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

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 denitrification to occur concurrently resulting in gaseous losses of N from both NH4+ and NO3-. The objective of the investigation was to test the hypothesis that both nitrification and denitrification can occur simultaneously under simulated field capacity conditions (similar to 5 kPa matric potential). Intact soil cores were taken from grassland subjected to both grazing and amenity use. The fate of applied NH4+ was examined during incubation. N-15 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+ disappeared over a period of 42 days from the soil mineral-N pool. Some of this N was evolved as N2O. Accumulation of NO3-N in the surface 0-2.5 cm indicated active nitrification. Addition of nitrapyrin increased N recovery by 26% and inhibited both the accumulation of NO3-N and emission of N2O. When intact field cores were incubated after addition of N-15-urea, all of the N2O evolved was derived from added urea-N. It was concluded that nitrification and denitrification do occur simultaneously in the top 7.5 cm or so, of the silty clay loam grassland topsoils of mid-Wales at moisture contents typical of field capacity. The quantitative importance of these concurrent processes to N loss from grassland systems has not yet been assessed.