Advances in Bioremediation of Agricultural Soil Contamination

. With the rapid advancement of industrialization and agricultural intensification, the issue of farmland pollution has garnered significant attention. This paper introduces the sources and hazards of farmland pollution, along with the types, principles, and technical advantages of bioremediation. Additionally, it makes a brief prediction of future farmland restoration efforts. Research indicates that the primary sources of farmland pollution currently include four factors: the natural environment, atmospheric deposition, irrigation water, and agricultural production activities. There are two primary methods of remediation for farmland contamination: phytoremediation and microbial remediation. Phytoremediation is mainly used in heavy metal pollution treatment and can be divided into hyper-accumulation plants and low-accumulation crops based on the remediation method. Microbial remediation, meanwhile, is primarily utilized to remediate organic pollution and assist phytoremediation.


Introduction
Due to the overuse of pesticides and residual decomposition,, countries worldwide are increasingly prioritizing the safety of agricultural products, and urgent remediation is required for large amounts of pesticidecontaminated farmland. China is among the largest agricultural countries worldwide, farmland pollution not only directly affects the physical and chemical properties of soil but also leads to transformations in farmland ecosystems. Bioremediation technology is more suitable for agricultural soil remediation due to its advantages of simple operation, low cost, and minimal impact on soil ecosystems. Nevertheless, bioremediation technology still faces many challenges.

Sources and hazards of farmland pollution
Farmland pollution is categorized into two main types: heavy metal and organic pesticide pollution. The sources of pollutants in agricultural land can be broadly classified into four categories according to the classification in Table 1, environmental natural occurrence, atmospheric deposition, irrigation water use and agricultural production activities. The environmental natural occurrence comprises natural soil formation resulting from rock weathering. Plants and animals in the natural environment release chemical pheromones, such as aromatic substances, during their life activities. Additionally, extreme weather conditions (such as volcanic eruptions, forest fires, and post-tsunami soils) can lead to varying degrees of contamination, thus threatening the soil quality of agricultural land.
The atmospheric deposition category refers to the process through which pollutants in the atmosphere deposit onto the ground through specific pathways, divided into dry and wet deposition. During the usage of fossil fuels, a large number of organic by-products are produced and released into the atmosphere and later enter agricultural soil via atmospheric deposition. With the renewal of chemical processes, new organic pollutants such as perfluorinated compounds have emerged, posing a new challenge to environmental capacity.
The irrigation water category involves the use of sewage for farmland irrigation in certain areas, resulting in dirty irrigation regions. As and Pb can penetrate the soil down to 30 meters in certain regions, seriously impacting soil quality and potentially changing the composition of heavy metals in the soil permanently [1] . Even after more than a decade has passed since sewage irrigation was stopped, the heavy metal content in farmland still exceeds the safety standards for soil environmental quality in China, such as Zhang Shi and Shenfu irrigation areas in Shenyang.
Agricultural production activities often involve the extensive use of pesticides and fertilizers to increase crop yield, resulting in exponential growth in their usage.
For instance, atrazine [2] , an environmental contaminant, can alter DNA methylation and histone modifications [3] . In China, farmers have traditionally used livestock manure as fertilizer for farmland to enhance soil fertility. Nevertheless, applying livestock manure to farmland brings not only pathogens but also triggers antibiotic contamination.

Phytoremediation
Plants have developed detoxification properties during their evolution to facilitate this process [4] . This is why some plants can grow in soil that has been contaminated with high concentrations of heavy metals. Phytoremediation can be categorized into above-ground and below-ground restoration, depending on the location and mode of restoration. Usually, multiple restoration mechanisms work simultaneously during the process of phytoremediation (As shown in table 2). Helianthus annuus L. -Cd Oxalic acid (OA), acetic acid (AA), tartaric acid (TA), malic acid (MA) and citric acid (CA) enhance Cd accumulation in sunflower [7] low-accumulation crops maize maize Pyrene and cadmium Pyrene and cadmium corn promote microbial exposition of pyrene, but pyrene contamination inhibits plant extraction of cadmium. Maize can grow normally in soils co-contaminated with high Cd and pyrene [8] Brassica napus L.

Brassica napus L. Cd
Acidic nitrogen fertilizer is more effective for soils with low Cd concentrations. Alkaline nitrogen fertilizer is more effective for soils with higher Cd concentrations [9] Agronomic

