由于其毒性和致癌性，砷（As ）在环境中受到 了 极大关注。水稻种植是我国土地的主要利用方式之一，我国的水稻产量居世界第一。由于定期的灌水排水等农业措施，水稻土会经历规律性的 氧化还原 条件 变 化。淹水条件下，氧化还原电位降低，稻田土壤中的砷活化后通过水通道被水稻根系高效吸收，并在籽粒中高效积累。目前水稻及水稻制品已经成为人们砷暴露的主要途径之一。
有机质是影响稻田土壤砷迁移转化的重要因素之一。秸秆经常被添加到水稻田来进行固碳或增加土壤肥力，从而增加稻田土壤有机质含量。 研究表明有机质添加可以促进土壤中的 砷释放和砷挥发。腐殖质是土壤有机质的主要成分，会 对砷迁移转化 产生显著影响 。 一方面 ，腐殖质 可以与砷络合或竞争铁矿物的吸附位点从而影响砷释放。另一方面，腐殖质可以为微生物提供能量，促进微生物生长，从而影响微生物介导的砷迁移转化。微生物在砷的生物地球化学循环中发挥了重要作用，但目前腐殖质添加条件下参与土壤砷循环的微生物并不清楚。水稻田是重要的甲烷代谢场所 ，每年排放大量的甲烷 。 在微生物作用下， 有机质 会厌氧 降解 最终生成甲烷 。 甲烷厌氧氧化在控制全球甲烷排放中起了重要的作用。 深古菌是一类具有甲烷代谢 潜力的古菌， 其在全球碳循环中 扮演着重要的角色 。但目前关于深古菌在 水稻田有机质添加下参与的甲烷代谢过程并不清楚。
采用砷污染土进行培养实验，探究添加不同剂量的胡敏酸和富里酸 (0.2%，0.5%，1.0%和1.5% (w/w))对土壤砷循环与微生物群落结构的影响。同时探究水稻田有机质降解过程中深古菌与甲烷代谢之间的耦合机制。具体研究内容和结果如下：
(1) 低剂量 (< 0.5%)的胡敏酸处理中土壤孔隙水中砷含量是空白处理的1.2倍，但胡敏酸添加对土壤中铁、锰和乙酸的浓度影响很小。高浓度 (> 1%)胡敏酸添加显著促进了砷甲基化。砷挥发量和土壤溶液中二甲基砷浓度随胡敏酸添加量的增加而增加。低浓度胡敏酸对微生物群落结构并无显著影响，而高浓度胡敏酸显著改变了土壤微生物群落结构，显著提高了Methanobacterium 的相对丰度 。
(2) 1.5%富里酸处理中土壤孔隙水中砷、铁、锰、乙酸浓度分别为空白的12倍、20倍、3倍、3倍。富里酸添加显著促进了砷甲基化，土壤溶液中一甲基砷浓度随富里酸的增加而增加。富里酸添加显著改变了微生物群落结构， Desulfitobacterium的相对丰度提高了41倍，且Bathyarchaeota得到了显著富集。
Arsenic (As) is of great concern in the environment due to its toxicity and carcinogenicity Paddy soil is one of the main land use modes in China, and the rice yield in China ranks first in the world. Paddy soils undergo regular redox environmental changes due to agricultural measures such as regular irrigation and drainage. It is known that under flooded condition, rice is efficient in accumulating As in its grains because of the high As availability i n soil and the efficient arsenite transport system. Rice consumption has become the main route for human exposure to As.
Organic matter is one of the key factors controlling As mobility in paddy soil. Straw is recommended to be incorporated to the soil for carbon sequestration and soil fertility. However, such practices could significantly increase the organic substances in the soil, and straw has been demonstrated to remarkably increase As mobility in soil and enhance As methylation Humic s ubstance s HS) is the main component of soil organic matter , which might signi ficantly affect As fate in soil On the one hand, HS might affect As release through forming As complex or competing for the adsorption sites on iron minerals. On the other hand, HS might also affect the biological cycle of arsenic throught providing energy for the growth of microorganisms H owever, so far it is still unknown what microorganisms are involved in these processes.
Rice field is an important place fo r methane metabolism which emits a large amount of methane every year. O rganic matter can degrade anaerobica lly and finally produce methane through microorganism s . Anaerobic oxidation of methane plays an important role in controlling global methane emissio ns. Bathyarchaeota is a group of archaea which has methane metabolism potential and plays an important role in the global carbon cycle. However, it is not clear about the coupling relationship betwen methane metabolism and Bathyarchaeota in paddy fields under the addition of organic matter.
In this study,microcosms were set up by incubating humic acid (HA) and fulvic acid (FA) with As-contaminated paddy soil. We explored t h e effects of HA and FA addition 0.2%, 0.5%, 1.0%, 1.5%) on As mobility and microbial community composition in paddy soil At the same time, we explored the coupling relationship between Bathyarchaeota and methane metabolism during the degradation of organic matter in paddy fields. Specific research results are as fo llows:
(1) Compared with the control, low dose (< 0.5%) of HA increased the concentrations of As (1.2-fold) in soil porewater, and had little effect on the concentration of Fe, Mn and acetic acid in soil porewater. High dose (> 1%) of HA significantly promoted arsenic methylation. The volatile As and the concentration of dimethylarsenic DMA) in porewater increased with the increase of HA addition . Low dose (< 0.5%) of HA addition had no significant effect on microbial community structure, while high dose (> 1%) of HA addition had significant effect on soil microbial
community structure , a nd significantly increased the relative abundance of Methanobacterium
(2) Compared with the control, FA addition significantly increased the concentrations of As (12-fold), iron (Fe; 20-fold), manganese (Mn; 3-fold) and acetic acid (3-fold) in porewater. The addition of FA promoted As methylation, and the concentration of mon omethyl arsenic MMA) in porewater increased with the increase of FA addition . The addition of FA significantly changed the microbial community structure. T he relative abundance of Desulfitobacterium increased 41 times and Bathyarchaeota was significantly enriched.
(3)Bathyarchaeota were not enriched at the end of culture in N 2 treatment but FA addition caused significant enrichment of Bathyarchaeota in air treatment . With the addition of FA , the number of mcrA gene cop ies showed the maximum on the 28th day.The inconsistency of mcrA gene and Bathyarchaeota indicated that Bathyarchaeota did not participate in methane production in this culture system.
In summary, our study showed that different organic matter HA vs. FA ) had different effects on arsenic migration in paddy soils. FA addition can promote As release in soil by modifying the microbial community st ru cture of soil , which might cause non-negligible environmental risks. Furthermore, the results showed that Bathy archaeota did not be involved in m ethanogenesis in this incubation system . This study could provide practical guide for the remediation of heavy metals and fertilization of organic matter in arsenic contaminated soils, and would also expand our understanding of the coupling relationship between arsenic and iron with HS through microorganisms in paddy soil.