Overestimation of gross N transformation rates in grassland soils due to non-uniform exploitation of applied and native pools

The study tested the validity of some of the assumptions in the N-15 pool dilution technique in short-term soil incubations. Microbial N transformation rates were calculated using N-15 pool dilution during 24 h in four grassland soils in April 1998. The change in concentration and enrichment of the NH4+-N and NO3--N pools was determined at 0, 1.5, 4, 10, 16 and 24 h following application of differentially N-15 labelled NH4NO3 in solution at a rate of either 2 or 15 mg N kg(-1) oven-dry soil and at an enrichment of 99.8 atom% excess. Rapid N-15 pool dilution occurred in all soils. Rates of gross mineralisation and NH4+ consumption were not constant during the 24 h incubation in contrast to nitrification rates. An application of 15 mg N kg(-1) decreased gross mineralisation and NO3- consumption and increased nitrification rates compared to an application of 2 mg N kg(-1). Applied (NH4+)-N-15 was rapidly nitrified with up to 55% of the added label recovered as (NO3-)-N-15 after 24 h. This rapid conversion of (NH4+)-N-15 to (NO3-)-N-15 occurred without a proportional and concurrent increase in the size of the unlabelled NO3- pool. Gross and net nitrification rates were significantly different due to (NO3-)-N-15 consumption. The results suggest that there was non-uniform exploitation of the N-14 and N-15 pools by soil microorganisms, invalidating one of the key assumptions in the N-15 pool dilution technique. Preferential consumption of applied NH4+ and NO3- led to an overestimate of gross mineralisation and nitrification rates due to the greater rate of decline of the N-15 enrichment of the added N pool. In future studies care should be taken to ensure that gross N transformation rates are not altered by the method used to quantify them. (C) 2000 Elsevier Science Ltd. All rights reserved.