Effect of Early Season Nitrogen on Nitrogen Fixation in a Ryegrass/ White Clover Pasture

The effect of rate and time of nitrogen fertilizer application on nitrogen fixation by white clover (Trifolium repens) in a ryegrass sward was studied under field conditions in two consecutive years. Very different weather conditions prevailed over the two years resulting in contrasting seasonal patterns of nitrogen fixation. In 1983 only 30% of the total annual fixation rate was achieved by rate June and this pattern was reversed in 1984. Nitrogen fertilizer affected nitrogen fixation more than clover growth in 1983 but in 1984 a combination of fertilizer and drought reduced both clover growth and nitrogen fixation. At the highest level of fertilizer used (100 kg N/ha) the clover retained 70% of its annual fixation capacity under the experimental conditions of this study. Accumulation of nitrate from added fertilizer was greater in grass than in clover and the grass also had a greater leaf nitrate reductase activity. This result was obtained both with mixed swards and monocultures of grass and clover. Rapid depletion following accumulation of tissue nitrate is attributed more to decreased ion uptake at this stage rather than increased metabolism via nitrate reductase.