Field H-2 Infusion Alters Bacterial And Archaeal Communities But Not Fungal Communities Nor Nitrogen Cycle Gene Abundance

Large amounts of H-2 are produced in the nodules of legume plants during N-2-fixation, which often subsequently leaks to the surrounding soil environment. This leaked H-2 activates H-2-oxidizing bacterial (HOB) communities and thus can alter microbial community structure and soil carbon and nitrogen turnover; a process that may explain part of the positive effect of crop rotations with legumes. We tested this hypothesis in the field by infusing H-2 in soil for 4 weeks and followed the effect on bacterial, archaeal and fungal communities by amplicon sequencing, as well as the abundance of genes involved in the nitrogen cycle using quantitative real-time PCR (qPCR). Soil exposure to elevated H-2 levels changed the capacity of soil to scavenge H-2 with a stimulation of low-affinity H-2-oxidizing activity. Activation of this functional group of bacteria led to significant increases in the relative abundance of Nitrososphaera, Gaiella and Gp16 in soil bacterial and archaeal communities, albeit without changing their diversity. In contrast, H-2 infusion had little effect on fungal community diversity, composition and taxa abundance. The abundance of selected functional genes encoding for bacterial and archaeal enzymes involved in the nitrogen cycle was not influenced by H-2 infusion, but the abundance of nifH was significantly affected by H-2 infusion x sampling date interaction. Variation in the high-affinity and low-affinity HOB, microbial community diversity and composition, and N-cycle gene abundance between sampling dates was attributed, in part, to significant differences in soil moisture during the study. Together, this work confirms that microbial taxa differ in their reaction to H-2 exposure in the field, and provides the first evidence that nitrifier and denitrifier microorganisms have uneven responses to elevated H-2 levels under field conditions.