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 KNO 3 and maintained at 20°C under anoxic conditions. Emission of N 2 O was measured from intact field cores equilibrated under conditions of simulated "field capacity" using glucose as C substrate. The rate of loss of NO 3 − –N decreased with depth and in the 0–2.5 cm layer all added NO 3 − –N was lost in 10 d incubation. Net mineralization of NH 4 + –N occurred at about one-sixth of the rate of NO 3− –N disappearance. The presence of NO 3 − reduced the rate of decrease in redox potential (E h ) and the E h did not fall below about +200 mV until all NO 3 − –N had been lost. Emission of N 2 O was greatest between 6 and 48 h and denitrification rate decreased with depth. Addition of glucose increased N 2 O 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. Key words: Compacted soil, denitrification, glucose, grassland, nitrous oxide