Mineralization of nitrogen in permanent pastures amended with fertilizer or dung

Nitrogen mineralization was measured in three permanent pastures - either fertilized or unfertilized grass, or a mixed grass-clover sward - which were further amended with either fertilizer or cattle dung over a summer growing season. Measurements were made at 4- weekly intervals from June to October. Rates of net mineralization were similar in each of the background treatments (overall mean 0.99+/-0.091 kg N ha(-1) day(-1)) and did not change markedly during the experiment. From the second sampling (July) onwards, rates of mineralization in all the dung treatments were higher than in the control by a factor of up to 2. In the fertilizer-amended treatments, rates were also consistently (but not significantly) higher than in the control. However, the relatively small effect of fertilizer detected at each sampling had a significant cumulative effect by the end of the experiment. There was no interaction between the background and current treatments. Potential mineralization, measured by anaerobic incubation, increased in all the treatments over the period of the experiment, showing an accumulation of readily mineralizable residues. Total N mineralized and the N accumulated during the experiment were calculated and compared. This approach suggests that potential measurements could provide a good estimate of changes in soil N supply that would not be otherwise detectable in changes in soil total N in the short-term.