Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative to Chemical Fertilizers

*Tuba Arjumend, Ercüment Osman Sarıhan and Mehmet Uğur Yıldırım*

## **Abstract**

Fertilizers have become a necessity in plant production to fulfill the rapid rise in population and, as a result, the increased nutritional needs. However, the unintended and excessive use of chemical fertilizers causes many problems and has a negative impact on agricultural production in many countries today. The inability to determine the amount, types, and application periods of the applied fertilizers adversely affects the natural environment, resulting in global warming and climate change, as well as the occurrence of additional abiotic stressors that have an impact on agricultural productivity. Hence, alternatives to chemical fertilizers and pesticides, such as the use of biofertilizers, must be explored for the betterment of agricultural production in a manner that does not jeopardize the ecological balance. Bacteria residing in the plant's rhizosphere can help with plant development, disease management, harmful chemical removal, and nutrient absorption. Introducing such phytomicrobiome into the agricultural industry is an effective approach as a result of its long-term and environmentally favorable mechanisms to preserve plant health and quality. Hence, this chapter aims at highlighting the deleterious effects of chemical fertilizers and providing a striking demonstration of how effectively plant-growth-promoting rhizobacteria (PGPR) can be used to increase the agriculture production in the context of climate change.

**Keywords:** chemical fertilizers and pesticides, climate change, global warming, biostimulants, phytomicrobiome, PGPR

### **1. Introduction**

Today's global population is estimated to be over 7.9 billion people, which is expected to reach 9.9 billion in 2050, 34% higher than it is now [1]. Developing countries will account for nearly all of this overpopulation. To feed this growing population, agricultural lands must be used considerably more effectively, and production should be boosted by 70% compared to today's values [2]. Besides, agricultural

production areas are unfortunately facing major ecological challenges, owing to human misapplications, natural calamities, as well as the impact of global climate change [3]. As a result of these factors, today the condition of our current lands is deteriorating leaving us with no choice but to grow nutrient-rich, chemical-free agricultural produce for human and animal use while using far less water and arable land than in the past. This is why a focus on both quality and quantity should be placed on food production without depleting natural resources. Developing and disseminating improved agricultural methods and technologies are equally critical.

Since cultivation areas are dwindling year after year, fertilizer mineral is a world market item that is vital to produce a higher plant yield per unit area and attain food security. It must be available in adequate quantities and in the proper balance to close the gap between nutrient supply from soil and organic sources and nutrient demand for optimal crop development [4]. Not just that, fertilizer is critical for the nation's economy to grow, as agriculture is the primary source of employment. By 2025, it will ensure food security for more than 8 billion people around the globe [5]. The increase in the use of chemical fertilizers by approximately 5 million tons in 10 years is a situation that should be considered while the agricultural areas are decreasing. However, it is more necessary to keep the soil's plant nutritional balance by considering climate, soil, and plant characteristics rather than the amount of chemical fertilizers utilized, and fertilizing based on soil analysis is critical.

## **2. The use of chemical fertilizers in agriculture**

Fertilizer is recognized as one of the most valuable agricultural production inputs, and synthetic fertilizers are becoming increasingly popular around the world. The global fertilizer market was valued at \$155.8 billion in 2019, with a compound annual growth rate (CAGR) of 3.8% predicted for the forecast period (2019–2024) [6]. Fertilizer consumption climbed from 10,777,779 million tons in 2015 to 14,495,815 million tons in 2020, a record high. The total global demand for fertilizers (N + P + K) was estimated at 198.2 million metric tons (mmt) in 2020/2021, according to the International Fertilizer Association (IFA). This was nearly 10 mmt, or 5.2% higher than in 2019–2020 and was the highest rise since the 2010–2011 fiscal year. Nitrogen experienced a 4.1% increase in demand to 110 mmt. Phosphorus demand increased by 7.0% (3.3 Mt), reaching 49.6 Mt., while demand for potash rose by 6.2% (2.2 Mt) to 38.5 Mt. [7]. In the last 50 years, the amount of chemical fertilizer used throughout the world has increased dramatically (**Figure 1**) [8].

