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 15N pool dilution technique in short-term soil incubations. Microbial N transformation rates were calculated using 15N 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 15N 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 15N 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 15NH4+ was rapidly nitrified with up to 55% of the added label recovered as 15NO3− after 24 h. This rapid conversion of 15NH4+ to 15NO3− 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 15NO3− consumption. The results suggest that there was non-uniform exploitation of the 14N and 15N pools by soil microorganisms, invalidating one of the key assumptions in the 15N 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 15N 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.