| The rapid development of industrialization has led to the aggravation of heavy metal pollution in paddy soil in the sOTUh of China.Therefore,it is important to explore the stability of organic carbon in paddy soil under this environmental stress and to further understand the effect of carbon sequestration and emission reduction of paddy soil under environmental changes.In paddy soil,the crop keeps the soil organic carbon renewal through the root and residue input in each season.The influence of the input of new carbon on the decomposition and transformation of the organic carbon in the soil has great significance to explore of the soil carbon cycle and soil organic carbon transformation in farmland under pollution stress.Therefore,as a subtopic of the mechanism of the effect of heavy metal pollution on the stability of organic carbon in paddy soil,this study through adding corn straw(C4 crop)to long-term rice(C3 crop)growth soil is contaminated by different levels of heavy metals(from the distance from the source of pollution to different distances)to explore the effects of different gradients of heavy metal-contamination on soil organic carbon mineralization.Through analyzing the change of active organic carbon pool,the change of the intensity of activated organic carbon and the structure of microbial community in the process of culture,they can provide some reference for the study of the stability mechanism of organic carbon in farmland soil under the environment of heavy metal pollution.The main conclusions were as follow:(1)The addition of fresh organic carbon can promote the mineralization of organic carbon in paddy soil both under the different levels of heavy metals for a long time and artificial adding different treatments of heavy metals,and significantly increase the cumulative mineralization of all soil samples.In heavy metals Polluted Paddy soil for a long term,the cumulative mineralization of organic carbon contaminated in different degrees increased by 120%,540%and 360%,respectively,and increased by 63.3%,84.2%and 62.8%in artificial heavy metals treatment.The addition of fresh organic carbon also increased soil organic carbon,its active carbon content,turnover rate of soil organic carbon and soil microbial biomass.(2)In the long-term heavy metal polluted paddy soil,low pollution can promote the mineralization of total organic carbon in paddy soil,especially new carbon.The high pollution rate of old carbon in paddy soil is significantly higher than low pollution,and the relative positive effect of high pollution is also significantly greater,but reduced the content of soluble organic carbon.In the soil samples artificial treated with heavy metals,the control and low concentration could also significantly promote the mineralization of new carbon in paddy soil.The mineralization rate of the old carbon and relative positive effect on high concentration were higher than low concentration and control treatment,and high concentration treatment increased the content of soil particulate organic carbon and soluble organic carbon.(3)The bacteria abundance in the long-term heavy metal pollution paddy soil decreased with the increasing of pollution degree.The Shannon-Winner index of P0 samples in the control group was 9.08%and 28%higher than P1 and P2,and the Shannon-Winner index of P0 samples was 8.57%and 19.6%higher than that of P1 and P2 under the addition of corn straw.There was no obvious abundance on the diversity of fungi.In the samples artificial treated with heavy metals,the abundance of bacteria and fungi in high concentration samples significantly decreased,and the Shannon-Winner index of P0 and P1 in the control group was 43.76%and 18.17%higher than P2,and the Shannon-Winner index of P0 and P1 samples was 44.76%and 20.73%higher than P2 under the addition of corn straw.Addition of corn straw under two different heavy metal treatments significantly reduced the fungi abundance.In the long-term heavy metal polluted paddy soil,fungi abundance decreased by 51.87%,61.75%and 43.65%,respectively.In the soil samples artificial adding different treatments of heavy metals reduced by 45.70%,47.15%,and 19.48%,respectively.The addition of corn straw didn't cause the significant effect on bacteria abundance.(4)Heavy metal pollution significantly affected the community structure of soil bacteria community structure,while addition of corn straw had no significant effect on bacterial community structure.The relative abundance of Proteobacteria in long-term heavy metal polluted soil was significantly higher in high pollution.Corn straw addtion increase the relative abundance of Proteobacteria and the trend is higher in high polluted soil samples.With artificial adding different metal treatments of addition of corn straw and control treatment,the relative abundance of Proteobacteria decreased significantly and the relative abundance of Actinomycetes increased.In two different heavy metal treatments,the relative abundance of Acidobacteria and Nitrospirae decreased in the high concentration samples,and the relative abundance was the lowest in the P2 with the high pollution.The Acidobacteria was significantly lower than the P0 and P1 samples in the samples of P2.The effect of corn straw addition and CK on the microbial community structure with artificial adding different metal treatments was higher than long-term heavy metal pollution.Heavy metal pollution significantly affected the community structure of soil fungi,and corn straw addition also had significant effects on fungi community structure.Humicola,Fusarium,Podospora and Nectria are dominant in the long-term heavy metal pollution paddy soil.The relative abundance of Humicola and Podospora in high pollution paddy soil was significantly lower than low pollution,and its relative abundance increased after corn straw addition.In addition,the relative abundance of Fusarium was significantly higher than low pollution,and it reduced by the addition of corn straw.However,in the soil samples artificial treated with heavy metals,the results were significantly different from the former.High concentration of heavy metals increased the relative abundance of Humicola,Fusarium,Penicillium and Trichoderma,and reduced the relative abundance of the Podospora and Mortierella.(5)RDA analysis showed that the compound pollution of heavy metals had a significant influence on the community structure both of bacteria and fungi.In the long term heavy metal polluted paddy soil s,four major heavy metal elements had synergistic effects on the bacteria and fungi community structure.P1 had the greatest impact of heavy metals on soil samples and the most sensitive bacteria species are Firmicutes and Actinomycetes.Pb and Cd also had synergistic effects in soil samples artificial treated with heavy metals,and the P2 samples are most 'affected by heavy metals,while the most sensitive fungi are the Eurotiomycetes and Leotiomycetes. 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