THE NITROGEN ECONOMY OF BROADACRE LUPIN IN SOUTHWEST AUSTRALIA

The time courses of above- and below-ground accumulation of biomass and N were followed in a crop of narrowleaf lupin (Lupinus angustifolius L. cv. Illyarrie) at Geraldton, W.A., and concurrent N-2 fixation assessed using the N-15 natural abundance technique. Crop biomass peaked at 10 t DM and 231 kg N ha(-1) with 13% of this N below ground. The crop accumulated the bulk (90 degrees) of its N through symbiotic N-2 fixation. Of the 164 kg total plant N ha(-1) remaining in recoverable biomass at maturity 44 degrees was recovered as grain, 49 degrees as other above-ground residues and 7% as roots. Despite a decrease in recoverable N of 67 kg ha(-1) between peak biomass and maturity, 96 kg N ha(-1) was returned as crop residues after grain harvest. Investigation of six farm crops in the study region gave values for nitrogen accumulation at peak biomass ranging from 199 to 372 kg ha(-1) of which, on average, 86% (222 kg ha(-1)) was fixed from the atmosphere. Predicted N returns to the soil from fixation averaged 65 kg ha(-1), though the range (32-96 kg ha(-1)) indicated that south-west Australian lupin crops provide somewhat variably sized pools of mineralizeable crop residues for following cereal growth.