RELEASE OF NITROGEN FROM PLANT AND ANIMAL RESIDUES AND CONSEQUENT PLANT UPTAKE EFFICIENCY

Plant N uptake and mineralisation were studied in a pot experiment using two soils of contrasting textures with a range of N-15-labelled manures: poultry manure, cattle slurry, fresh swine manure, composted cattle manure, sewage sludge, straw, cabbage residues, pea residues and grass-clover turf. A range of manure and soil properties were determined before the incubations to examine if any combination of these properties would be useful to predict the amount and timing of N supply. Three replicate pots with and without ryegrass were used. Soil mineral nitrogen and ryegrass yield, N uptake and N-15 enrichment were measured throughout the 16 week incubation. The total amount of mineral N produced in pots in the absence of ryegrass ranged from 46 to 165 mg N kg(-1). Nitrogen uptake by the ryegrass ranged from 17 to 89 mg N kg(-1). Stepwise regression procedures identified the most important of the measured factors for predicting N release from manures: NO3- concentrations of the soil-manure mixtures at the beginning of the incubations; C:N ratio and the N concentration of the manure. However, none of the chemical indices tested were found to be valuable for the prediction of N release and no acceptable model could be produced. Our understanding of the biological processes occurring in soil needs to be much improved, before we are able to model adequately the N release from manures and the consequent uptake efficiency of this N for crop growth.