Evidence of carbon stimulated N transformations in grassland soil after slurry application

High nitrification rates which convert ammonium (NH4+) to the mobile ions NO2− and NO3− are of high ecological significance because they increase the potential for N losses via leaching and denitrification. Nitrification can be performed by chemoautotrophic or heterotrophic organisms and heterotrophic nitrifiers can oxidise either mineral (NH4+) or organic N. Selective nitrification inhibitors and 15N tracer studies have been used in an attempt to separate heterotrophic and autotrophic nitrification. In a laboratory study we determined the effect of cattle slurry on the oxidation of mineral NH4+-N and organic-N by labelling the NH4+ or NO3− pools separately or both together with 15N. The size and enrichment of the mineral N pools were determined at intervals. To calculate gross N transformation rates a 15N tracing model was developed. This model consists of the three N-pools NH4+, NO3− and organic N. Sub-models for decomposition of degradable carbon in the soil and the slurry were added to the model and linked to the N transformation rates. The model was set up in the software ModelMaker which contains non-linear optimization routines to determine model parameters. The application of cattle slurry increased the rate of nitrifcation by a factor of 20 compared with the control. The size and enrichment of the mineral N pools provided evidence that nitrification was due to the conversion of NH4+ to NO3− and not the conversion of organic N to NO3−. There was evidence that slurry-enhanced oxidation of NH4+ to NO3− was due to a combination of autotrophic and heterotrophic transformations. Slurry application increased the mineralisation rate by approximately a factor of two compared with the control and the rate of immobilisation of NH4+ by approximately a factor of three.