A Comprehensive Analysis of Agricultural Non-Point Source Pollution in China: Current Status, Risk Assessment and Management Strategies

By inergency On Mar 18, 2024

[ad_1]

1. Introduction

In recent years, with the improvement of point source pollution control levels, agricultural non-point source (AGNPS) pollution arising from various sources is already more and more serious, which has become a key factor affecting water environment security [1]. In an environment where point sources are effectively controlled, non-point source (NPS) pollution is considered to be the main factor causing the deterioration of water quality [2]. NPS pollution has gradually become a major regional and global environmental problem, which has attracted more and more attention around the world [3,4,5,6]. NPS pollution is a concept relative to point source pollution, which refers to soluble pollution produced in rural life or production activities, such as heavy metal in soil, excess nitrogen and phosphorus carried by fertilizers, pesticides residues and livestock manure, as well as the pollution caused by bringing pollutants into water bodies (rivers, lakes, reservoirs, bays) through surface runoff [7,8,9]. The widespread presence, complex hydrodynamics and great difficulty in handling of NPS pollution has made it an important source of pollution in water environment [10].

In addition, AGNPS pollution presents characteristics such as complexity, hysteresis, multiple sources of pollution, wide distribution, difficulty in control, etc. [11,12]. Many studies have reported that chemical fertilizers, soil particles, microplastic particles, livestock manure and pesticides can enter soil surface runoff and underground runoff under rainfall conditions. Excessive input of phosphorus and nitrogen into water bodies leads to eutrophication [13,14,15,16]. The widespread use of plastic films in agriculture leads to the degradation and formation of small plastics and microplastics, which accumulate in the soil, causing a decrease in soil fertility and pollution [17]. These particles are also transported to nearby water bodies, causing developmental delays and weakened immune systems in aquatic animals [18]. However, the uncertainty of NPS pollution discharge path leads to the difficulty of water pollution traceability [19]. At the same time, the lack of relevant monitoring data also limits the research and control of NPS pollution [20]. In view of the limitations of NPS pollution-related data, scholars have studied and developed many assessment models to determine the key source areas in the watershed by analyzing the spatial distribution of NPS pollution [21]. Up to now, the research on spatial distribution and risk assessment of AGNPS pollution has made great progress, the assessment model has provided scientific basis for the prevention and control of AGNPS pollution in the basin, and has also played a positive role in achieving the goal of water pollution control in the basin [22,23].

In summary, this paper primarily focuses on several key aspects: providing a comprehensive summary of the sources of AGNPS pollution; examining the recent situation in AGNPS pollution within China; and introducing and summarizing the most current methods for managing AGNPS pollution, specifically targeting the control of nitrogen, phosphorus, microplastics, and heavy metals in the most heavily affected provinces in China. This article will serve as a foundational reference for future efforts in AGNPS pollution control in China.

2. Materials and Methods

2.1. Study Area

China, situated on the eastern Eurasian Plate, features a varied landscape. As the world’s third-largest country by land area, at 9.6 million square kilometers, China is surpassed only by Russia and Canada. Moreover, China is the second most populous country after India, comprising about 17.72% of the global population [24]. Despite this, only 7% of the world’s arable land is in China, a limitation that leads to high usage of fertilizers and pesticides, ranking it among the world’s foremost consumers. The vast population contributes to increasing agricultural and livestock waste, intensifying AGNPS pollution.

This study is designed to provide an accurate assessment of the current state of AGNPS in China. Our analysis centers on three key contributors to AGNPS across China’s 31 provinces: total nitrogen (TN) and total phosphorus (TP), heavy metals, and microplastics (resulting from the degradation of plastic films). The objective is to evaluate the risk associated with present NPS levels.

2.2. Methodology

2.2.1. TN and TP Assessment

In this study, the factors of TN and TP were determined by analyzing the concentrations of nitrogen and phosphorus in major rivers, reservoirs, and lakes situated in proximity to agricultural regions across all 31 provinces. The evaluation of water quality is detailed in Table S1.

