Nitrogen Interactions Cause Soil Degradation in Greenhouses: Their Relationship to Soil Preservation in China

Abstract

Proper greenhouse fertilization is crucial for establishing high-quality yields, particularly as food demand grows. In this review, the effect of fertilizers, specifically nitrogen, on greenhouses and degradation caused by nitrogen interactions are critically evaluated based on a literature analysis. Nitrogen (N) fertilizers, which represent reactive or biologically accessible nitrogen in soil, are currently used in agricultural systems. Soil, water, and air are endangered by reactive nitrogen pollution. Increasing food demand causes a rise in N fertilizer use, which harms the environment and living organisms. In developing countries, more N is used per capita than in underdeveloped countries. Greenhouse agriculture accounts for 3.6% of total agricultural production. It was revealed that greenhouses in China often get 13–17 times as much nitrogen fertilizer as traditional farming. N was overused abundantly throughout the year, which led to soil acidity, nutritional imbalance, and secondary salinization. Studies on soil salinization and secondary salinization in China date back 70 years. This review attempts to draw attention to the soil damage in greenhouses caused by excessive nitrogen. Nitrate leaching and soil acidity received special attention in this review. Numerous eco-friendly techniques for avoiding soil degradation brought on by the execessive use of fertilizer are also discussed.

1. Introduction

Agricultural production has become more intensive as a consequence of rising consumer demand for more nutritious foods [1], which promotes greenhouse production [2]. China grows 35% of its veggies in greenhouses [3]. The major causes of soil deterioration include an overuse of fertilizer and conventional farming [4]. Proper greenhouse crop fertilization is critical for developing high-quality, high-yielding plants, especially as the demand for food increases. By 2050, the world’s population is expected to rise to 9 billion, imposing more strain on today’s agricultural system [5]. Continual reliance on fertilizers is essential for agriculture as food demand grows [6]. Chemical fertilizers have brought agricultural production and scientific practice to a new level [7]. Chemical fertilizers have been proven to boost crop yields and accelerate agricultural growth when used fully and strategically [8]. Increased N fertilizer applications will result in increased yield enhancement. The current study investigated the effects and the molecular mechanisms of a legume-derived pH under optimal and sub-optimal nitrogen (N) concentrations (112 and 7 mg L⁻¹, respectively) in tomato (Solanum lycopersicum L.) [9]. The Food and Agriculture Organization (FAO) investigated that most contemporary crop production relies on nitrogen fertilizer to maximize yields and costs, whether grown in normal tile-drained fields or in greenhouses [10]

The intensive chemical fertilizer usage, particularly nitrogen, raises nitrate levels in plants, a chemical component present naturally in plants, in order to improve crop output [11]. Nitrate is found in abundance in many plants, particularly in vegetables; nitrate contents in leafy and kitchen produce, grains, and tuber vegetables were 130, 48, 101, and 61 ppm, respectively [12].

Half to two-thirds of the anions in greenhouse soils are derived from nitrate, which is produced when excess N is oxidized, resulting in a decrease in soil pH [13]. Plants may be more vulnerable to physiological disorders, such as a nutritional disorder resulting from the soil’s excessive nitrate concentration and acidity. Besides, N reduces helpful microorganisms, accelerates microbial communities from bacterial to fungal dominance, and, hence, exacerbates soil-borne illnesses. One of the world’s most pressing ecological problems is the runoff of nutrient-rich water that releases reactive nitrogen molecules like nitrate (NO₃⁻) and ammonium (NH₄⁺) into aquatic ecosystems [14].

