In the Midwestern U.S., soil redox potential varies across seemingly homogenous landscapes, with shallow topographic depressions experiencing transient anaerobic conditions following intense rainfall. These topographic depressions may have high rates of dentification, an anaerobic microbial reaction that produces nitrous oxide (N2O), a potent greenhouse gas. However, depressional soils that experience dynamic redox potential can have distinct communities of denitrifying microorganisms that may mediate rates of denitrification in response to rainfall. As the spatial extent of ponding increases due to amplification of the hydrological cycle, rates of denitrification may depend on how historical soil drainage has shaped the contemporary microbial community. Thus, the goal of this study was to determine if soil drainage history selects for distinct microbial communities that differentially mediate N2O production via denitrification. To answer this question, we used stable isotope tracers to measure rates of N2O production from denitrifiers and nitrifiers from three paired upslope and depressional soils in Champaign county, Illinois in response to flooding treatments in the laboratory. We also characterized the community composition of denitrifying microorganisms by sequencing genes regulating nitrite reduction (nirS, nirK) and nitrous oxide reduction (typical and atypical nosZ). This was done using Illumina MiSeq sequencing of Fluidigm-generated amplicons.
Results/Conclusions
We found that upslope and depressional soils have distinct denitrifier communities, and that these communities likely contribute to different rates of N2O production via denitrification in response to ponding. Under ponded conditions, upslope soils had higher rates of net N2O efflux (p < 0.01, paired t-test). Based on stable isotope 15N-NO3- and 15N-NH4+ tracers, the majority (60 %) of this N2O was derived from denitrifiers. The denitrifier microbial community composition was correlated to topographic position after site variance was removed (p < 0.05, partial correspondence analysis). Upslope soils had higher relative abundance of Ca. Koribacter, a genus of bacteria which has genes encoding for the reduction of nitrate, nitrite, and nitric oxide, the initial steps of denitrification that leads to N2O production. Furthermore, depressional soils had higher relative abundance of Anaeromixobacter, a genus of bacteria that has genes encoding for the reduction of N2O to N2, but not the initial steps of denitrification. Together, these results suggest that depressional and upslope soils have distinct communities of denitrifying microorganisms that lead to differential rates of N2O emissions in response to ponding. This may result in higher N2O emissions from upslope soils that experience increased ponding as a result of rainfall intensification.