Dominance of bacterial ammonium oxidizers and fungal denitrifiers in the complex nitrogen cycle pathways related to nitrous oxide emission

Organic compounds and mineral nitrogen (N) usually increase nitrous oxide (N 2 O) emissions. Vinasse, a by‐product of bio‐ethanol production that is rich in carbon, nitrogen, and potassium, is recycled in sugarcane fields as a bio‐fertilizer. Vinasse can contribute significantly to N 2 O emissions when applied with N in sugarcane plantations, a common practice. However, the biological processes involved in N 2 O emissions under this management practice are unknown. This study investigated the roles of nitrification and denitrification in N 2 O emissions from straw‐covered soils amended with different vinasses ( CV : concentrated and V: nonconcentrated) before or at the same time as mineral fertilizers at different time points of the sugarcane cycle in two seasons. N 2 O emissions were evaluated for 90 days, the period that occurs most of the N 2 O emission from fertilizers; the microbial genes encoding enzymes involved in N 2 O production (archaeal and bacterial amoA , fungal and bacterial nirK , and bacterial nirS and nosZ ), total bacteria, and total fungi were quantified by real‐time PCR . The application of CV and V in conjunction with mineral N resulted in higher N 2 O emissions than the application of N fertilizer alone. The strategy of vinasse application 30 days before mineral N reduced N 2 O emissions by 65% for CV , but not for V. Independent of rainy or dry season, the microbial processes were nitrification by ammonia‐oxidizing bacteria ( AOB ) and archaea and denitrification by bacteria and fungi. The contributions of each process differed and depended on soil moisture, soil pH , and N sources. We concluded that amoA ‐ AOB was the most important gene related to N 2 O emissions, which indicates that nitrification by AOB is the main microbial‐driven process linked to N 2 O emissions in tropical soil. Interestingly, fungal nirK was also significantly correlated with N 2 O emissions, suggesting that denitrification by fungi contributes to N 2 O emission in soils receiving straw and vinasse application.