Consuming vegetables in a semi-closed greenhouse vegetable production (GVP) environment poses a risk to human health due to contamination of the soil with pollutants. Ref. [15], has provided an overview of the changes in soil characteristics, nutrient balances, and other agrochemicals (such as fungicides and phthalic acid esters) under GVP in Eastern Chinese provinces. Ref. [16], they discovered that the soils utilized for GVP had more salinity and acidity, as well as a buildup of fertilizers and other agrochemicals, than open-field soils. In a pot study of soils from different-aged GVP units on Shouguang soils in Shandong Province, they discovered that 70-day urea fertilizer nitrification released H+, acidifying soil [17]. GVP cropping periods increased soil acidity and salinity in Yixing County, Jiangsu Province, combined with nitrate buildup from yearly N application exceeding 1300 kg ha⁻¹ in chicken dung, composite fertilizer, urea, and ammonium bicarbonate [16]. In a suburb of Nanjing, Jiangsu Province, soil pH under GVP with three to 10 cropping years dropped by 0.57–1.17 pH units compared to vegetable cultivation in open fields [18].

Three N rates ((urea) 0, 600, and 1200 kg N ha⁻¹) were applied in five soils with different greenhouse cultivation years to evaluate soil acidification and salinization rates induced by the nitrogen fertilizer pH decrease from 0.45 to 1.06 units, whereas the electrolytic conductivity increased from 0.24 to 0.68 mS/cm⁻¹. For every mol of NO2⁻ + NO3⁻-N buildup in soil, 0.92 mol of H⁺ was produced. Under N1 and N2 rates, the proton loading from nitrification was 14.3–27.3 and 12.1–58.2 kmol H⁺ ha⁻¹ in the core of Shandong Province, respectively. On the other hand, the proton loading from lettuce’s absorption of extra bases was only 0.3–4.5% of that from nitrification [3]. Several research studies have been conducted to investigate the effects of excessive N application. Ref. [19], determined that 31 and 19 percent of total applied N accumulated in soils utilized for GVP and open-field soils, respectively. GVP seems to have lower rates of N usage efficiency than open-field vegetable cropping methods [19]. In Yixing, Jiangsu Province, greater rates of N application resulted in a decrease in N usage efficiency [20].

Heavy metal (HM) buildup is another major concern in greenhouse soils in Eastern China as a result of human activities such as excessive fertilizer, manure, pesticide usage, irrigation, and atmospheric deposition [21]. Ref. [15], documented the occurrence of significant quantities and transfer factors of heavy metals along China’s Yellow Sea. Cd, Zn, Ni, and Cu were shown to be the primary metal elements producing GVP soil contamination in parts of China’s Yangtze River Delta [22]. Ref. [23], recommended reducing the fertilization frequency and rate in three typical GVPs in Jiangsu and Shandong Provinces to prevent heavy metal pollution of soils and vegetables. Zinc, lead, cadmium, chromium, and arsenic in fertilizer denature soil minerals and cause nutrient losses. Thus, biological soil degradation and toxicities from fertilizer application exacerbate soil deterioration. Excessive use of chemical and manure fertilizers, as well as insecticides, are the primary sources of heavy metals [24]. Additionally, GVP cultivation that lasts longer than five years often results in greater levels of heavy metal buildup [25]. In Siping, Jilin Province, Northeast China, the heavy metal levels in soils used for GVP were 1.6 and 1.9 times higher than in open field agriculture (maize) and forest soils, respectively [26].

With 51.6% of the world’s total output of vegetables, China is the country with the highest production [27]. In moderately temperate to warm temperate regions, solar greenhouses (SG) and plastic greenhouses (PG) are the two most common forms of greenhouse [28]. Greenhouse agriculture accounts for 3.6 percent of global agricultural production. Regionally, the numbers vary greatly, ranging from 2.5 to 26 percent in areas of extensive greenhouse farming in china, correspondingly. Chinese soils used for GVP and greenhouse vegetables have accumulated pollutants and nutrients as a result of intensive production. The relevance of greenhouse farming was emphasized further due to an increase in self-sufficient regional food production after current challenges with global commerce caused by the coronavirus pandemic in 2019 [29].

