Mechanistic insights into N2O emission mitigation by nitrification inhibitor dicyandiamide (DCD) in a tropical sandy soil after six years of manure amendment

Abstract

Organic amendments (OM) can profoundly affect soil nitrous oxide (N₂O) emissions via changing nitrogen (N) cycles. However, mechanistic insights into how nitrification inhibitors modulate the responses of soil N₂O emissions to successive applications of OM are currently insufficient. In this study, we performed a laboratory experiment to examine N₂O emissions from a tropical vegetable soil subjected to six years of chemical fertilization (CF) and chemical fertilization combined with manure application (CFM) and evaluate the mitigation effectiveness of nitrification inhibitor dicyandiamide (DCD) under each management regime. Isotopocule mapping showed that bacterial nitrification and/or fungal denitrification accounted for 77.4%–88.5% of total N₂O production across treatments during the emission peak. The cumulative N₂O emissions from the CFM-treated soil were nearly 8-fold of those from the CF-treated soil. The CFM treatment stimulated N₂O production from bacterial nitrification and denitrification by increasing the abundance of genes linked to nitrifiers (ammonia-oxidizing bacterial (AOB) amoA and total comammox amoA) and denitrifiers (nirK, nirS, and qnorB), respectively. Importantly, DCD decreased cumulative N₂O emissions by an average of 73.3%, with better mitigation performance observed in the CFM-treated soil than in the CF-treated soil due to stronger inhibited nitrification and increased abundance of the nosZ gene, and altered bacterial community composition. The 16S rRNA sequencing further revealed that adding DCD to the CFM-treated soil resulted in declines in the abundances of bacterial phylum Actinobacteria and Chloroflexi that positively affected N₂O emissions; the opposite pattern prevailed for Gemmatimonadetes that negatively affected N₂O emissions. This study highlights the potential of manure application, when coupled with nitrification inhibitors, to achieve the dual goals of enhancing soil fertility and reducing environmental risk associated with N₂O emissions in tropical agricultural soils.