Long-term effects of multiple applications of nitrogen fertiliser on grazed dryland perennial ryegrass/white clover dairy pastures in south-west Victoria. 1. Nitrogen fixation by white clover

Abstract

A 3-year experiment determined the impact of multiple applications of different rates of nitrogen (N) fertiliser, applied over autumn and winter in 1997, 1998, and 1999, on N₂ fixation in grazed dryland perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) dairy pastures. Four treatments replicated 3 times in a randomised block design comprised: 0 N (A); 3 applications of 25 kg N/ha (B); 3 applications of 50 kg N/ha (C); and 3 applications of 75 kg N/ha (D). Urea (46% N) was the N source and applications were from autumn to late winter. ¹⁵N abundance methods, based on herbage samples, were used to estimate N₂ fixation. Percentage N derived from the atmosphere (%Ndfa) through white clover N₂ fixation decreased as the rate of N fertiliser application increased during 1997, but remained unaffected by treatment during 1998 and 1999. Slight decreases in %Ndfa, associated with an increase in soil nitrate, were offset by the positive association between clover dry matter (DM) and soil ammonium. The amount of N₂ fixed by white clover decreased from 43 kg N/ha.year (A) to 23 kg N/ha.year (C) during 1997. The amount of N₂ fixed by white clover during 1998 and 1999, however, was unaffected by increasing rates of N fertiliser application and averaged 17 and 12 kg N/ha.year in 1998 and 1999, respectively. White clover sward composition (percentage of DM) was unaffected by the rate of N fertiliser application. The average white clover composition across treatments declined from 23% DM in 1997 to 12% DM in 1999. It is concluded that the DM yield production of dryland perennial ryegrass/white clover dairy pastures is likely to be restricted by available soil N, as the amount of N₂ fixed annually by clover is low (approx. 20 kg N/ha.year). There is, therefore, scope to supplement pasture N nutritional requirements with fertiliser N. Further, application of N fertiliser may restrict the ability of white clover to fix N₂; however, under the grazing management adopted, N fertiliser had no effect on the percentage of white clover.