GROSS NITROGEN TRANSFORMATIONS IN GRASSLAND SOILS AS AFFECTED BY PREVIOUS MANAGEMENT INTENSITY

Gross transformations of nitrogen were studied in a grassland soil whi ch 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 N-15-labelled NH4NO3 applied in solution at a rate of 57 mu mol N g(-1) oven-dried soil and at an enrichment of 1 0 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. U sing N-15 pool dilution gross rates of nitrification, mineralization a nd immobilization were calculated. The results showed that the mineral ization-immobilization turnover in January was similar on ail soils, d espite their different management histories. There was little net mine ralization of organic N throughout the incubation. Changes in the N-15 enrichment of the NO3--N pool showed that there was substantial nitri fication during the incubation and that it was greater in soils with a history of high N inputs compared to those receiving low N inputs. Cu mulative 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 inp uts of 100, 200, 300, 400 and 500 kg N ha(-1) y(-1), respectively. The N2O and N-2 concentrations were low in all soils. The average daily N 2O 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 N-2 flux was lower than that of N2O. Evidence suggested that the N2O produced was solely from denit rification which was proportionately greater on soils with a history o f high N applications. Nitrification and denitrification in grassland soils is, therefore, highly influenced by previous management intensit y. (C) 1998 Elsevier Science Ltd. All rights reserved.