Effects Of Acid Mine Drainage Pollution On Methanogen And Sulfate-Reducing Bacteria In Paddy Soil

A contaminant that significantly damages the environment is acid mine drainage（AMD）.It alters the soil’s physicochemical properties when it penetrates farmland soil,resulting in heavy metal pollution and further disrupting the microbial populations in the soil.Methanogens and sulfate-reducing bacteria are crucially important functional microorganisms in the transition of soil carbon to sulfur,which might indicate soil fertility,quality,and pollution.Analyzing the variety,community structure,and ecological roles of soil methanogens and sulfate-reducing bacteria can lead to new insights and recommendations for soil environmental remediation.In this study,the research location was long-term AMD contaminated farmland and typical unpolluted farmland close to a mining region in Tongling City,Anhui Province.Soil samples were collected at different periods（pre-planting,mid-planting and mature stages of rice）and at different profile depths（0-20 cm,20-40 cm and 40-60 cm）.The temporal and spatial changes of physicochemical properties as well as heavy metal elements,in addition the response of methanogens and sulfate-reducing bacteria communities in AMD contaminated soil were investigated.（1）The temporal and spatial changes of physicochemical properties and heavy metal content in AMD contaminated soil and unpolluted soil were elucidated.Compared with the unpolluted soil,the p H value of polluted soil was lower,ranging from 4.49 to 4.81.The content of SO₄^2-（sulfate）in polluted soil was 1.39～2.48 times higher than that in unpolluted soil.The contents of OM（organic matter）and TN（total nitrogen）in the contaminated soil gradually increased from the pre-planting stage to the maturity stage.The content of SO₄^2-in the unpolluted soil was lower in the pre-planting and mid-planting stages,and was highest in the mature stage,while the contents of OM and TN in the unpolluted soil increased first and then decreased from the pre-planting stage to the mature stage.In the vertical direction,the contents of OM,TN,TP（total phosphorus）,NO₃^-（nitric nitrogen）and SO₄^2-in polluted soil and unpolluted soil decreased with increasing depth.The contents of heavy metals Cd,Cu,Pb and Zn in polluted soil were significantly higher than those in unpolluted soil.The Cd content of contaminated soil increased gradually from surface to deep layer,while Cu and Pb were mainly enriched in surface layer,and their contents decreased with the increase of depth.（2）The distribution of methanogenic archaeal communities in AMD contaminated soil and unpolluted soil and its correlation with soil environmental factors were elucidated.The dominant genera of methanogenic archaea in paddy soils mainly included Methanobacterium,Methanocella,Methanosaeta and Methanosarcina.Methanobacterium was the main methanogenic archaea in the contaminated soil,while Methanocella was the main methanogenic archaea in the unpolluted soil.The relative abundance of Methanocella decreased and then increased,while that of Methanobacterium increased and then decreased from the pre-planting to the maturity stage of rice.The relative abundance of Methanocella increased firstly and then decreased with increasing depth at pre-planting and mature stages,and decreased firstly and then increased at middle planting stage.Methanobacterium and Methanocella both belong to hydrotrophic methanogens,suggesting that H₂/CO₂ reduction is the main methanogenic pathway in rice fields.In the contaminated soil,the predicted methanogenic gene abundance before planting was much higher than that at the middle planting and maturity stages.The abundance of methanogenic functional genes in the contaminated soil was higher than that in unpolluted soil before rice planting,but lower in unpolluted soil at middle planting and maturity stage.OM,TN and NO₃^-are the key factors affecting methanogens.（3）The distribution of sulfate-reducing bacteria community in AMD contaminated soil and unpolluted soil and its correlation with soil environmental factors were clarified.Desulfobacca,Desulfomonile,Desulfovibrio,Desulfovirga and Desulfosporosinus are predominant in the sulfate reducing bacterial community in the paddy field soil.Desulfobacca was the dominant sulfate-reducing bacteria in the contaminated soil and the unpolluted soil.The relative abundance of Desulfosporosinus was higher in the contaminated soil compared to the unpolluted soil.The relative abundance of Desulfobacca in contaminated and unpolluted soil is slightly lower in the mid-planting and maturity compared with pre-rice planting.Desulfobacca is displayed in a vertical distribution as high in the surface and low in the deep.The abundances of predicted sulfate-reducing functional genes in contaminated soils increased first and then decreased from pre-planting to maturity.In each period,the abundance of sulfate-reducing functional genes in 0-20 cm soil layer was higher than that in unpolluted soil layer.SO₄^2-,p H and heavy metals（Cd,Cu,Pb and Zn）are the key factors affecting sulfate-reducing bacteria.（4）The interactive relationship between methanogenic and sulfate-reducing bacteria in paddy soils was elucidated.The molecular ecological network composed of methanobacteria and sulfate-reducing bacteria in AMD contaminated soil and unpolluted soil was mainly synergistic,and the proportion of such synergistic relationship was higher in the contaminated soil.The molecular ecological network composed of methanobacteria and sulfate-reducing bacteria in the contaminated soil also has more complex structure and stronger ability to resist environmental changes.The predicted interaction between methanogenesis and sulfate reduction functional genes suggests that sulfate reduction functional genes play an important regulatory role.The dsr A and dsr B genes played a strong role in the unpolluted soil,whereas the apr A and apr B genes played a strong role in the contaminated soil.