Previous research has found total mercury (THg) and methylmercury (MeHg) levels increase with litterfall decay, thus suggesting litterfall decomposition plays an essential role in the biogeochemical transformation of mercury (Hg). However, it remains unclear how Hg accumulates in the decaying litter, how bacterial taxa networks vary and what roles various microorganisms play during litterfall decomposition, especially nitrogen (N)-fixing, MeHg-degrading and Hg-methylating microbes. Here, we demonstrated as degradation proceeded, a gradually-complex network evolved for litterfall bacteria for the subtropical mixed broadleaf-conifer (MBC) forest, whereas a relatively static network existed for the evergreen broadleaf (EB) forest. N-fixing and MeHg-degrading bacteria dominated throughout litterfall decomposition process, with relative abundances of N-fixing genera and nifH copies maximum and relative abundances of MeHg-degrading bacteria and merAB copies minimum in summer. Hence, N-fixing bacteria likely mediate THg increase in the decomposing litterfall, while MeHg enhancement may be regulated by aerobic MeHg-degrading microbes which can transform MeHg to inorganic divalent Hg (Hg²⁺) or further to elemental Hg (Hg⁰). Together, this work elucidates variations of N-fixing and MeHg-degrading microbes in decaying litterfall and their relationships with Hg accumulation, providing novel insights into understanding the biogeochemical cycle of Hg in the forest ecosystem.

先前的研究发现，总汞 (THg) 和甲基汞 (MeHg) 水平随着凋落物衰减而增加，因此表明凋落物分解在汞 (Hg) 的生物地球化学转化中起着至关重要的作用。然而，目前尚不清楚汞如何在腐烂的垫料中积累，细菌分类群网络如何变化以及各种微生物在垫料分解过程中所起的作用，尤其是固氮 (N)、降解甲基汞和甲基化汞的微生物。在这里，我们证明了随着退化的进行，亚热带阔叶针叶混交林 (MBC) 的凋落物细菌进化出一个逐渐复杂的网络，而常绿阔叶 (EB) 森林则存在一个相对静态的网络。固氮和降解甲基汞的细菌在凋落物分解过程中占主导地位，在夏季， nifH复制甲基汞降解细菌的最大和相对丰度，而merAB复制最小。因此，固氮细菌可能会介导分解垃圾中 THg 的增加，而 MeHg 的增强可能受到好氧 MeHg 降解微生物的调节，这些微生物可以将 MeHg 转化为无机二价 Hg (Hg ²⁺ ) 或进一步转化为元素 Hg (Hg ⁰ )。总之，这项工作阐明了腐烂枯枝落叶中固氮和降解甲基汞的微生物的变化及其与汞积累的关系，为理解森林生态系统中汞的生物地球化学循环提供了新的见解。