Denitrifier abundance and community composition linked to denitrification activity in an agricultural and wetland soil

We examined denitrifier community abundance and composition in an agricultural and a wetland soil. Both soils were subjected to manipulations of pH and moisture content in a microcosm experiment, to understand if similar environmental and biotic factors governed denitrification rates in the two ecosystems, and how these factors were linked to denitrification rates. Bacterial communities containing nirK, nirS, and nosZ-I all had distinct composition in the agricultural compared to the wetland soil, with nirK and nirS community composition being particularly distinct compared to that of nosZ-I. Lowering soil pH decreased gene abundance and altered the composition of nirK and nirS in both soils and increased the N2O: (N-2 + N2O) ratio in agricultural soil. Re-wetting soil for the experimental manipulations led to increased denitrification rate in the agricultural soil but had no effect on the denitrification rate measured in the wetland soil. We found that the combination of abiotic variables, gene abundance, and diversity measures predicted denitrification better than any of these factors alone, showing the importance of including microbial measures for predicting denitrification. The manipulations of these soils provided evidence for niche differences between the homologous nirK and nirS genes. Notably, nirS abundance was more highly correlated to activity in higher pH soils, while nirK abundance was more correlated to activity in low pH soils. We show that nirK and nirS do not behave and contribute equally to denitrification under these varying conditions.