2.2.2. Microplastics Ecological Risk Assessment

In this study, the microplastic risk in water bodies across Chinese provinces was assessed using two indices: ecological risk index (H) and the regional pollution loading index (PLI). The risk index (H) is derived from the toxicity of polymers, serving as a measure in microplastic risk assessment. This index encapsulates the cumulative toxic impact of different polymer types [25]. The formula used is as follows:

H is the ecological risk index of microplastics, P_i is the proportion of each plastic, S_i is the risk score of each type of plastic polymer. Polyethylene (PE), polypropylene (PP), polyamide (PA), polyethylene terephthalate (PET), polystyrene (PS) as the main components of agricultural plastic films, with risk scores of 11, 1, 50, 4, and 30, respectively [26].

The regional pollution loading index (PLI) uses microplastic abundance as an indicator of the agricultural NPS pollution—microplastics status of the region [27].

$P L I_{a l l} = \sqrt{P L I_{1} \times P L I_{2} \times P L I_{3} \times \dots \times P L I_{n}}$

(4)

CF_i is the pollution coefficient, which is the ratio of the abundance of microplastics at each point (Ci) to the background value of microplastic abundance (C_b), C_b refers to the predicted no-effect concentration as the reference value, and the reference value of the water body is 6650 n·m⁻³ [28]. PLI_i is the pollution loading index of a certain point, PLI_all is the pollution loading index of the whole region, n is the number of points. The risk classification is shown in Table S2.

2.2.3. Heavy Metal Ecological Risk Assessment

Heavy metals possess distinct physicochemical properties that impact the environment, leading to ecological pollution. Consequently, the ecological risk index formula is employed to quantify the extent of heavy metal contamination [29]. The formula is as follows:

$R I = \sum_{i = 1}^{n} E_{r}^{i} = \sum_{i = 1}^{n} (T_{r}^{i} \times P_{i}) = \sum_{i = 1}^{n} (T_{r}^{i} \times \frac{W_{i}}{B_{i}})$

(5)

RI represents the comprehensive ecological risk index. E_i^r denotes the potential ecological risk index for a specific heavy metal, T_i^r signifies the toxicity response coefficient of i; P_i is the enrichment coefficient of i; W_i refers to the measured value of i; and Bi represents the background value of i. Research has established that the toxicity response coefficients for As, Cd, Cr, Cu, Hg, Ni, Pb and Zn are 10, 30, 2, 5, 40, 5, 5, and 1, respectively [30]. The risk classification is shown in Table S3.

2.2.4. Heavy Metal Health Risk Assessment

The assessment of health risks posed by heavy metals primarily involves quantifying both non-carcinogenic and carcinogenic risks to humans, which occur through direct oral intake, dermal contact, and inhalation. In this study, we employed the human exposure risk assessment model endorsed by the United States Environmental Protection Agency (USEPA) to determine the carcinogenic and non-carcinogenic risks to both adults and children in the study area [29]. The formula for this calculation is as follows:

$A D I_{d o i} = \frac{C_{s o i l} \times I n g R_{r i c e} \times E F \times E D}{B W \times A T}$

(6)

$A D I_{r i} = \frac{C_{s o i l} \times I n h S \times E F \times E D}{P E F \times B W \times A T}$

(7)

$A D I_{s c} = \frac{C_{s o i l} \times S A \times A B S \times E F \times E D \times A F}{B W \times A T}$

(8)

ADI_doi, ADI_ri, and ADI_sc represent the acceptable daily intake (ADI) values for direct oral intake, respiratory inhalation, and skin contact, respectively, expressed in mg·(kg·d)⁻¹. C_i denotes the concentration of heavy metals, measured in mg·kg⁻¹. The definitions and values of additional parameters can be found in Table S4.

