Effects Of Straw,cow Dung And Its Composting Products On Soil DOM Characteristics And Bio-mercury Enrichment In Paddy Fields

Mercury and mercury-containing compounds are persistent,global,and biologically toxic heavy metal pollutants,especially in flooded and low-oxygen environments such as wetlands and lakes.They can be easily transformed to methylmercury（Me Hg）by anaerobic microorganisms such as Sulfate-Reducing Bacteria.Me Hg,which is neurotoxic and bioaccumulative,threatens human health through food chain amplification effects.Paddy field is one of the critical places for mercury methylation.Rice shows a strong ability to accumulate Me Hg,ingestion of wheat is,therefore,an important way for Me Hg to enter the food chain.Returning agricultural organic wastes to the field,such as straw,can reduce the atmospheric environmental problems caused by straw burning and significantly improve soil fertility.However,it may also affect the structure and composition of soil DOM,there by promoting the methylation of soil mercury in rice fields and increasing the rice enrichment of soil Me Hg.The underlying mechanisms of these processes remain elusive.Understanding the impact of organic fertilizer input such as straw and cow manure on the structure and composition of soil DOM,as well as on the environmental behavior of mercury in rice fields,is crucial for the harmless use of straw and the reduction of mercury exposure risks for residents in mercury-contaminated areas.This study took the mercury-contaminated rice field in Wanshan,Guizhou Province as the research object and compared the changes in the concentration,structure,and composition of soil DOM with the addition of straw,composted straw,cow dung,and composted cow dung.Mercury’s environmental behavior in rice fields and the changes in soil Me Hg and rice Me Hg enrichment mechanisms were preliminarily investigated by analyzing soil DOM concentration,structure,and composition.The main conclusions are as follows:（1）Compared with the control group without addition of any organic matters,amendments of straw,compost straw,cow dung or composting cow dung largely increased the concentration of soil dissolved organic carbon（DOC）at the early stage of cultivation,while had less impacts at the middle and later periods.Meanwhile,inputs of these organic materials reduced the protein-like components in the soil DOMs at the early stage of cultivation by 80.23%,83.89%,37.21%and 86.68%,respectively,but they generally increased the aromatic and hydrophobic components while their composting products synchronously elevated the humification levels of soil DOMs（p<0.05）throughout the incubation period.With the extending of incubation time,the DOC concentrations,protein-like and humic-like components in the DOMs reduced at different levels in view of the all-treatment groups（straw,cow dung and their composting products）,and the decreasing capacity of protein-like components was much higher than that of humic-like components.Furthermore,compared with the straw-added group,the treatment with addition of its compost had lower DOC concentrations and protein-like components in the soil DOMs throughout the incubation period.Meanwhile,the treatment with addition of composting cow dung had less difference in the DOC concentrations with cow dung,but a decrease in the protein-like component,while an increase in the aromaticity,hydrophobic components and humification levels in the soil DOMs.（2）Compared to control group(66.13±6.76 ng L^-1),the addition of straw,compost straw,cow manure and compost cow manure all increased the content DHg pore water at tillering stage,and the increase rates were 18%,37%,9%and 193%,respectively.Compared with straw and cow manure raw materials,compost straw only slightly increased the content DHg soil pore water,and the increase rate was16%,while the DHg content of pore water in compost cow manure treatment group increased significantly（p<0.05）,and the increase rate reached 168%.During the whole culture period,the pore water DHg of each treatment group decreased gradually with the culture time.Compared to control group(16.65±0.38 ng L^-1),The content of pore water in tillering stage was increased in the range of 57%-208%by adding straw,compost straw,cow dung and compost cow dung,among which,the content DMe Hg pore water was significantly increased（p<0.05）,and the increase rate reached 208%.Compared with straw and cow manure raw materials,compost straw only slightly increased the content DMe Hg pore water,the increase rate was 21,while compost cow manure significantly increased the content DMe Hg pore water（p<0.05）,the increase rate was 97.During the whole culture period,the pore water DMe Hg of each treatment group decreased gradually with the culture time.the addition of straw,compost straw,cow manure and compost cow manure increased the soil Me Hg content in each period compared with the control group,in which the composting product promoted the soil Me Hg content better than the corresponding raw materials.on the time scale,the soil Me Hg content decreased with the culture time.Finally,compared with the control group,the soil Me Hg content of straw,compost straw,cow manure and compost cow manure treatment group increased by 40%,70%,59%and 75%respectively.Compared with straw and cow manure raw materials,compost straw increased soil Me Hg content in each period,especially in tillering stage,soil Me Hg increased significantly,and the increase rate reached 136.The increase of compost cow manure Me Hg content mainly occurred at tillering stage.And the increase range is small,only 15.（3）Compared with the control group,the addition of straw and compost straw significantly increased the content of methylmercury in rice by 83%and 49%,respectively,and the addition of cow manure and compost cow manure also increased the content of methylmercury in rice,but the increase effect was not significant,by22%and 18%,respectively.The input of compost straw reduced the Me Hg content of rice and the reduction rate was 19%,while the compost cow manure only reduced the Me Hg content of rice.