Soil nitrogen and water dynamics in crops following perennial pastures under drought conditions

Perennial pasture phases may increase the sustainability of cropping rotations by increasing out of season water use, reducing leakage to the water table, and improving ground cover over the summer months. The dominant perennial pasture species used is lucerne (Medicago sativa). The perennial grasses phalaris (Phalaris aquatica) and cocksfoot (Dactylis glomerata) are broadly adapted but have been studied less extensively in cropping rotations. The effects of lucerne, phalaris, cocksfoot and mixtures of these species, on soil mineral N and water was studied in the field at Junee Reefs in southern NSW (530 mm annual rainfall) by deep coring at sowing and harvest. Potential N mineralisation was assessed by incubation studies in the laboratory.During three dry seasons (2001-3) soil water was very low for most treatments, having been depleted prior to the beginning of the experimental period. Nitrogen mineralisation dynamics were strongly influenced by residue decomposition, explained by time of pasture removal and summer rainfall. In 2001 the rate of N mineralisation was significantly higher following pastures containing cocksfoot. In following seasons substantial amounts of N were mineralised following all perennial pastures. The rates were not significantly different. There was little correlation between potential mineralisation measured in the laboratory and actual mineralisation measured in the field. Drought conditions prevented soil N mineralisation over summer. In-crop mineralisation and crop N accumulation was high after lucerne, leading to haying-off of wheat crops, but perennial grass pastures provided sufficient mineral N to following wheat crops under drought conditions.