Gross nitrogen transformations in grassland soils as affected by previous management intensity

Gross transformations of nitrogen were studied in a grassland soil which had received N inputs of 100, 200, 300, 400 or 500 kg N ha−1 y−1 for the past 7 y. Paired soil incubations were undertaken in January 1996 using differentially 15N-labelled NH4NO3 applied in solution at a rate of 57 μmol N g−1 oven-dried soil and at an enrichment of 10 atom % excess. The change in concentration and enrichment of the NH4+-N, NO3−-N and N2O pools was determined at 0, 1, 3, 7, 14 and 21 d. Using 15N pool dilution gross rates of nitrification, mineralization and immobilization were calculated. The results showed that the mineralization–immobilization turnover in January was similar on all soils, despite their different management histories. There was little net mineralization of organic N throughout the incubation. Changes in the 15N enrichment of the NO3−-N pool showed that there was substantial nitrification during the incubation and that it was greater in soils with a history of high N inputs compared to those receiving low N inputs. Cumulative gross nitrification after 21 d was equivalent to 44.0, 84.2, 106.8, 165.7 and 181.7 kg N ha−1 for the soils with a history of inputs of 100, 200, 300, 400 and 500 kg N ha−1 y−1, respectively. The N2O and N2 concentrations were low in all soils. The average daily N2O flux over the incubation was 0.38. 0.27, 0.38, 0.64 and 1.07 mg N2O-N kg−1 d−1 for soils previously receiving 100, 200, 300, 400 and 500 kg N ha−1 y−1, respectively. The N2 flux was lower than that of N2O. Evidence suggested that the N2O produced was solely from denitrification which was proportionately greater on soils with a history of high N applications. Nitrification and denitrification in grassland soils is, therefore, highly influenced by previous management intensity.