2. Growing Population and Rising Food Demand

The rapid expansion of vegetable farming is a direct response to the rising need for food for the world’s growing population. Greenhouse production increases vegetable yields, cropping indices, and farmer income. Non-point source pollution of water resources is caused by the overuse and improper use of chemical fertilizers and manure in greenhouse vegetable growing.

2.1. Application of Nitrogen Fertilizer and (GVPS)

A number of initiatives to modernize the agricultural industry via the use of greenhouse technology have been developed due to rising food demand. Different management methods are included in the greenhouse production system, including greenhouse construction and design, pre- and post-harvest cultural activities, and the monitoring of environmental practices, such as temperature and fertilization, etc. [30]. Greenhouse vegetable production (GVP) has expanded quickly and now accounts for a significant portion of China’s vegetable supply [31]. To compete with other agricultural production systems, GVPS uses higher N application rates. Groundwater nitrate contamination is a worldwide problem, and it is related to the overuse of nitrogen fertilizer in greenhouse vegetable production systems (GVPS) [32]. Greenhouse vegetable systems typically require between 1600 and 2400 mm of water per year for irrigation [33]. Because vegetable systems typically absorb less than 400 kg ha⁻¹ N, the subtraction approach for calculating fertilizer N usage efficiency may provide less than 10% results when utilizing standard management techniques [34].

Consequently, it’s crucial to create BMPs (Best Management Practices) for the long-term growth and prosperity of GVPS’s. Testing has been done to determine the most effective strategies for N management in GVPS to minimize N losses while maintaining consistent crop yields, namely optimal (or reduced) application of N fertilizer [32], straw assimilation [35], planting catch crop [36], and substituting chemical N fertilizer. Short-term, high crop yields may be maintained by the use of soil residual N and may lower N losses if N applications are cut down. However, long-term crop output may be negatively affected by the persistent depletion of soil N at a lower N treatment rate. Consequently, it is important to study the impacts of decreased N application on soil N losses and vegetable output over the long term [37].

2.2. Plastic Greenhouse Production (PGP)

Worldwide, plastic greenhouse production (PGP) systems have expanded rapidly to meet the rising demand for fruits and vegetables. PGP and open fields have different cultivation models. Due to the prolonged confined or semi-closed environment, PGP systems are significantly impacted by human activity [15]. Agrochemical input and the multiple-crop index in PGP were likewise considerably higher than those in open fields. PGP has at least two cropping seasons, as opposed to open fields with grain crops, which have one or two seasons at most per year. Plastic greenhouse production (PGP) systems face significant soil fertility and environmental issues as a result of their heavy reliance on pesticide inputs [38]. Soil acidification [39], soil secondary salinization [40], and imbalances of macronutrients (nitrogen, phosphorous, and potassium) [41] all result from poor nutrient management in PGP systems and have a major impact on soil fertility quality. Soil quality evaluations must include both fertility and environmental condition to deploy PGP systems effectively.

The amount of N, P, and K rose when the greenhouse soils were utilized for growing vegetables for longer periods of time. Similarly, soil salinity increased more in greenhouse soils than in open-field soils. The outcomes indicate that the maintenance of soil quality can be achieved through a balanced NPK fertilizer regime for the growth of sustainable vegetable production systems.

China accounts for over 80% of the world’s cultivation area of plastic-shed vegetable, where plastic sheds are commonly employed for vegetable production. However, soil salinization poses a serious danger to the existing soil’s sustainability in plastic sheds in China. Soils in plastic sheds were significantly salinized in comparison to soils in neighboring open fields. North China and the Bohai Rim had the biggest percent change in soil organic matter (68.5%), while having the least amount of soil impacted by salt (1.84 g kg1) [42]. Soil salinization is caused mostly by nitrate (60–70% of anions), which, in turn, lowers vegetable quality and production [42]. The human body gets more than 80 percent of its nitrates from veggies. Nitrate is rapidly transformed into nitrite, which is harmful to humans for both short- and long-term exposure [43]. Because of the devastating consequences that high levels of nitrate in the soil may have on crop yields and human health, finding a solution to this issue is an urgent priority.

