Grazing effects on denitrification in a soil under pasture during two contrasting seasons

The present study was designed to measure the effects of grazing events on denitrification in a pasture. A poorly-drained silt loam soil was used. The experiments were conducted both in the moist-cool winter, when a pasture was grazed with cows at a high stocking rate (about 300 cows ha−1) for 24 h, and in the dry-warm summer, when the pasture was grazed with cows at a low stocking rate (about 40 cows ha−1) for about 12 h. Denitrification rate was measured using the acetylene inhibition technique, by incubating soil cores (0–75 mm depth) in a closed system. In the moist-cool winter, the effect of grazing on denitrification was significant. An increase in nitrogen loss through denitrification generally occurred between 3 and 14 days after grazing, with the highest denitrification rate on d 10 following grazing. However, the measured total N loss through denitrification induced by grazing during that period was still very small, with less than 1% of the N returned in urine by the grazing animals being lost through denitrification in the 0–75 mm topsoil in the 2 weeks following grazing. Soil nitrate concentrations showed the same pattern as did denitrification rate. Grazing also had a temporary stimulating effect on denitrification enzyme activity (DEA). In the dry-warm summer, no systematic effect of grazing events at a low stocking rate on denitrification was observed, even though slightly higher concentrations of soil NO3− persisted for a long period after the grazing event. These results indicated that the effect of grazing on denitrification was influenced by soil conditions, particularly soil moisture content, in different seasons.