Linking nitrogen load to the structure and function of wetland soil and rhizosphere microbial communities

Wetland ecosystems are important reservoirs of biodiversity and significantly contribute to emissions of the greenhouse gases CO2, NO2 and CH4. High anthropogenic nitrogen (N) inputs from agriculture and fossil fuel combustion have been recognized as a severe threat to biodiversity and ecosystem functioning such as control of greenhouse gas emissions. Therefore it is important to understand how increased N input into pristine wetlands affects the composition and activity of micro-organisms, especially in interaction with dominant wetland plants. In a series of incubations analyzed over 90 days, we disentangle the effects of N fertilization on the microbial community in bulk soil and the rhizosphere of Juncus acutiflorus, a common and abundant graminoid wetland plant. We observed an increase in greenhouse gas emissions when N is increased in incubations with J. acutiflorus, changing the system from a greenhouse gas sink to a source. Using 16S rRNA amplicon sequencing and metagenomics, we determined that the bacterial orders Opitutales, Subgroup-6 Acidobacteria and Sphingobacteriales significantly responded to high N availability and we hypothesize that these groups are contributing to the increased greenhouse gas emissions. These results indicated that increased N input leads to shifts in microbial activity within the rhizosphere, severely altering N cycling dynamics. Our study provides a framework for connecting environmental conditions of wetland bulk and rhizosphere soil to the structure and metabolic output of microbial communities.