Chemical fertilizers have also become more popular in Turkey in recent years, where the cultivation areas are decreasing every year, the need for fertilization is increasing, since more plant production per unit area is required. According to TUIK (Turkish Statistical Institute) 2021 statistics, both the use of fertilizers and nitrogen fertilizers has increased in agricultural production in Turkey in the last 10 years. TUIK statistics showed that annual fertilizer use in Turkey increased from 9,074,308 tons to 14,495,815 tons between 2010 and 2020, and the use of nitrogenous fertilizers increased from 5,995,500 tons to 9,774,691 tons within these values. The amount of fertilizer per unit production area is 107 kg/ha. The use of chemical fertilizers in agricultural inputs accounts for a share of 15–20% [9].

Advances in fertilization and agricultural applications have led to a significant increase in crop productivity in many regions, including Turkey. The most important chemical fertilizers applied to obtain more efficiency in plant production are those

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

#### **Figure 1.**

*Global usage of chemical fertilizer since 1970 [8].*

containing nitrogen, phosphorus, and potassium. Nitrogen fertilizers (N), however, are the most widely used chemical fertilizers in the world, as well as in Turkey, and play a unique role in plant production. Potassium fertilizers (K2O) are the second most consumed after nitrogen, followed by phosphorus fertilizers (P2O5) [8].

It has been determined that 87% of agricultural lands in Turkey have poor organic matter content [10]. Therefore, agricultural production is supported by fertilization, and nitrogen fertilizers constitute an important part of the total fertilizer applied. According to TUIK data, nitrogenous fertilizer usage rates as a percentage of total fertilizer use have shifted between 65 and 69% in the last 10 years [9]. Fertilizer use benefits plants in a variety of ways, including being a less expensive source of nutrients, having significant nutrient content and solubility, making it easily available to plants, and requiring less fertilizer, hence making it more suited than organic fertilizer [11]. Despite these advantages, mineral fertilizer has a number of negative environmental consequences as a result of rising consumption and decreased nutrient utilization efficiency. As a result, in intensive agricultural production systems, integrating intense cultivation with high nutrient utilization efficiency is a key difficulty.

### **3. Harmful effects of unnecessary chemical fertilizer use**

Though conscious fertilization is desirable, the use of improper fertilizers can be extremely harmful, posing severe problems for current and future generations [12]. Sometimes, unfortunately, a wrong perception occurs among the producers of chemical fertilization. It is thought that more efficiency can be obtained by using more chemical fertilizers. Contrary to popular belief, the "LAW OF DECREASING PRODUCTION" is valid in fertilization. That is, the benefit derived from fertilization rises up to a point, after which continuing to apply fertilizer causes harm rather than a benefit.

The unintended and excessive use of chemical fertilizers to boost yields, as well as rising reliance on them, has a negative impact on the agricultural production system's sustainability as well as financial losses in many countries today [13]. Certain factors, such as changes in fertilizer type, variations in application times, the producer's lack of understanding in this area, and improper fertilizer applications, in particular, have been found to have quite substantial environmental consequences and threatening effects on the health and life of living creatures [14]. The inefficient and not demand-oriented fertilization applications in agricultural production can lead to soil acidity and soil crust, low organic matter and humus content, heavy metal accumulation, decrease in pH values, soil salinity, plant nutritional imbalances, limited plant growth, erosion, a decline in microbial activity and efficacy and emission of gasses containing substances that damage the atmosphere and the ozone layer, and eventually the greenhouse effect [15].

The issues at the forefront of the detrimental environmental effects of chemical fertilizers are highlighted here.

#### **3.1 Increased acidity of the soil**

Excessive soil acidity induced by fertilizers is a major cause of soil degradation across the world. Fertilizers, especially nitrogen, acidify soil when applied in excess. This scenario has negative consequences, such as the crops' incapacity to absorb phosphate, the proliferation of hazardous ion concentrations in the soil, hindrance of crop development, and suppression of microorganism activity [16]. If ammonium sulfate fertilizer is given to acidic soil, for example, the acidity level will become even higher. One-way ammonium sulfate fertilization of tea, according to research conducted in the Rize province of Turkey, considerably increased the acidity of low-pH soils. Currently, 85% of the land has fallen below pH 4, which is deemed critical. Likewise, in Nevsehir province, the pH of the soil has dropped to 2 as a result of nitrogen fertilization of potatoes grown in 100-fold increasing acidity over the last 25 years [17].