Greenhouse Vegetable Production in China

China has experienced a considerable transition in vegetable production from open fields to greenhouses over the last two decades, because of increased customer demand for vegetables and high profit margins for greenhouse vegetable cultivators [44]. Presently, greenhouses generate almost one-third of all the vegetables farmed in China [45]. On a worldwide scale, China’s utilization of greenhouses for vegetable cultivation is noteworthy. In 2018, it was estimated that there were 5.6 million hectares of greenhouses worldwide [46]; China accounts for around 83 percent of this area [47].

For example, in North China, the average yearly N inputs from chemical fertilizers and manures were 1358 kg N ha⁻¹ and 1881 kg N ha⁻¹, or a combined total of almost 3000 kg ha⁻¹ N, to attain a high level of productivity [48]. From 2013 to 2017, China covered over 1 million hectares of cropland annually. To build PGP systems in a sustainable manner, an assessment approach that considers soil fertility and environmental security is necessary. The use of greenhouses for the cultivation of fresh fruits and vegetables is expanding in many parts of the agricultural world [49].

The greenhouse cultivation area is rapidly expanding with the rising demand for vegetables and horticultural goods. According to the PRC Ministry of Agriculture and Rural Affairs, the entire area of planting facilities, frequent plastic greenhouses, solar greenhouses, and verandas, will be amended at more than 2 million hectares by 2025. It was reported that the total area of greenhouse soil reached 4.708 million hectares in 2019 [50]. The production of vegetables by China in 2020 is shown in Figure 1. Production has demonstrated somewhat of a declining trend during the last two years. COVID-19, which has a significant negative impact on vegetable output both domestically and internationally, might be the cause.

3. Soil Acidification as a Result of Nitrogen/Nitrate Leaching Causing Soil Degradation

Acidification of the soil is a major contributor to soil deterioration [51]. In a controlled agricultural production system, the transformation of carbon (C), nitrogen (N), and sulfur (S) releases protons (H⁺) into the soil solution, causing soil acidification. Reduced pH from soil acidification has a negative impact on soil microbes and plants [52]. The nitrogen absorption, nutrient activity, and all other phenomena that occur in soil are substantially impacted by the upper lower pH levels in the soil solution [53]. Beside this, leaching removed considerable quantities of NO₃⁻ and NH₄^{+ −}NH from the soil. The loss of nitrogen from fertilizer in the form of NO₃⁻ via leaching and subsequent dispersion into the environment has a negative impact on soil productivity, food safety, water quality, and air quality [54]. Ref. [55], noted this loss of NO₃^{¯ I} indirectly. Gao et al. noted that greenhouses in China often get 13–17 times as much nitrogen fertilizer compared to normal farming.

A significant portion of non-point source pollution is caused by nitrogen leaching losses from intensive vegetable systems [56]. The need to precisely measure N leaching from agriculture has arisen in response to rising public concern about the adverse consequences of excessive N fertilization and irrigation and the rising danger of groundwater contamination [57]. Improving yields while decreasing N leaching is the goal of realistic N management methods [58].

Furthermore, crop absorption of N as ammonium also contributes to soil acidification, as does the leaching of nitrate, which may reduce a soil’s acid-neutralizing capacity (ANC) [39]. Soil acidity, nutritional imbalance, and secondary salinization arose from overusing nitrogen fertilizers in China’s plastic greenhouse vegetable field, where N was applied excessively all year [39]. Soil acidification occurs naturally but is hastened by farming practices that disrupt the N and C cycles [59]. When nitrate concentrations in the soil rise beyond the levels required for plant uptake, leaching occurs, disrupting the N cycle. This is a key contributor to soil acidity in greenhouse systems [60]. Soil acidity may result from the disruption of the C cycle brought on by harvesting and product removal [61].

