Assessment of the contribution of denitrification to N losses from compacted grassland soil by NO3- disappearance and N2O production during anaerobic incubation

Decreases in herbage production and of N uptake and utilization have been observed on Denbigh series soils in mid-Wales after several years in permanent pasture. Laboratory experiments were conducted to examine the contribution of denitrification to N loss from these wet grassland soils. Denitrification capacity was measured in seived soil following the addition of KNO3 and maintained at 20 degrees C under anoxic conditions. Emission of N2O was measured from intact field cores equilibrated under conditions of simulated "field capacity" using glucose as C substrate. The rate of loss of NO3--N decreased with depth and in the 0-2.5 cm layer all added NO3--N was lost in 10 d incubation. Net mineralization of NH4+-N occurred at about one-sixth of the rate of NO3--N disappearance. The presence of NO3- reduced the rate of decrease in redox potential (E-h) and the E-h did not fall below about +200 mV until all NO3--N had been lost, Emission of N2O was greatest between 6 and 48 h and denitrification rate decreased with depth. Addition of glucose increased N2O emission in the 2.5-5.7 cm layers indicating that C limitation to denitrification may occur at shallow depths in the soil profile of compacted grassland. On average, the total denitrification ranged between 15 and 20 kg N ha(-1), equivalent to 20-30% of applied N. The potential rates of denitrification change markedly over quite shallow depths in these compacted grassland soils. Furthermore, since denitrification occurred at substantial rates under simulated field capacity, conditions conducive to denitrification are likely to persist for quite long periods in the moist climatic conditions.