Hao et al. [18], carried out a field experiment to measure soil acidification rates in response to varied fertilizer sources and N rates, including control, optimal urea, conventional urea, optimized NH4Cl, and conventional NH4Cl plots, nitrogen addition resulted in average H<sup>+</sup> production of 4.0, 8.7, 11.4, 29.7, and 52.6 keq ha−1 yr.−1, respectively. This was followed by a 1–10% decrease in soil base saturation and a 0.1–0.7 unit decrease in soil pH in the topsoil (0–20 cm). In a greenhouse study conducted to evaluate the effect of conventional nitrogen fertilizer on soil salinity and acidity, a significant rise in both soil acidity and salinity was witnessed as N input increased after one season, with pH decrease ranging from 0.45 to 1.06 units [19]. Moreover, after 21 years of application, chemical N fertilizer dropped the soil pH from 6.20 to 5.77, a 0.02 pH unit drop per year [20]. In another study, an evaluation of the impact of long-term fertilizing techniques on soil samples revealed a fall in soil pH from 8.4 to 7.5 [21]. Because nutrients are less available to plants in acidic soil, serious plant nutritional deficiencies are prevalent, resulting in overall crop reduction.

#### **3.2 Deposition of heavy metals**

Heavy metal deposition in soils is mostly caused by the manufacture and consumption of industrial products, although fertilizers and pesticides used in agriculture also contribute significantly. Arsenic (As), copper (Cu), nickel (Ni), cadmium (Cd), and uranium (Ur), among other heavy metals, can build up in the soil following repeated chemical fertilizer applications, particularly phosphorus (P) fertilizers and their source material [22–24]. These toxic heavy metals not only pollute the environment, but they may also cause soil degradation, plant development retardation, and perhaps impair human health through food chain contamination harming the central nervous system, circulatory system, excretory system, and cardiovascular system, as well as cause bone damage, endocrine disruption, and possibly cancer [25].

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

Phosphorus (P) fertilizer is widely utilized in agriculture due to its vital function in crop growth and production [26]. However, P fertilizer has been recognized as the predominant cause of HMs pollution in soil when compared to potassium (K) and nitrogen (N) fertilizers [27]. According to a 10-year field trial, P fertilization aided Zn, Pb, Cd, and As buildup in the topsoil. With increasing P application, the threshold cancer risk (TCR) associated with As and Cd increased [28]. Likewise, another experiment concluded that frequent application of P fertilizer and the extended residence period of HMs may generate a large accumulation of HMs in soils [29].

Heavy metals are concentrated in agricultural soil as a result of improper application of commercial fertilizers, manure, sewage, or sewage sludge [30]. The results of the study conducted by Huang and Jin [31] suggested that the long-term usage of exaggerated synthetic fertilizers and organic manures contributed to the heavy metals (HMs) accumulation in the soils. Research carried out by Atafar et al. [32], confirmed that the fertilizer use enhanced the amounts of Cd, Pb, and As in cultivated soils. Before fertilization, the Cd, As, and Pb concentrations in the studied location were 1.15–1.55, 1.58–11.55, and 1.6–6.05 mg/kg, respectively; after harvesting, values were 1.4–1.73, 26.4 5.89, and 2.75–12.85 mg/kg soil for Cd, As, and Pb, respectively. The findings of another study concluded that chemical fertilizer usage increased the availability of Cu, Ni, Pb, and Zn as well as the buildup of Cd, Cu, and Zn in the greenhouse soil [33].

#### **3.3 Salinity of the soil**

Salts are a common component of chemical fertilizers and are considered destructive to agriculture because they harm soil and plants. Increases in the salinity of the soil can be seen by natural or artificial means. Artificially induced salinity is the result of the accumulation of fertilizers used in large quantities over long periods of time in areas where intensive farming is practiced, making the soils unsuitable for production [22, 34, 35]. Following one season of conventional nitrogen fertilizer, electrolytic conductivity increased from 0.24 to 0.68 mS cm−1 [19]. Long-term intensive farming raised soil electrical conductivity (ECe), which rose from "low salinity" (1.5 dS m−1 0.49) to "highly saline" (6.6 dS dS m−1 1.35) levels [21].

