Variable response of nirK and nirS containing denitrifier communities to long-term pH manipulation and cultivation

Denitrification is a key process responsible for the majority of soil nitrous oxide (N2O) emissions but the influences of pH and cultivation on the soil denitrifier community remain poorly understood. We hypothesised that the abundance and community structure of the total bacterial community and bacterial denitrifiers would be pH sensitive and that nirK and nirS containing denitrifiers would differ in their responses to change in pH and cultivation. We investigated the effect of long-term pH-adjusted soils (ranging from pH 4.2 to 6.6) under different lengths of grass cultivation (one, two and three years of ley grass) on the general bacterial and denitrifier functional communities using 16S rRNA, nirK and nirS genes as markers. Denitrifier abundance increased with pH, and at pH below 4.7 there was a greater loss in nirS abundance per unit drop in pH than soils above this threshold pH. All community structures responded to changes in soil pH, while cultivation only influenced the community structure of nirK. These differences in denitrifier responses highlight the importance of considering both nirK and nirS gene markers for estimating denitrifier activity. Identifying such thresholds in response of the microbial community to changes in pH is essential to understanding impacts of management or environmental change.