Moisture Management
Oryza sativa L.； Oryza sativa L. and Ipomoea aquatica Forsk

Cd
The intercropping pattern resulted in a lower concentration of seed grain Cd Higher biomass than the original production mode has the function of remediation of Cd-contaminated soil。 [10] Planting density Solanum nigrum L. Cd By changing agronomic practices, biomass production per unit of time can be increased and the restoration process can be accelerated [11] Above-ground part remediation includes phytoextraction [8] , phytoaccumulation [7] and phytovolatilization remediation. For instance, tobacco roots are highly enriched in cobalt, nickel, and cadmium, while the leaf part has a higher tendency to accumulate cadmium but comparatively less zinc, selenium, and mercury [12] . Yang et al [13] collected plants from rotations of sunflowers, peanuts, and sesame used for phytoremediation of Cd and Pd contamination in agricultural fields. The oil was then extracted using hexane to meet minimum standards, or crops were treated with potassium tartrate to meet feed standards. Through this process, contaminants can be effectively removed from the ecological material cycle.
Below-ground part remediation includes the combined action of plant root filtration and rhizosphere [9] . In addition, the joint action of the rhizosphere refers to plants secreting nutrients such as amino acids, carbohydrates and flavonoids during the growth process [14] , which enhances the metabolic activities of microorganisms around the roots and accelerates the degradation of organic pollutants in the soil. Another option for farmland restoration is to look for low-accumulation crops. Studies have shown that there are significant differences in the accumulation of di(2ethylhexyl) phthalate (DEHP) in different rice varieties, in different parts of the same species, and different growth stages of the same species [15] . Therefore, crops with low accumulation of contaminants should be grown in contaminated farmland to ensure that the heavy metal content in edible parts is lower than the food hygiene standards or feed standards, to achieve safe cultivation and safeguard human health. In the contaminated Cd and Pd farmland, a comparison was made by planting the restoration plants Hylotelephium spectabile, amaranth and rapeseed/corn rotation, and it was found that the farmland soil was better restored by rapeseed/corn rotation planting, and its by-products rapeseed meal and straw also met Chinese national standards for organic fertilizer and feed [5] ( As shown in table 2).
Low-accumulation crops should be considered not only for the safety of agricultural products but also for their economic viability. Phytoremediation using Southeast Sedumandoilseed rape, approach in a test field with a Cd contamination [6] , the study also proved the economic feasibility of this phytoremediation strategy through NPV and BCR analysis.
During phytoremediation, modifying plant cultivation practices, such as moisture management [10] , altering fertilizers, planting density [11] , and determining the optimal time for harvesting, can all significantly influence the progress of the restoration(As shown in table 2).

Microbial remediation
The process of enhancing the metabolic activity of soil microorganisms to efficiently degrade toxic and harmful substances, or render them less toxic via various technical means, is known as microbial remediation [16,17] . Generally, these methods fall into three categories: microbial transformation, microbial adsorption and enrichment, and microbial leaching(As shown in table 3).
Microbial adsorption is the process by which microbial cell structures capture heavy metals and subsequently adsorb them to binding sites on the cell wall. The final destination of microbial-adsorbed pollutants can be divided into two types: vivo enrichment and vitro precipitation.
Microbial enrichment refers to the accumulation of heavy metal ions partially adsorbed on the surface of microbial cells, which enter the cells through the action of carrier proteins and bind to each other with proteins in the microbial body to avoid contact with other components of the cells [18] . In general, metals sequestered by microbial sorption include Cd(II), Cu(II), and Zn(II), which contribute to the reduction of heavy metals in the biologically active state of the soil. In recent years, the study of microbial-induced carbonate precipitation (MICP) has been widely concerned and deeply explored [19] , Many different types of bacteria were found to induce this precipitation. Among the mineralizing microorganisms, urease-producing bacteria (UPB) is one of the most common strains. Chen investigated the effectiveness of MICP remediation on lead-contaminated soil and examined the resulting changes in soil quality [20] . The study found that after MICP remediation, the leaching of heavy metal lead in soil was reduced by 76.34%. Additionally, the soil quality was significantly improved with a 71.43% increase in agglomerates of large particles and a 73.78% increase in porosity.
Bioleaching is a process in which metallic minerals are dissolved and associated metals are released by microbial action. Acidophilic bacteria including T. thiooxidans, T. acidophilus, and T. ferrooxidans can be used for leaching when pyrite and sulfides are present in coal [18] . Microbial remediation has the advantages of a high capacity for adsorption and degradation of pollutants and a low risk of secondary contamination, so it can be used as one of the means of agricultural soil remediation. The toxicity of contaminants in the soil can directly affect the biological enzymes, thus weakening the microbial remediation effect [21] . Atrazine degrading bacteria

Soybeans atrazine
The removal rate of atrazine in soil up to 95% or more [2] Soil microorganisms Zea mays L.and Glycine max L. Merr Lindane Lindane had no significant effect on soil capacitance, microbial diversity and population size in the inter-and intra-rhizosphere of plants. [17]

Summary
Farmland pollution has become a significant global problem, exacerbated by population growth and industrial development. This paper introduces the primary sources of farmland pollution, discusses the general principles, technical advantages, and disadvantages of bioremediation technologies for farmland, and emphasizes the importance of focusing on the soil's environmental values and considering the physicochemical properties of farmland soils. Ultimately, the goal of farmland remediation should be to restore soil environmental functions. New bioremediation technologies that explore the interplay between farmland soil environment, crops, and soil microorganisms are continuously being developed to complete farmland remediation with lower costs, more convenient means, and minimal environmental impact.