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Details are presented of a simple mathematical framework that allows 15N tracer experiments to be interpreted in terms of the main processes of the soil/plant nitrogen cycle. The calculations, all of which can be performed on a scientific calculator, yield the rates of gross mineralization and nitrification and the crop nitrogen uptake occurring as ammonium and nitrate. Two procedures are presented. One requires paired experiments with labelled ammonium and unlabelled nitrate as one treatment, and unlabelled ammonium and labelled nitrate as the other. The second procedure requires only the labelled ammonium, unlabelled nitrate treatment. Example calculations are presented using actual experimental data.The interpretative procedure uses the fact that the rate of isotopic dilution in an ammonium pool labelled with 15N is a function of the rate at which unlabelled ammonium is introduced into the pool via mineralization. Similarly, the rate of isotope dilution in an ~SN labelled nitrate pool is a function of the rate at which unlabelled nitrate is introduced into the pool via nitrification.

A 15N labelling technique was used to measure N20 and N 2 emissions from an undisturbed grassland soil treated with cow urine and held at 30 cm water tension and 20°C in a laboratory. Large emissions of dinitrogen were detected immediately following urine application to pasture. These coincided with a rapid and large increase in soil water-soluble carbon levels, some of this increase being attributed to solubilization of soil organic matter by high pH and ammonia concentrations. Emissions of nitrous oxide generally increased with time in contrast to dinitrogen fluxes which decreased as time progressed. Estimated losses of NzO and N 2 over a 30 day period were between 1 to 5% and 30 to 65% of the urine N applied plus N mineralized from soil organic matter, respectively. Most of the N 2 and N20 originated from denitrification with nitrification-denitrification being of minor significance as a source of NzO. Comparisons of the 15N enrichments in the soil mineral N pools and the evolved N 2 0 suggested that much of the N20 was produced in the 5-8 cm zone of the soil. It is concluded that established grassland soils contain large amounts of readily-oxidizable organic carbon which may be used by soil denitrifying organisms when nitrate is non-limiting and soil redox potential is lowered due to high rates of biological activity and high soil moisture contents.

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The effects of urinary chloride and nitrogen concentration and osmotic pressure on the nitrification of ammonium in a calcareous soil treated with cow urine were examined. Urinary chloride concentrations of up to 7.4 g L -I had no effect on the rate of nitrification, as determined by the accumulation of soil nitrate. Osmotic stress, generated using a mixed salt solution, had an inhibitory effect on nitrification at soil osmotic pressures lower than or equal to -1.0 PMa. Nitrification was completely inhibited at a soil osmotic pressure of -2.6 MPa. Accumulation of nitrate after a lag phase of 18 days was noted in the -2.0 MPa soil osmotic pressure treatment, indicating some degree of adaptation or osmo-regulation within the nitrifying population at this stress level. High urine-N concentrations resulted in considerable nitrite accumulations and reduced nitrification activity through the effect of free ammonia. It is concluded that in most temperate grassland soils at near-neutral pH, urinary chloride and nitrogen are unlikely to reduce nitrification rates, except where urine-N concentrations exceed 16 g N L-~. Inhibition due to osmotic stress will be directly related to soil moisture status and may be particularly severe in dry, light-textured soils.

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9New and emerging policies that aim to set standards for protection and sustainable use of soil 10 are likely to require identification of geographical risk/ priority areas. Soil degradation can be 11 seen as the change or disturbance in soil quality and it is therefore crucial that soil and soil 22This method to systematically prioritise areas where detailed investigation is required, using a 23 ranking based against a desired level of action, could be relatively quick, easy and cost 24 effective. As such this has potential to fill in gaps and compliment existing monitoring programs 25 and assist in development and implementation of current and future soil protection legislation. 26