Soil salinity is a major global issue that has a negative impact on agricultural output. Salinization of agricultural land diminishes economic advantages greatly, as demonstrated by Welle and Mauter [36] in California, where salinization lowered overall agricultural income by 7.9%.

#### **3.4 Nutritional inadequacy**

Inorganic fertilizers used recklessly can cause nutritional imbalances in the soil, thus limiting the intake of other essential nutrients. If the common NPK type is frequently used, secondary and micronutrient deficiencies occur in the soil and crop. Excess nitrogen and phosphate fertilizers, for instance, enable the plant to absorb more potassium than it requires. In acidic soils, lime and lime-containing fertilizers lead to the retention of micro plant nutrients, such as P, B, Fe, and Zn in the soil. Over-applied phosphorus fertilizers also prevent the uptake of nutrients, such as Ca, Zn, and Fe, and reduce their efficacy [22, 37].

#### **3.5 The influence on soil friability**

Soil compaction is a key component of the land degradation syndrome and a serious issue for modern agriculture, negatively impacting soil resources [38]. Overuse of fertilizers for extended periods of time and intensive cropping are two of the main causes of compaction. Chemical fertilizers damage soil particles, resulting in compacted soil with poor drainage and air circulation [39]. Reduced soil aeration has an impact on soil biodiversity. Microbial biomass may be diminished as a result of severe soil compaction. Soil compaction may not affect the amount of macrofauna, such as earthworms, but it does affect the distribution of macrofauna, which is important for soil structure.

Soil compaction leads to high soil strength and bulk density, poor drainage, poor aeration, limited root growth, erosion, runoff, and soil deterioration, hence resulting in low permeability, hydraulic conductivity, and groundwater recharge [40, 41]. High soil compaction stifles root growth, reducing the plant's ability to absorb nutrients and water. Compaction, according to reports, reduces root growth and yield by more than 80% [42]. As the soil bulk density increases, nitrification drops by 50%, and plants use less N, P, and Zn from the soil [43]. The findings of the research conducted by Massah and Azadegan [44] suggested that in non-compacted and severely compacted soils, bulk density increased from 1.34 to 1.80 Mg.m−3, and penetration resistance increased from 0.89 to 3.54 MPa, respectively. Soil compaction reduced permeability by 81.4%, accessible water by 34%, and yields by 40%.

#### **3.6 Soil structure and microbial activity deterioration**

In agricultural production, the unintentional, excessive, or random application of chemical fertilizers and pesticides degrades the chemical, biological, and physical structure of the soil, resulting in a rise in pathogen and pest populations [45, 46]. Moreover, with intensive and unconscious chemical fertilizer applications, the amount of organic matter in the soil decreases, which adversely affects the microorganism activities and causes the structure of the soil to deteriorate. If the same fertilization errors are repeated, soil structures will deteriorate with each passing year, plant growth will slow as fertilizer doses are increased, and the overall amount of product obtained will decrease. Some of the fertilizers will not be able to hold on to the soil and will be removed with the water. The conversion of nutrients into forms that plants can benefit from will be reduced.

Soil microbial activity is a crucial component of soil health, and soil organisms serve as a mechanism for nutrient recovery, as well as provide a variety of other environmental functions. Chemical fertilizer misuse can have a detrimental and lethal effect on soil quality and microbial community structure, including earthworms, and other soil inhabitants. Prolonged consumption of chemical fertilizers can cause a significant drop in soil pH, which has been associated with a decrease in bacterial diversity and major changes in bacterial community composition [47]. Nitrogen usage in agriculture has a deleterious influence on the nitrogen cycle and the activities of related bacterial communities, including nitrogen-fixing microorganisms such as Rhizobium sp. [48]. Besides, excess nitrogen fertilizers limit the activities of nitrifying bacteria [49].

