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.