THE NITROGEN ECONOMY OF BROADACRE LUPIN IN SOUTHWEST AUSTRALIA

The time courses of above- and below-ground accumulation of biomass an d N were followed in a crop of narrowleaf lupin (Lupinus angustifolius L. cv. Illyarrie) at Geraldton, W.A., and concurrent N-2 fixation ass essed 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 cr op accumulated the bulk (90 degrees) of its N through symbiotic N-2 fi xation. Of the 164 kg total plant N ha(-1) remaining in recoverable bi omass at maturity 44 degrees was recovered as grain, 49 degrees as oth er above-ground residues and 7% as roots. Despite a decrease in recove rable 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 o f six farm crops in the study region gave values for nitrogen accumula tion at peak biomass ranging from 199 to 372 kg ha(-1) of which, on av erage, 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 ra nge (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.