However, long-term research on the effects of regulating irrigation and fertilization to reduce nitrate leaching are uncommon. For a wide variety of needs, many locations throughout the globe must rely only on groundwater supplies [62]. The health of local inhabitants is threatened [63] by nitrate pollution of groundwater, which is mostly produced by agricultural nitrate leaching due to the rising world population and food demand [64]. Figure 2 depicts the nitrogen cycle, which shows the leaching of excess nitrogen from various sources.

3.1. Salinization in PVPs

The salinization also changes the state of the soil’s microorganisms [65], which has an indirect negative impact on the environment’s ecology and impedes the sustainable growth of agricultural output. Findings from [66], show that the removal rate of nitrate was highest in soils with a medium secondary salinization degree, which decreased microbial diversity, increased microbial stability, and altered the composition of microbial communities. Godfray anticipated that by 2050, worldwide agricultural output would rise by 70–100 percent. [67], necessitating increased dependence on nitrogen fertilizer synthesis using the industrial Haber–Bosch process to sustain a global population of 9 billion people [68].

Fertilization and irrigation for vegetable growing led to soil nutrient buildup, which caused soil salinity [69]. To better understand the accumulation and changing features of soil ion compositions in the field, Wu et al. conducted a study in Chinese greenhouse horticulture. The results indicate that excessive fertilizer increased greenhouse soil nutrients, causing secondary salinization. Greenhouse soil exhibited high nitrate and ammonium nitrogen contents. The soil profile showed salt accumulation, pH decrease, and varied acidification following greenhouse cultivation [70].

Poor agricultural practices in greenhouses were identified to cause soil salinization [71]. A survey was conducted by Chen et at. on current fertilizer practices and their effects on soil fertility, and soil salinity was examined from 1996 to 2000 in Beijing Province, a major vegetable production area in the North China Plain [69]. The inputs of main nutrients (NPK), as well as fertilizer application techniques and sources, were assessed for various vegetable species and field conditions. Examination of the seasonal subtleties in soil nutrients, acidification, and salt accumulation in vegetable fields in Southeastern China shows that, in addition to causing soil salinization and acidification, nitrate buildup in vegetable production soil under alternating open-field and greenhouse settings also poses a significant risk of groundwater contamination [16]. Thus, the soil in the greenhouses gathered a significant amount of mineral N, and leaching took place farther down in the soil. The pH of the soil in vegetable fields was substantially lower than in wheat–maize crops. Due to excessive fertilizer application in greenhouse vegetable production in Northeast China, soil quality may degrade more rapidly than in traditional wheat–maize rotations due to the accumulation of high salt and nitrate concentrations [72].

Soil enzymes and microorganisms can potentially convert nitrogen fertilizer in the soil into NH₄^{+ −}N and NO₃⁻-N, respectively, after mineralization. Microbes and plants would take up this inorganic nitrogen and incorporate it into their own tissues [73]. However, when the adsorption capacity of NH₄⁺ in the soil has been reached, NH₄^{+ −}N may leak from the soil via infiltration, notwithstanding the significant adsorption effect of soil colloids [74]. Topsoil exhibits anomalous phenomena due to salinization, including whitening and reddening as a result of salt buildup (Figure 3). In Figure 3, the soil composition is 6–7% reddening of soil and 3–4% white frost could be observed. Surfaces began to “redden” when the presence of purple-appearing Porphyridium spp. was detected and the salt level was 6–10% [13].

Soil enzymes and microorganisms may convert NH₄^{+ −}N to ammonia, which then escapes into the atmosphere; NH₄⁺ can be oxidized to NO₃⁻-N through nitrification, with N₂O as a byproduct. Nitrification plays a significant role in the loss of nitrogen fertilizer, accounting for almost 70% of the total [75]. Increasing nitrogen-use effectiveness (NUE) and mitigating nitrogen’s harmful effects on the environment have risen to the forefront of environmental concerns across the world [75].

