Nitrogen Immobilisation and Leaching in Pasture Soils

Keywords: Grassland fertilisation, Nitrogen immobilisation, N Mineralisation, Nitrate leaching, Nitrification, Urine N.

Soil factors influencing nitrate leaching were examined as part of an investigation of N fluxes in a recently sown pasture and in an upland permanent pasture. Direct measurement of changes in soil N content demonstrated that the accumulation of N in the soil accounted for the major portion of the N input to the recently sown pasture. The application of artificial urine to field plots in autumn resulted in high concentrations of residual nitrate in the soil at the end of the growing season and most of this N was leached during the winter. Urine applied earlier in the summer left little residual nitrate in the soil and contributed little to leaching losses. Laboratory incubation experiments provided no evidence of net immobilisation of ammonium- or nitrate-N when added to soils at similar concentrations to those expected to occur in urine-affected areas of pasture, even when grass residues were incorporated into the soil. There was evidence that the addition of high concentrations of ammonium ions inhibited nitrification in soil from the permanent pasture. Greater quantities of N were mineralised in soil from this site than in soil from the recently sown pasture.

S P Cuttle and P C Bourne, AFRC Institute of Grassland and Environmental Research (IGER), Plas Gogerddan, Aberystwyth, Dyfed, UK.

31 pages, 6 figures, 5 tables, 29 references.