Health risks are categorized into non-carcinogenic and carcinogenic types. The non-carcinogenic health risk is assessed using HQ or HI, which are defined as follows:

$H Q_{i j} = \frac{A D I_{i j}}{R_{f} D_{i j}}$

(9)

HQ_ij represents the non-carcinogenic risk index for individual heavy metals, while HI denotes the collective non-carcinogenic health risk index for multiple heavy metals. The reference dose, RfD_ij, in mg·(kg·d)⁻¹, is detailed in Table S5. A value of HQ_ij or HI below 1 indicates an insignificant non-carcinogenic health risk, deemed acceptable. Conversely, values exceeding 1 signify a notable non-carcinogenic risk.

The carcinogenic risk index is determined as follows:

$C R_{i j} = A D I_{i j} \times S F_{i j}$

(11)

CR_ij refers to the individual carcinogenic risk index, while SF_ij is the slope factor in mg·(kg·d)⁻¹, and TCR represents the total carcinogenic risk for multiple heavy metals. CR_ij value below 10⁻⁶ implies negligible carcinogenic risk. In contrast, values ranging from 1 × 10⁻⁶ to 1 × 10⁻⁴ denote an acceptable level of carcinogenic risk.

2.3. Data Screening and Processing

In this study, we systematically collected and analyzed literature on agricultural water and soil pollution published between 2018 and 2023. To ensure a clear research focus on agricultural nonpoint source pollution, we applied rigorous screening criteria, prioritizing studies that directly address the impacts of pollution from agricultural activities. In addition, we paid particular attention to studies that provided a clear methodological approach to differentiate between pollution from agricultural sources and pollution from urban sources. This approach allows us to more accurately define and understand the extent and impacts of agricultural nonpoint source pollution, while excluding, to the extent possible, interference from specific urban sources. For detailed data, please refer to the Supplementary Materials (Tables S6–S8).

The ArcGIS 10.7 software was utilized to create the sampling distribution map. Data processing and descriptive statistical analysis were conducted using Origin 2019 software, while correlation analysis to assess agricultural surface source pollution in each Chinese province was performed with SPSS 22.0 software.

2.4. Use of AI-Assisted Technology

In this study, we integrated the GPT-4.0 AI model provided by OpenAI and the Write tool from DeepL to improve the writing quality and language accuracy of the paper. The specific applications are as follows:

We use GPT-4.0 to touch up academic papers. By entering the command “Below is a paragraph from an academic paper. Then polish the writing to meet the academic style, improve the spelling, grammar, clarity, concision and overall readability. Then polish the writing to meet the academic style, improve the spelling, grammar, clarity, concision and overall readability.”, GPT-4.0 automatically optimized the text for spelling, grammar, clarity and overall readability. This process enhances the academic style and clarity of presentation of the paper.

In order to further enhance the quality of the text, we used DeepL Write for text proofreading. DeepL Write proofreads the text for spelling and grammar through its advanced text processing functions, and suggests optimization of the language style and expression.

All content touched up using GPT-4.0 and DeepL Write was reviewed and edited in detail by the authors to ensure accuracy, appropriateness, and compliance with academic integrity. Relevant ethical standards and publication guidelines were strictly adhered to throughout the research and writing process to ensure quality and academic integrity.

5. Discussion

The AGNPS pollution control technology can be succinctly described by the 3R principle of ‘source reduction, interception, and repair’ [87]. This technology has specific applicability and limitations, thus, selecting appropriate control methods based on distinct pollution sources and regional pollution levels is essential.

5.1. Mitigating Nitrogen and Phosphorus Pollution

In 31 provinces across China, levels of TN and TP are significantly elevated. To mitigate the risk of excessive nitrogen and phosphorus concentrations in aquatic environments, a comprehensive, multifaceted approach to treatment is essential. Consequently, this paper outlines control measures focusing on three primary pollution sources: the management of fertilizers, the handling of livestock waste, and the effective management of crop residues.

