High-Resolution Denitrification Kinetics in Pasture Soils Link N(2)O Emissions to pH, and Denitrification to C Mineralization

Denitrification in pasture soils is mediated by microbial and physicochemical processes leading to nitrogen loss through the emission of N(2)O and N(2). It is known that N(2)O reduction to N(2) is impaired by low soil pH yet controversy remains as inconsistent use of soil pH measurement methods by researchers, and differences in analytical methods between studies, undermine direct comparison of results. In addition, the link between denitrification and N(2)O emissions in response to carbon (C) mineralization and pH in different pasture soils is still not well described. We hypothesized that potential denitrification rate and aerobic respiration rate would be positively associated with soils. This relationship was predicted to be more robust when a high resolution analysis is performed as opposed to a single time point comparison. We tested this by characterizing 13 different temperate pasture soils from northern and southern hemispheres sites (Ireland and New Zealand) using a fully automated-high-resolution GC detection system that allowed us to detect a wide range of gas emissions simultaneously. We also compared the impact of using different extractants for determining pH on our conclusions. In all pH measurements, soil pH was strongly and negatively associated with both N(2)O production index (IN(2)O) and N(2)O/(N(2)O+N(2)) product ratio. Furthermore, emission kinetics across all soils revealed that the denitrification rates under anoxic conditions (NO+N(2)O+N(2) μmol N/h/vial) were significantly associated with C mineralization (CO(2) μmol/h/vial) measured both under oxic (r(2) = 0.62, p = 0.0015) and anoxic (r(2) = 0.89, p<0.0001) conditions.