Effects of nano-selenium addition on the nitrogen cycle during composting: Mechanistic regulation of denitrification and organic nitrogen metabolism

Nano-selenium organic fertilizers are a key component in building selenium-enriched functional agriculture, but the impact of nano-selenium addition on nitrogen transformation during composting remains poorly understood. In this study, 0 μg/kg (CK), 75 μg/kg (Se_L), 150 μg/kg (Se_M), and 300 μg/kg (Se_H) nano-selenium were added to cattle manure to investigate the effects on different nitrogen metabolic pathways. The results show that organic nitrogen metabolism and denitrification are major pathways by which aerobic composting microorganisms participate in the nitrogen cycle. Nano-selenium significantly upregulated the gltB gene expression during the thermophilic phase and inhibited ammonium-nitrogen (NH₄⁺) accumulation by promoting L-glutamate synthesis. The NH₄⁺ contents in the Se_M and Se_H groups were 0.21 g/kg and 0.43 g/kg lower than in the CK group at the end of composting (p < 0.05). The medium concentration of nano-selenium favored a reduction in the synergistic effects among nitrogen metabolic functions. The downregulation of nirK and upregulation of the remaining denitrification genes in the Se_M group improved the nitrate-to-nitrogen conversion and effectively reduced nitrous oxide (N₂O) emissions. After three days of composting, the N₂O emissions from the Se_M group were only 36.46 % of that in the CK group (p > 0.05). In conclusion, adding appropriate concentrations of nano-selenium to the aerobic composting process of cattle manure is an effective strategy to regulate the nitrogen cycle and reduce the accumulation of NH₄⁺ as well as gaseous nitrogen losses.