The treatment technologies for chemical fertilizers primarily encompass three approaches: soil fertilization, the application of broad-spectrum fertilizer synergists, and the use of organic fertilizers in crop cultivation [45]. (1) Soil fertilization: this strategy entails the strategic use of organic fertilizers, customized to meet the specific nutrient needs of crops, the nutrient supply capacity of the soil, and the efficiency of the fertilizer [88,89]. (2) Broad-spectrum fertilizer synergists: These synergists can be combined effectively with a range of fertilizers, including organic ones, farm manure, and traditional chemical fertilizers. Their usage markedly improves fertilizer efficiency and meets the diverse nutritional needs of crops across various growth stages [90,91]. (3) Organic fertilizers: these fertilizers are rich in beneficial microorganisms, when applied under optimal conditions, enable the proliferation and secretion of hormones, elements, and enzymes advantageous to crops [92].

The Clean Breeding Project is designed to prevent and control pollution from livestock and poultry breeding, primarily focusing on the treatment of manure and sewage. This initiative involves four applications of harmless livestock and poultry manure: as feed, fertilizer, fuel, and raw materials. Given the integration of end-of-pipe and source treatments, the application of modern technology for treating livestock and poultry wastewater merits consideration [45]. Currently, manure composting and biogas treatment are the predominant technologies. (1) Manure composting technology: this involves the microbial degradation of organic matter in solid manure under controlled conditions of moisture, carbon-nitrogen ratio, and aeration, resulting in mineralization, humification, and detoxification [93]. (2) Biogas treatment technology: Generating biogas from livestock manure is a viable resource utilization approach. In biogas digesters, organic matter from manure is anaerobically converted into renewable resources like biogas, biogas slurry, and biogas residue, thereby facilitating extensive utilization in the ecological chain [94].

The primary approach to managing crop straw involves its recycling and reuse, encompassing five key methods: transforming straw into fuel [95], fermenting straw for fertilizer production [96,97,98], processing straw into feed for livestock and poultry [99], converting straw into industrial raw materials, and utilizing it as a planting substrate [100]. These methods of recycling not only effectively mitigate the pollution caused by crop residues in their natural state, but also significantly contribute to the sustainable cycle of agricultural resources.

5.2. Control of Microplastic Pollution

The widespread production and utilization of agricultural plastic films have led to significant microplastic pollution, adversely affecting the sustainability and health of agricultural ecosystems due to the accumulation of used films and the proliferation of microplastics in soil and water bodies. To address the NPS pollution caused by agricultural plastic film, urgent reform and innovative solutions are required, focusing on reduction, substitution, and recycling strategies.

Reduction: The cultivation of various crops under different ecological conditions should be optimized based on the technical standards of film mulching and specific operational techniques. By considering factors such as land, light, water, and fertilizer, and through diverse plastic film applications, it is possible to enhance the efficient use of plastic films, mitigate their pollution impact, and adjust their usage standards appropriately.

Alternatives: Plastic mulch films are categorized into non-degradable and degradable types, with the development of the latter being actively pursued globally Currently, the more advanced technologies encompass biodegradable mulch films [101], photodegradable mulch films [102], paper mulch films [103], and liquid spray degradable mulch films [104].

Recycling: Waste agricultural film needs to be thoroughly recycled and reused. Polyethylene and polypropylene, the primary constituents of agricultural film, are valuable renewable resources. They can be transformed into recycled plastic particles and other products through specific processing methods [72]. There is a non-woven mulch film made of coarser chemical fiber that has the property of being easily recycled, which can reduce the difficulty of recycling [105]. In addition, there is a need for mandatory legislation and policy support through the government to ensure the effective recycling of plastic films.

5.3. Prevention and Control of Heavy Metal Pollution

Heavy metals commonly found in agriculture, such as As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, pose significant environmental and health risks when present in high concentrations in water and soil. Consequently, our study emphasizes analyzing the sources of these elements and devising strategies for their mitigation. According to our relevant analyses (refer to Figure S1 for detailed information), As, Cd, Cu, Hg, Ni and Zn originate partially from pesticides, Pb partially from pesticides and plastic films, and Cr partially from pesticides and crop residues. The management of plastic films and crop residues has already been discussed, and the control of pesticide use, along with soil remediation strategies, will be examined subsequently.