3.2. Problems of Heavy Metals

A key factor in the buildup of nutrients and heavy metal contamination that led to soil deterioration in greenhouses was the overuse of fertilizers. Wan et al. studied the levels of heavy metals and nutrients in the topsoil and subsoil of East Chinese greenhouses and farmland [76]. The study was carried out in Shouguang, Country China, a major greenhouse vegetable producing area. The results showed that greenhouse soils were much more fertile and heavy metal concentrations did not substantially change with soil depth. There was a positive correlation between fertilizer and agrochemical usage and increases in greenhouse soil nutrient concentrations. The greenhouse ridge acts as a protective barrier, shielding soils from air deposition caused by pollutants from surrounding industrial complexes, resulting in lower soil Pb and Cr concentrations in greenhouses [76].

The availability of nutrients, including phosphorus, iron, manganese, copper, and zinc, may be adversely affected by soil pH shifts, which can negatively impact plant development [72]. The soil of vegetable plantations in the middle area was mostly contaminated by As and Cd, with some Hg, Zn, and Cu as well. Pollution was also seen to some extent in Western China. The industrial/sewage irrigation vegetable land had the greatest concentration of As, Cd, Hg, and Zn, notably for Hg with 2.36 mg kg⁻¹. Greenhouse vegetable land soils had the most Cu and Cr, while urban vegetable land soil had the most Pb. With the exception of Pb, which showed a slightly greater concentration in the suburb area, soil for the basis of vegetable production had relatively low heavy metal concentrations [77].

Heavy metals have harmful effects on the soil biota because they interfere with several microbial functions and make soil microorganisms less active and abundant. In a study, the Eastern Chinese vegetable fields were found to have higher frugal “heavy metal” pollution side by side, with Cd, Hg, and Zn being the predominant contaminants [77].

4. Influence of Synthetic Fertilizer (Nitrogen) on Soil

To investigate the effects of synthetic N fertilizer on salinity and soil acidity, Han et al. performed a greenhouse pot experiment in the central Shandong Province. Three N rates (0, 600 and 1200 kg ha⁻¹ N) were applied to examine the effects of nitrogen fertilizer on soil acidification and salinization. As a result, soil acidity decreased and soil salinity rose after a single growing season. Nitrification of excess N fertilizer has been blamed for the rise in soil acidity and salinity. Greenhouse soils were acidified mostly by nitrification rather than by the proton loading achieved with lettuce [17].

Soil organic matter is essential for maintaining soil quality to enhance the physicochemical and biological characteristics of the soil. Organic fertilizers were able to provide enough nutrients to reach desirable outcomes comparable to those achieved with additional inorganic fertilizer, as well as enhanced physical conditions, such as a larger proportion of solid soil aggregates and a greater water holding capacity. Inorganic fertilization increases the chemical, physical, and microbiological qualities of soil while gaining desirable results and lowering the danger of nutrient loss in lettuce crop growth [78]. The traditional fertilizers can be encapsulated within shell materials such as sulfur, thermoplastics, ethylene-vinyl acetate, surfactants, etc., known as controlled-release fertilizers, to improve their efficiency and also to reduce environmental contamination. The efficiency and productivity of agricultural plants may be increased while phytotoxicity and environmental pollution are reduced by using controlled-release fertilizers, deep-band placement, and foliar spray [79].

Previous studies in Beijing’s suburbs compared the ecological effects of conventional tomato farming to those of organic farming. Results indicated that organically growing tomato had a 54.87 percent lower environmental effect index than conventionally growing tomato, with significant environmental advantages [80]. The use of conventional fertilizers leads to greater than required expenditures for farmers and more degradation of the environment. In order to lessen the still-noticeable effects of manure application on the soil’s ecotoxicity, additional efforts should be undertaken to assist organic farmers in applying organic fertilizers more effectively. In order to quantify the environmental performance and explore prospective improvement possibilities associated with conventional and organic apple outcome systems in two key apple-producing locations (Shandong Province and Shaanxi Province) in China, Zhu et al. utilized a life cycle assessment (LCA). The findings showed that, despite lower output, organic apple production systems versus conventional systems could help reduce environmental impacts in the majority of the impact categories studied. The output and use of organic manure in organic apple production systems contributed significantly to total environmental impacts [81].

