Multivariate and Multiscale Approaches for Interpreting the Mechanisms of Nitrous Oxide Emission during Pig Manure-Wheat Straw Aerobic Composting

Nitrous oxide (N2O) emission during corn-posting causes nitrogen loss and air pollution. The interpretation of N2O emission mechanisms will help to customize composting strategies that mitigate climate change. At pile and particle scales, this study characterized N2O emission-related variables (gases, ions, and microbes) and their correlations during pig manure-wheat straw aerobic composting. Pile-scale results showed that N2O emission mainly occurred in mesophilic, thermophilic, and cooling phases; the nitrification by ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) coexisted with the denitrification by denitrificans (DEN); the major NOB and DEN were Nitrobacter (NOB_Nba) and Thiobacillus denitrificans (DEN_Tb), respectively. The mechanisms of nitrification, nitrifier denitrification, and anaerobic denitrification in composting particles were initially visualized by confocal laser scanning microscopy: Betaproteobacteria (AOB_Beta) sporadically distributed on the outer area of the particles, NOB_Nba internally attached to AOB_Beta, and Nitrosomonas europea/Nitrosomonas eutropha (AOB_eu) and DEN_Tb concentrated in the interior. Correlation analysis of the variables showed that the distribution area of AOB_eu was proportional to N2O emission (R-2 = 0.84); AOB not only participated in nitrification but also nitrifier denitrification, and N2O formation was mainly from nitrifier denitrification by AOB_eu during the mesophilic-thermophilic phase and from denitrification by AOB_eu and DEN during the cooling phase.