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

Abstract

The pool of nitrate‐N (NO3–‐N) in the soil is more prone to losses than that of ammonium‐N (NH4+‐N) so any shift towards NO3–‐N dominance in the soil pools, caused by management intensity, could have environmental implications. The change in the balance of soil NH4+‐N and NO3–‐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 NH4+‐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 NO3–‐N and NH4+‐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 NO3–‐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 NO3–‐N occurred during the soil incubations. Past management history can play an important role in determining soil NO3–‐N content and hence potential losses of N to the environment.