In greenhouse vegetable production, it is standard practice to use excessive amounts of fertilizers, insecticides, pesticides, and irrigation water [82], which may lead to soil nutrient imbalances and the accumulation of heavy metals [21]. Several investigations have highlighted a considerable decrease in soil pH, which is already noticeable in both the upper (the top 30 cm) and lower (the lower 90 cm) layers of the soil, as deep as 3 m [83]. A number of environmental effects, such as the eutrophication and acidification of aquatic and terrestrial ecosystems, threaten the health of ecosystems and, ultimately, humans. Greenhouse vegetable cultivation often requires a greater degree of management and a larger input of nutrients and irrigation. New research has provided enough evidence of the significance of greenhouse soil deterioration. For example, based on findings, Tang et al. concluded that secondary soil salinization affected 78.26% of 30 greenhouse planting locations in Shanghai, China [84]. Greenhouse vegetable production may enhance local farmers’ revenue by almost 20 times per hectare compared to conventional grain agriculture [85]. Nitrification of excess N fertilizer has been linked to elevated soil acidity and salinity. Nitrification causes higher greenhouse soil acidity than lettuce proton loading [17].

5. Approaches against Soil Degradation

Improving N usage efficiency, crop N absorption, and soil fertility and lowering environmental concerns all depend on lowering nitrogen (N) loss from fertilizer [86]. ¹⁵N tracking studies state that soil climate and agronomic management approaches significantly impact N fertilizer’s destiny. Inputs and outputs of fertilizers in the soil–crop system have an effect on the amount of nitrogen in the soil. The surplus or deficit in N is used to assess the system’s productivity, soil fertility, and environmental consequences using a nitrogen budget or N balance approaches [87]. Research was carried out by Liang et al. to demonstrate the inevitable deterioration of soil. Simultaneously, greenhouse output can be maintained at a sustainable level while reducing soil damage [88]. Most of the crops had the greatest yields in their respective cropping models. Planting cowpea or other leguminous crops after harvesting cucumbers is used to halt soil degradation [89]. Highly targeted agricultural output, stable soil fertility, and reduced environmental risk are the objectives of better N management. N loss and environmental pollution may result from an overusage of N fertilizer. Conversely, insufficient N application might compromise agricultural production and soil fertility. A rational consideration of the many pathways by which nitrogen can be lost, such as NH₃ volatilization, denitrification, leaching, and surplus, is essential for effective N management.

In soil-cropping systems, crop N absorption still originates from the soil, regardless of the amount of nitrogen fertilizer utilized. Without fertilizer N or other sources of N, the soil’s N supply will diminish. Increases in crop productivity and total N intake were shown when nitrogen was managed optimally, as opposed to the more commonplace scenario in which N was mostly taken from the soil’s N pool. Therefore, long-term crop output and soil N fertility need a positive balance in the soil’s N economical value. Ju and Gu assessed the productivity, environmental consequences, soil fertility, and sustainability of soil-cropping systems, requiring nitrogen budgets that factor in all N inputs and outputs [84]. In soil cropping systems, nitrogen (N) assessment has the potential to reveal issues and enhance N management. Based on a literature analysis, Ju et al. retrieved N nitrogen from rice and wheat in a summer rice–winter wheat rotation system in paddy soil in China’s Taihu Lake area. While using in-situ data collected over the course of many years in different locations, they compared the effects of conventional and optimal N management on the N budget [86]. Research indicates that NH₃ volatilization and NO₃⁻ leaching were two primary N loss mechanisms in the system, despite the fact that fertilizer N application rates under traditional farmer’s practice were exceptionally high and N deposition was becoming an important input item.

