Response of the Abundance of Key Soil Microbial Nitrogen-Cycling Genes to Multi-Factorial Global Changes

Multiple co-occurring environmental changes are affecting soil nitrogen cycling processes, which are mainly mediated by microbes. While it is likely that various nitrogen-cycling functional groups will respond differently to such environmental changes, very little is known about their relative responsiveness. Here we conducted four long-term experiments in a steppe ecosystem by removing plant functional groups, mowing, adding nitrogen, adding phosphorus, watering, warming, and manipulating some of their combinations. We quantified the abundance of seven nitrogen-cycling genes, including those for fixation (nifH), mineralization (chiA), nitrification (amoA of ammonia-oxidizing bacteria (AOB) or archaea (AOA)), and denitrification (nirS, nirK and nosZ). First, for each gene, we compared its sensitivities to different environmental changes and found that the abundances of various genes were sensitive to distinct and different factors. Overall, the abundances of nearly all genes were sensitive to nitrogen enrichment. In addition, the abundances of the chiA and nosZ genes were sensitive to plant functional group removal, the AOB-amoA gene abundance to phosphorus enrichment when nitrogen was added simultaneously, and the nirS and nirK gene abundances responded to watering. Second, for each single- or multi-factorial environmental change, we compared the sensitivities of the abundances of different genes and found that different environmental changes primarily affected different gene abundances. Overall, AOB-amoA gene abundance was most responsive, followed by the two denitrifying genes nosZ and nirS, while the other genes were less sensitive. These results provide, for the first time, systematic insights into how the abundance of each type of nitrogen-cycling gene and the equilibrium state of all these nitrogen-cycling gene abundances would shift under each single- or multi-factorial global change.多种协同发生的环境变化正影响着土壤氮循环过程，而该过程主要由微生物介导调控。尽管各类氮循环功能类群对这类环境变化的响应模式可能存在差异，但目前对其相对响应程度的认知仍极为有限。本研究在草原生态系统中开展了4项长期定位试验，设置了植物功能群移除、刈割、施氮、施磷、增水、增温以及部分处理组合调控等实验处理。我们定量分析了7类氮循环功能基因的丰度，包括固氮相关基因（nifH）、有机氮矿化相关基因（chiA）、氨氧化相关基因（氨氧化细菌amoA（AOB）或古菌amoA（AOA））以及反硝化相关基因（nirS、nirK和nosZ）。首先，针对每一类氮循环基因，我们比较了其对不同环境变化的响应敏感性，结果发现各类基因的丰度分别对不同的调控因子具有敏感性。总体而言，几乎所有基因的丰度均对氮富集处理表现出敏感性。此外，chiA和nosZ基因的丰度对植物功能群移除处理具有敏感性；AOB-amoA基因丰度在同时施氮的条件下对磷富集处理敏感；而nirS与nirK基因丰度则对增水处理产生响应。其次，针对每一种单因子或多因子环境变化，我们比较了不同基因丰度的响应敏感性，结果发现不同的环境变化主要对特定的基因丰度产生调控作用。总体而言，AOB-amoA基因丰度的响应敏感性最高，其次为反硝化基因nosZ与nirS，其余基因的敏感性相对较低。本研究结果首次系统揭示了在各类单因子或多因子全球变化情景下，每一类氮循环基因的丰度以及所有氮循环基因丰度的平衡状态将如何发生变化。