Denitrifiers in the surface zone are primarily responsible for the nitrous oxide emission of dairy manure compost

Abstract

During the dairy manure composting process, significant nitrous oxide (N2O) emissions occur just after the pile turnings. To understand the characteristics of this N2O emission, samples were taken from the compost surface and core independently, and the N2O production was monitored in laboratory incubation experiments. Equal amounts of surface and core samples were mixed to simulate the turning, and the 15N isotope ratios within the molecules of produced N2O were analyzed by isotopomer analysis. The results showed that the surface samples emitted significant levels of N2O, and these emissions were correlated with NOx−-N accumulation. Moreover, the surface samples and surface-core mixed samples incubated at 30°C produced N2O with a low site preference (SP) value (−0.9 to 7.0‰) that was close to bacteria denitrification (0‰), indicating that denitrifiers in the surface samples are responsible for this N2O production. On the other hand, N2O produced by NO2−-amended core samples and surface samples incubated at 60°C showed unrecognized isotopic signatures (SP=11.4–20.3‰). From these results, it was revealed that the N2O production occurring just after the turnings was mainly derived from bacterial denitrification (including nitrifier denitrification) of NOx−-N under mesophilic conditions, and surface denitrifying bacteria appeared to be the main contributor to this process.