Change in the balance of ammonium-N and nitrate-N content in soil under grazed grass swards over 7 years
1999
Watson, C.J. | Poland, P.
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 effects of past management on net nitrification of 400 microgram 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.
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