Resource Availability Effects on Nitrate-Reducing Microbial Communities in a Freshwater Wetland

Abstract

Microbial communities in freshwater wetland soils process nitrate via denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA). Because the processes generate different end products (N-gas versus NH4 +), the relative dominance of DNF versus DNRA has implications for ecosystem nitrogen cycling, greenhouse gas production, and downstream eutrophication. To examine how resource availability affects these two microbial groups, wetland soil was supplemented with labile (compost) or recalcitrant (wood) organic matter (OM) and/or potassium nitrate fertilizer. Following a three-month in situ incubation, the abundance and composition of the DNF- and DNRA-capable microbes were examined via quantitative polymerase chain reaction (qPCR) and terminal restriction fragment length polymorphism (T-RFLP) using process-specific functional genes (DNF: nirS qPCR, nosZ T-RFLP; DNRA: nrfA qPCR and T-RFLP). Denitrifer abundance was positively related to OM lability and simultaneous nitrate amendment enhanced OM effects, while DNRA abundance varied little across treatments. For both groups, community structure showed an interactive response to OM type and nitrate availability, even when abundances did not change. This work highlights the importance of considering co-varying resource gradients, and the differential responses of DNF and DNRA communities to resource manipulation provides insight into the environmental regulators of ecosystem nitrate removal in wetlands.