Short-term effects of ammonium nitrogen in upland pasture soil affected or not affected by cattle

The short-term effects of excessive NH (4) (+) -N on selected characteristics of soil unaffected (low annual N inputs) and affected (high annual N inputs) by cattle were investigated under laboratory conditions. The major hypothesis tested was that above a theoretical upper limit of NH (4) (+) concentration, an excess of NH (4) (+) -N does not further increase NO (3) (-) formation rate in the soil, but only supports accumulation of NO (2) (-) -N and gaseous losses of N as N2O. Soils were amended with 10 to 500 mu g NH (4) (+) -N g(-1) soil. In both soils, addition of NH (4) (+) -N increased production of NO (3) (-) -N until some limit. This limit was higher in cattle-affected soil than in unaffected soil. Production of N2O increased in the whole range of amendments in both soils. At the highest level of NH (4) (+) -N addition, NO (2) (-) -N accumulated in cattle-affected soil while NO (3) (-) -N production decreased in cattle-unaffected soil. Despite being statistically significant, observed effects of high NH (4) (+) -N addition were relatively weak. Uptake of mineral N, stimulated by glucose amendment, decreased the mineral N content in both soils, but it also greatly increased production of N2O.