Feasibility Study And Safety Assessment Of In Situ Inhibition Of Soil Nitrate Leaching By Sponge Iron Coupled With Biochar

The high amount of nitrogen input caused by human activities significantly disturbed the geochemical cycle of nitrogen,and most of the nitrogen activated by humans eventually entered and accumulated in the soil and water environment,forming nitrogen pollution.Numerous studies have pointed out that agricultural non-point pollution is the main source of nitrate in groundwater,which is closely related to the increase of fertilizer application.In China,the problem of fertilizer use is the high application intensity and low utilization rate.The long-term excessive application of nitrogen fertilizer led to the accumulation of large amounts of nitrate in the soil,which caused nitrate leaching and diffusion under the action of rainfall,irrigation and drainage.Therefore,to reduce the pollution of aquatic systems by nitrate from agricultural non-point pollution,it is necessary to focus on solving the long-term legacy of chemical fertilizers as well as improving the nitrogen retention capacity of agricultural soils.In-situ soil remediation can inhibit agricultural non-point nitrate pollution at the source by blocking the migration of nitrate to groundwater bodies,and the search for cost-effective and biosecurity in-situ soil remediation materials is particularly critical.In this study,sponge iron was used as an inorganic electron donor to stop the diffusion of nitrate leaching caused by excessive accumulation and concentrated drainage in the soil by using its reducing ability.With the help of biochar,a more suitable environment for nitrate retention and reduction was created,forming an in-situ denitrification system based on sponge iron-biochar,which is applicable to scenarios of nitrogen pollution accumulation such as agricultural drains.The ecological effects of the system were examined in multiple aspects.The main conclusions are as follows.（1）Sponge iron and biochar could significantly inhibit soil nitrate leaching,and the inhibition effect was positively correlated with the amount of sponge iron added.At 5%addition,the inhibition rate of nitrate leaching was up to 99.31%,and the maximum amount of nitrate intercepted was 408.85 mg L^-1,and the iron content of leachate was stable between 0.07-0.20 mg L^-1,which did not cause secondary pollution to groundwater.The transfer of electrons from Fe⁰to NO₃^--N occurred in the system,and the inhibition of soil nitrate leaching was mainly reflected in the retention and storage when the addition of sponge iron was less than or equal to 0.5%.when the addition of sponge iron was greater than 0.5%,denitrification mainly occurred,and N₂and NH₄⁺-N were produced.NH₄⁺-N could be retained by soil colloid adsorption,making soil N loss lower.Sponge iron was oxidized and lost electrons in this process,and the main product formed was Fe₃O₄.（2）The addition of 0.1-5%sponge iron and 2%biochar in the system both enhanced the germination speed and germination rate of spinach and Pak choi seeds,and significantly promoted the growth of Pak choi.When sponge iron was added at 1%,the fresh weight of Pak choi increased by 88.24%.Sponge iron promoted the synthesis of photosynthetic pigments in Pak choi leaves,increased leaf stomatal conductance and photosynthetic intensity,and promoted the uptake of iron and other nutrients by Pak choi.In addition,the addition of sponge iron and biochar activated the antioxidant enzyme system of Pak choi and increased SOD and POD enzyme activities,and this stimulation positively promoted the growth and antioxidant mechanism of Pak choi.The system did not show phytotoxicity.（3）Neither the addition of 0.1-5%sponge iron nor 2%biochar in the system exhibited acute toxicity to earthworms,nor did they significantly inhibit earthworm growth or cause significant earthworm avoidance behavior（NR<80%）.However,the average body weight growth rate of most earthworms in the experimental group was reduced compared to the control.Soil containing sponge iron and biochar induced oxidative stress in earthworms,significantly stimulated the increase of SOD activity,and showed a low-dose promotion and high-dose inhibition of CAT activity.The addition of sponge iron and biochar affected the original state of the soil but did not exhibit biotoxicity to soil animals.（4）Neither 0.1-5%sponge iron nor 2%biochar in the system damaged the soil buffering properties.Soil p H fluctuated between 7.1 and 7.9 in each installation during the experiment,and conductivity increased with the application of NO₃^--N water fertilizer.The addition of sponge iron increased the soil iron content,and when the addition was greater than 1%,the soil changed from an oxidized to a reduced state under regular leaching conditions,which was more favorable for nitrogen retention in the soil.The biochar stimulated the soil urease activity,and the sponge iron slightly inhibited the urease activity,while significantly increased the dehydrogenase activity under regular leaching conditions.The system did not change the structure of the soil microbial community,but affected the relative abundance of the dominant flora,and the addition of sponge iron increased the Chao1 index by 45.19%and enriched species diversity under synergistic plant action.The relative abundance of Proteobacteria and Actinobacteria in the soils of the experimental group increased,the former being closely associated with microbial denitrification and the latter with plant growth promotion.In summary,sponge iron and biochar can be used as in-situ soil remediation materials for the prevention and control of agricultural non-point nitrate pollution,and the reasonable application rate of sponge iron is 3%considering the nitrate interception and transformation efficiency as well as the impact on soil,plants and animals.