Transfer of nitrogen from three pasture legumes under periodic defoliation in a field environment

Subterranean clover (Trifolium subterraneum), white clover (T. repens) and lucerne (Medicago sativa) were each grown in association with cocksfoot (Dactylis glomerata) for three years in field microplots with periodic removal of herbage. The three legumes produced quite different effects on the nitrogen uptake of the grass and the nitrogen content of the underlying soil. The total nitrogen input to the plant-soil system by subterranean clover was only about half that by either of the two perennial legumes. However, subterranean clover continually donated the most nitrogen to the grass. White clover also increased the nitrogen yield of the grass during the second year, but in the third year, severe competition caused a 45 per cent mortality of grass plants and nitrogen transfer was reduced. Both clovers released about 40 per cent of their apparent nitrogen inputs, distributed between the grass and the soil ; thus soil nitrogen under white clover increased by 160 kg N ha-1 year-1. Lucerne put the most nitrogen into the system but produced only transient increases in grass nitrogen yield. Twenty-one per cent of the apparent nitrogen input of lucerne was donated to the soil.