#### **3.7 Contamination of water bodies and nitrate accumulation**

It is critical to emphasize the importance of understanding how to apply chemical fertilizers properly. Chemical fertilizers, as part of their larger threat to the *Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

environment, animals, and human health, eventually leak into our water bodies, such as ponds, streams, and groundwater, contaminating water supplies, exposing humans and animals to a variety of short- and long-term hazardous chemical effects on their health and bodies. In ideal conditions, it is estimated that roughly 2–10% of fertilizers interfere with surface and groundwater [50]. The accumulation of nitrates in the water emerges as a result of the use of N fertilizers in the agricultural field, which is increasing day by day. Even under ideal conditions, only 50% of the nitrogen fertilizer given to the soil can be taken up by plants; 2–20% evaporates, 15–25% combines with organic compounds in the clay soil, and 2–10% is discharged into streams, rivers, and streams with surface runoff [50, 51]. Nitrate, a frequent contaminant of surface and groundwater, can cause serious health concerns, including inflammation of the colon, stomach, and urine systems. Furthermore, these compounds have been reported as carcinogens that can have a harmful impact on human health. They also have the potential to induce disorders in infants, such as methemoglobinemia, a condition in which the blood carrying capacity is limited due to a decrease in hemoglobin.

## **4. Agriculture and fertilizers' contribution to global warming and climate change**

Though the rise in agricultural productivity alleviated poverty, it also posed a threat to the ecosystem due to its negative consequences. Rising levels of synthetic fertilizer application for agricultural production, for instance, increase greenhouse gas emissions, eroding the protective ozone layer, and exposing humans to harmful ultraviolet radiation [52]. Above all, agriculture is responsible for a major fraction of the greenhouse gas (GHG) emissions that are driving climate change, accounting for 17% directly from agriculture activities and another 7–14% through land-use changes.

During the production of nitrogenous fertilizer, greenhouse gases, such as CO2, CH4, and N2O are released. Moreover, nitrous oxide emissions from soils, fertilizers, manure, and urine from grazing animals, as well as methane generation by ruminant animals and paddy rice agriculture, are the most significant direct agricultural GHG emissions. Both of these gases have a far larger potential for global warming than carbon dioxide.

Agriculture is the primary source of anthropogenic N2O emissions, accounting for 60% of total emissions. It has a 310-fold greater global warming potential than carbon dioxide. Excess nitrogen fertilizer application results in nitrogen oxide emissions (NO, N2O, NO2), which cause serious air pollution [51]. The primary issue with nitrous oxide emissions is the impact of global warming and the function of nitrous oxides in ozone degradation, encouraging the decomposition of the ozone layer [53] and resulting in atmospheric "holes," exposing humans and animals to excessive UV radiation [54]. Water vapor, hydrogen sulfide, and chloro-fluoro hydrocarbons are among the other gases that contribute to ozone depletion [55].

After being volatilized or released from fertilized fields, ammonia is deposited in the atmosphere and oxidized to generate nitric and sulfuric acids, resulting in acid rain. Acid rain has the potential to harm flora, buildings, and species that live in lakes and reservoirs [56]. Methane emissions from transplanted paddy fields are also a major concern, as methane is a powerful greenhouse gas whose concentration is doubled when ammonium-based fertilizers are used. These gases all contribute to global warming and climate change [57].

Climate change is gaining traction, resulting in major global temperature spikes, as well as the prevalence of additional abiotic stressors that are reducing crop output. Significant production losses in major grain crops have been attributed to climate change, resulting in 3.8% yield reductions for maize and 5.5% for the wheat [58, 59].

Fertilization, which is one of the most essential inputs in agricultural operations, increases productivity on the one hand, but its overuse might have negative consequences on the other. Excessive usage of agricultural chemicals jeopardizes the long-term viability of agriculture. Today, the fast expansion in agricultural productivity has begun to slow down [45, 56]. Clean food production becomes inevitable with a healthy and reliable agriculture system that does not require chemicals.

Given that chemical fertilization cannot be completely eliminated in agricultural applications, in this scenario, sustainability initiatives and the usage of ecologically sound technologies can help achieve the goal of enhancing healthier crop productivity whilst eliminating unnecessary input and thereby mitigating harsh weather conditions, as well as improving soil health by sequestering carbon and retaining organic material and mineral nutrients in the soil [60]. Hence, it is vital to use alternatives, such as Plant-Growth-Promoting Rhizobacteria (PGPR), to support sustainable agricultural productivity and everlasting soil fertility and to build production strategies that will aid in the proliferation of beneficial soil microorganisms activities.
