Denitrification and associated N2O emissions are limited by phosphorus availability in a grassland soil

•Cumulative N2O emission was significantly increased after N addition as NH4+ but not as NO3−.•P addition significantly increased cumulative N2O and CO2 emissions only in the NO3− treatment.•Potential denitrification marginally increased with P addition.•P addition can result in large N2O emission from denitrification under high N fertilisation condition.

Effects of phosphorus (P) availability on nitrogen (N) loss through microbial nitrification and denitrification processes remain poorly understood. We conducted an incubation experiment to study the responses of N2O production after P (0 and 3 mg P kg− 1 soil) addition with and without NH4+ or NO3− (0 and 15 mg N kg− 1 soil) in a high P-fixing grassland soil. The N addition included a 15N tracer (1 mg 15N kg− 1). In a separate experiment, we also investigated the effect of P availability on potential nitrification and denitrification. We hypothesised that the addition of P in the soil would increase N2O emission from microbial nitrification and denitrification and thereby less soil 15N-NH4+ and 15N-NO3− would be recovered. We found that P addition only significantly increased cumulative N2O emission and respiration in the NO3− treatment, suggesting stimulated activities of denitrifying microorganisms due to their relief from P limitation. However, a decrease in soil 15N recovery after P addition in the NO3− treatment was not detected because of the very small loss of N as N2O (< 0.3% of the applied 15N tracer). Potential denitrification marginally increased with P addition, while potential nitrification was not affected by P addition. We conclude that P addition may increase gaseous loss of N as N2O in P-poor soils through microbial denitrification rather than nitrification when inorganic N is abundant.