Change in the balance of ammonium‐N and nitrate‐N content in soil under grazed grass swards over 7 years

The pool of nitrate‐N (NO 3 – ‐N) in the soil is more prone to losses than that of ammonium‐N (NH 4 + ‐N) so any shift towards NO 3 – ‐N dominance in the soil pools, caused by management intensity, could have environmental implications. The change in the balance of soil NH 4 + ‐N and NO 3 – ‐N content with time was studied using grazed grass swards receiving different fertilizer N inputs. In addition, the effect of past management on net nitrification of 400 μg NH 4 + ‐N g –1 was investigated in a soil incubation study. Mineral N was determined at frequent intervals (at least every 2 weeks) throughout the year in the top 5 or 7·5 cm of a sandy clay‐loam soil at the Agricultural Research Institute of Northern Ireland at Hillsborough, County Down, for a 7‐year period (1989–90 to 1995–96). The treatments were a perennial ryegrass–white clover sward receiving no fertilizer N, together with perennial ryegrass swards receiving 100, 200, 300, 400 or 500 kg N ha –1  year –1 as calcium ammonium nitrate. The plots were continuously grazed by beef steers from April to October to maintain a constant sward height of 7 cm. There was little or no change in average soil NO 3 – ‐N and NH 4 + ‐N content from 1989–90 to 1995–96 on the grass–clover sward and plots receiving 100 and 200 kg N ha –1  year –1 . However, with the plots receiving 300, 400 and 500 kg N ha –1  year –1 NO 3 – ‐N became progressively more dominant with time. The incubation study confirmed that this was due to an increase in net nitrification rate. There was evidence that rapid microbial assimilation of NO 3 – ‐N occurred during the soil incubations. Past management history can play an important role in determining soil NO 3 – ‐N content and hence potential losses of N to the environment.