Bioaugmentation is another approach, used to accelerate pollutant degradation, which is promising for bioremediating organically damaged soils (Figure 4). Bacillus subtilis, which can be isolated from salty soil, has a high tolerance for the salty environment and produces fermentation products that lower the pH of the soil solution and boost the concentration of active phosphates [90]. Bacillus megatherium was observed in another investigation to ameliorate soil salinity, boost nutrient availability, and boost plant biomass [91].

Finding and expressing nitrogen-uptake genes is another genetic approach used to improve soil deterioration and boost agricultural productivity. Four Arabidopsis’ nitrogen transporter gene families were revealed through studies: NO₃⁻ transporter1/peptide (NPF), NO₃⁻ transporter 2 (NRT2), chloride channel (CLC), and slow anion channel associated 1 homologue (SLAC/SLAH) [92]. A number of other genes, which were over-expressed in KZ15 under stressful circumstances, encoded members of other transcription factor families (WRKY, MYB, bHLH, ethylene-responsive factor, and nuclear factor-Y or NF-Y). When subjected to nitrogen stress, both varieties showed an increased expression of the genes involved in glutathione metabolism. The expression of genes in two Sorghum bicolor (L.) Moench cultivars was analyzed by Yang et al. [93]. Keza15, which is tolerant of low nitrogen supplies, and Suiza7, which is sensitive to low external nitrogen concentrations), were grown hydroponically and were either given a high dose of Ca (NO₃)² (4 mmol L⁻) or were subjected to nitrogen stress (0.0 mmol L⁻). Nod (nodulation factors) and Myc (regulator genes) factors (legume–rhizobia symbiosis) are comparable; hence, Nod genes might be induced in cereals [94]. However, rhizosphere microorganisms and grains were shown to have robust signal-based communication [95]. These lab experiments aren’t generalized to the field.

6. Perspective

Plants suffer from soil degradation caused by excessive nitrogen, including stunted foliage growth (which delays the growth of other essential parts of the plant), high salt concentration (which causes leaf burning), stunning root growth, and groundwater pollution (as nitrogen leaches out and mixes with the water). Besides these considerations, excessive nitrogen also effects the performance and growth of microbes in the soil. High N fertilizer input resulted in high nitrate content, which was negatively correlated with anammox bacterial abundance and influenced their diversity. Excessive N fertilizer usage results in high nitrate seeping into the subsoil, inhibiting bacterial development and N-transforming capacity. However, social, economic, and political factors are important in facilitating the transition to sustainable agricultural practices [96].

Important strategies for fertilizing greenhouses and reducing soil degradation risks are:

• Organic production of leafy green vegetables lowers the nitrate content of the vegetables compared to conventional production, if other variables such as seasonality and production systems are controlled.
• Viable measures include rational fertilization regimens, decreased nitrogen leaching, and boosting soil macro-aggregates to create artificial symbiotic crops through genetic engineering, which are some of the possible ventures that may be undertaken.
• Genetic engineering makes it feasible to develop high-yielding crops that need less nitrogen fertilizer through genetic improvements by incorporating the desired genes. Through genetic engineering, we can change crops to fix nitrogen. Under nitrogen-limited conditions, bacteria (Rhizobia) in the soil produced Nod factor signaling molecules in response to plants exuding flavonoids. Root hairs are transformed by nod factors into nodules, where nitrogen-fixing bacteria resides.
• Bacteria and plant legumes have a symbiotic interaction for the reduction of oxygen levels. Crops can be genetically engineered by inducing the expression of Nod genes in them. QTLs (quantitative trait loci) were mapped to find associated markers with the gene that regulates the NUE characteristic). Whenever the nitrogen level exceeds its limits, leghemoglobin creates signals to the bacteria to fix the excessive nitrogen in the soil and decompose it.
• Reduced soil nitrogen fertilizer loss through crop rotation or intercropping plants to increase NUE production is also compatible with the current trend in ecologically intensive agriculture.

The ultimate objective should be to implement a comprehensive and integrated strategy for managing soil resources.