbon (SOC), value of SOM can be used to determine the value of SOC and vice versa. The values of SOM and SOC are indicators of availability of nutrients in the soil as carbon is the major components of plants and other lifeforms.

Soil microbial community includes bacteria, fungi, protozoa, earthworms, insects, reptiles, and other small creatures. In fact, the microbes contributed to formation of the soil itself by etching away rocks with their acid attacks. Their metabolic wastes and dead bodies constitute nutrients for plants. Humus which

#### *Leveraging Soil Microbes with Good Farming Practices for Higher Soil Carbon Sequestration… DOI: http://dx.doi.org/10.5772/intechopen.105201*

contributes to stability of soil is made by soil microbes and mostly from necro mass or dead bodies of microbial population. In other words, soil microbes give their life to ensure soil health and fertility while working hard during their lifetime.

Bacteria and fungi are main microbes that play significant role in maintaining soil health. The Rhizobia, azobacter, and azospirillum are popular names of useful soil bacteria that help build soil structure and maintain soil health and fertility. Most bacteria and fungi have symbiotic relationship with plants. The symbiotic association of fungi with plants is called mycorrhiza while these fungi are called mycorrhizal fungi (MF). The host plant roots grow exudates outside main roots to attract MF to their roots resulting in much higher root biomass. The MF grow hyphae, the thread like structures on their body which extend to far distances forming mycelium network to mobilize nutrients and to work as communication network connecting plant and microbes. The MF are classified as under:


Endomycorrhizal fungi include arbuscular mycorrhizae fungi (AMF) which develop unique "arbuscular" structures at hyphae to enclose plant roots. They produce "glomalin" protein which binds soil particles into aggregates stabilizing soil. Major part of the hyphae lies within intercellular spaces of roots of the host plant for exchanging nutrients while only a small part lies on the surface. Nutrient exchange is a fascinating process. On sensing nearby presence of AMF, the root creates a structure to let in the AMF's hyphal tip up to cell wall and merge with it. A cavity is formed in the merged entity to receive payloads of nutrients from both sides under control of the plant cell membrane.

Ectomycorrhizal fungi form a mantle on the surface of the root. The root cells secrete sugars and other food ingredients into the intercellular spaces to feed the fungal hyphae. Effectively, the hyphae increase surface of the root many times resulting in higher absorption of nutrients. They orchestrate exchange of nutrients from humus/soil to plant. They secrete antimicrobial substances which protect roots from attack of pathogens. Symbiosis of these MF is generally plant specific.

#### **2.4 Nutrients and their exchange**

Carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulphur, calcium, magnesium, chlorine, iron, copper, boron, zinc, nickel, selenium, manganese, cobalt, molybdenum, silicon, and sodium are the well-known nutrients. Carbon which is the major building block of all living systems constitutes more than 50% of plant biomass while nitrogen is about 40% of plant biomass. The plants take "C" and "O" from atmosphere and "H" as water/moisture from atmosphere/soil. Since absorption of "CHO" require no human intervention, these are termed as "basic" while others are termed "non-basic." Among non-basic nutrients, the N, P, K, S, Ca, and Mg are called macronutrients since they exist in major proportion in the plants and constitute structure of the plants. Other nutrients are micronutrients.

All nutrients existing in soil are compounds in the solution form. These are absorbed by the roots directly or indirectly. Most plants absorb nutrients indirectly via soil microbes. The area around the roots where nutrient exchange takes place is called rhizosphere. Though the plants are not mobile, they can acquire macro and micronutrients from distances by means of different mechanisms like changes in root structure and establishment of symbioses. Since deficiency of some nutrients in some soils is always a possibility, plants have evolved nutrient uptake strategies to cope with different situations and nutrient limitations. Changing the root structure is one such strategy adopted by plants to increase the overall surface area of the root and to increase nutrient acquisition to access new nutrient sources [5].

Nitrogen and phosphorus are among the elements most limiting to plant growth and productivity because these nutrients are often present in small quantities or not in bioavailable form. So, plants do form symbiotic relations with soil microorganisms like bacteria and fungi. Use of nitrogen fertilizers is harmful as excess nutrients turn into insoluble form and pollute ground water systems. Interaction of plants and symbiotic microorganisms is quite interesting. When the plant releases compounds called flavonoids into the soil, the bacteria are attracted to the roots. Then bacteria release compounds called nod factors that cause local changes in the structure of the root and root hairs to envelop the bacteria in a small pocket. Further details are skipped to avoid distraction from the main subject.

#### **2.5 Replenishment of nutrients in soil**

As growing plants absorb nutrients from soil, the nutrient reserves in soil get depleted which is made up by farmers by adding organic matter, farmyard manure, compost, or synthetic fertilizer. The synthetic fertilizers with inorganic nutrients do increase yield of crops but not without harmful effects.

Biofertilizers (BF) containing live microorganisms addressing the issue of harmful effects can replace or supplant the synthetic fertilizers. As they contain microorganisms of select bacteria, fungi, or algae, they restore nutrient cycles in soil just as the soil microbes do. As the nutrient replenishment in soil takes place as a natural process, no harmful effects are associated with the use of BF. Being natural, eco-friendly, renewable, and cost-effective the BF are considered as the most sustainable soil solutions [6]. Hence rest of this section covers an elaborate discussion on BF only.

The facts that atmospheric nitrogen can be used by plants through biological nitrogen fixation (BNF) by certain microorganisms and that insoluble soil nutrients can be converted into soluble form through activities of certain other microorganisms are used in formulating biofertilizers. Since most of the phosphorus and potassium nutrients exist in insoluble form, they are not available to plants. Use of certain specific microorganisms can make those nutrients water soluble and bioavailable to plants. Microorganisms that produce plant growth promoting compounds are also used in BF formulations. As microorganisms mainly belong to bacteria and fungi groups, BF are also classified as bacterial and fungal BF as described below.

#### *2.5.1 Bacterial BFs*

Bacterial BF include both nitrogen and phosphorus fertilizers as discussed below.

#### *2.5.1.1 Nitrogen fixation*

The nitrogen fixation process is operationalized by the nitrogenase enzyme which is present in diazotrophic microorganisms such as symbiotic and freeliving nitrogen fixing bacteria. The nitrogen fixing process involves conversion of atmospheric nitrogen into ammonia (NH3) which is bioavailable for plants. Such biological nitrogen fixation (BNF) can meet up to 50% of the demand of all plants though actual nitrogen fixation depends on the plant species and environmental factors. The nitrogen BFs contain nitrogen fixers like Rhizobia which are symbiotic with legumes. As symbiotic relation is between specific bacteria strain and specific crop, the specific strain suitable for a particular crop is selected. Free living bacteria like Azotobacter and Azospirillum which establish loose symbiotic relation with non-legume cereals are also used. As these bacteria also produce growth promoting compounds, these BF are also used as plant growth promoters (PGP).
