SOME CHEMICAL AND PHYSICAL FACTORS AFFECTING THE RATE AND DYNAMICS OF NITRIFICATION IN URINE-AFFECTED SOIL

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-1 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-1. Inhibition due to osmotic stress will be directly related to soil moisture status and may be particularly severe in dry, light-textured soils.