Pesticide reduction is typically achieved through integrated pest management, encompassing phytochemical, biological, and physical control techniques. (1) Botanical insecticidal techniques: these involve the use of pesticides derived from secondary metabolites produced by plants, animals, and microorganisms, which are noted for their lower toxicity and enhanced safety for humans and the environment, compared to traditional chemically synthesized pesticides [106]. (2) Biological control technology: this approach employs natural predators to manage pests, where each pest typically has one or more natural enemies that can effectively inhibit their overpopulation [107,108]. (3) Physical control technology: this method includes the application of physical measures to control diseases and pests, such as solar frequency vibration insecticidal lamps and insect-prevention yellow plates, which are effective in eliminating larvae or adult pests [45].

Soil remediation entails the restoration of the environmental functions of soils contaminated by various pollutants through diverse technical approaches. In addressing heavy metal contamination in soils, the primary goal is to either reduce the concentration of these metals or diminish their bioavailability. (1) Chemical stabilization: This approach involves adding certain compounds (such as lime, phosphates, or silicates) to the soil, which react with heavy metals to form insoluble or barely soluble compounds. This process significantly reduces the mobility and bioavailability of the heavy metals [109,110]. (2) Phytoremediation: This employs specific plants, known as hyperaccumulators, for their capacity to absorb and accumulate heavy metals from the soil. These plants extract heavy metals from contaminated soils, and their subsequent harvesting and safe disposal effectively remove these metals from the environment [111]. (3) Microbial remediation: This method leverages the metabolic functions of certain microorganisms to degrade, convert, or stabilize heavy metals in the soil. These microorganisms, either naturally occurring or genetically engineered for enhanced efficiency, play a crucial role in mitigating soil heavy metal contamination [112,113].

6. Conclusions

AGNPS pollution has been an important source of environmental pollution. Due to the shortage of cultivated land and the lack of environmental protection measures, AGNPS pollutants carrying a large number of nitrogen, phosphorus, heavy metals, microplastics and other elements, which enter the soil and water bodies along with surface runoff and underground runoff, posing a serious threat to China’s environmental security.

Our review’s statistical analysis reveals that TN and TP pollution in the water bodies of various Chinese provinces is severe. Provinces including Jilin, Hainan, Inner Mongolia, Heilongjiang, Gansu, Hunan, Jiangsu, Liaoning, and Anhui exhibit elevated microplastic pollution. Furthermore, Hubei, Gansu, Liaoning, Guizhou, Hunan, Jilin, Fujian, and Ningxia face severe comprehensive heavy metal pollution, notably involving As, Cd and Hg, indicating significant impacts on the agricultural environment. Concurrently, this study has identified that in Gansu Province, children are exposed to a significantly elevated non-cancer risk attributed to heavy metal pollution, while adults in Hubei and Gansu Provinces face a markedly high risk of cancer.

In response to this challenge, we recommend various prevention and control measures for AGNPS pollution, tailored to specific pollutants including nitrogen and phosphorus, microplastics, and heavy metals. In regions with high TN and TP contamination, strategies include the judicious management of fertilizers, proper handling of livestock waste, and effective management of crop residues. In areas with elevated microplastic concentrations, reduction, substitution, and recycling strategies are advocated. For provinces facing significant heavy metal contamination, particularly with As, Cd and Hg, an integrated management approach is advised. This approach encompasses the management of crop residues and plastic film, stringent control of pesticide use, and comprehensive soil remediation strategies.

The implementation of these AGNPS pollution prevention and control measures, in conjunction with corresponding risk assessments, focuses on key aspects such as source reduction, pollution interception, and remediation. These strategies aim to reduce the discharge of various pollutants, ultimately fostering sustainable agricultural development, improving the aquatic environment, and mitigating the overall AGNPS pollution scenario.

[ad_2]