Ammonium enrichment caused by ammonia mitigation can reduce N2O emissions from subsequent manure management

The implementation of ammonia mitigation measures in livestock production results in ammonium (NH4+) enrichment in the manure, but there is a lack of mechanistic understanding of such enrichment on nitrogen (N) cycling and gaseous N emissions from subsequent manure management. Here, we performed a series of manure incubations in a novel system enabling simultaneous determination of N2, N2O, NO and NH3 emissions, combined with 15N labelling and metagenomic sequencing, to investigate the impacts of NH4+ enrichment of manure on gaseous N emissions. A low level of NH4+ enrichment (<30g/kg) stimulated ammonia oxidation and nitrite accumulation, whereas these were inhibited at a high level of enrichment (50g/kg). The accumulated nitrite was mainly reduced through bacterial denitrification; however, total gaseous N emissions decreased with increasing NH4+ enrichment, mainly because NH4+ enrichment triggered carbon limitation by increasing the stoichiometric carbon:nitrogen imbalance and reducing the abundance of keystone nirK- and norB-bacterial denitrifiers and N2O production. These findings demonstrate that NH3 mitigation can lead to the reduction of multiple gaseous N emissions from manure management, and highlights a stoichiometry-controlled mechanism that underlies microbial N transformations and reactive N emissions from livestock manure.