Estimation of simultaneous nitrification and denitrification in grassland soil associated with urea-N using 15 N

A low efficiency of use of N fertilisers has been observed in mid-Wales on permanent pasture grazed intensively by cattle. Earlier laboratories studies have suggested that heterogeneity in redox conditions at shallow soil depths may allow nitrification and deni- trification to occur concurrently resulting in gaseous losses of N from both NH4 c and NO3 . The objective of the investigation was to test the hypothesis that both nitrification and denitrification can occur simultaneous- ly under simulated field capacity conditions (F5 kPa matric potential). Intact soil cores were taken from grassland subjected to both grazing and amenity use. The fate of applied NH4 c was examined during incuba- tion. 15 N was used as a tracer. Nitrapyrin was used as a nitrification inhibitor and acetylene was used to block N2O reductase. More than 50% of N applied as NH4 c