NITROGEN ACCUMULATION IN SOLE AND MIXED STANDS OF SWEET-BLUE LUPIN (LUPINUS-ANGUSTIFOLIUS L), RYEGRASS AND OATS

Nitrogen fixation was measured in monocropped sweet-blue lupin (Lupinus angustifolius), lupin intercropped with two ryegrass (Lolium multiflorum) cultivars or with oats (Avena sativa) on an Andosol soil, using the N-15 isotope dilution method. At 117 days after planting and at a mean temperature below 10-degrees-C, monocropped lupin derived an average of 92% or 195 kg N ha-1 of its N from N2 fixation. Intercropping lupin with cereals increased (p<0.05) the percentage of N derived from atmospheric N2 (% Ndfa) to a mean of 96%. Compared to the monocropped, total N fixed per hectare in intercropped lupin declined approximately 50%, in line with the decrease in seeding rate and dry matter yield. With these high values of N2 fixation, selection of thc reference crop was not a problem; all the cereals, inter cropped or grown singly produced similar estimates of N2 fixed in lupin. It was deduced from the N-15 data that significant N transfer occurred from lupin to intercropped Italian ryegrass but not to intercropped Westerwoldian ryegrass or to oats. Doubling the N-15 fertilizer rate from 30 to 60 kg N ha-1 decreased % Ndfa to 86% (p<0.05), but total N fixed was unaltered. These results indicate that lupin has a high potential for N2 fixation at low temperatures, and can maintain higher rates of N2 fixation in soils of high N than many other forage and pasture legumes.