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

Abstract

Nitrous oxide (N₂O) emission during composting 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 ( R² = 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.