NITROGEN-FIXATION IN CHICKPEA .1. INFLUENCE OF PRIOR CROPPING OR FALLOW, NITROGEN-FERTILIZER AND TILLAGE

N fixation was measured in chickpea in a field experiment on the Darling Downs, Queensland. The N-15 natural abundance method was used with barley as a non-fixing control crop. Chickpea was grown with 0, 50 and 100 kg/ha of applied N after pretreatments of either fallow or grain sorghum where sorghum stubble was either incorporated by cultivation, removed or retained on the surface with zero tillage. When chickpea was grown after sorghum both the percentage and amount of N in the tops derived from fixation were larger than after fallow. N fertilizer additions reduced per cent but not total N2 fixation. Stubble management had no influence on total N2 fixed. There was a close inverse relationship (R2 = 0.95) between soil nitrate measured at establishment of chickpea and per cent of N derived from fixation. All treatment differences in per cent N2 fixation whether due to prior cropping, fallowing, N fertilizer or tillage could be explained through treatment influences on soil nitrate. Chickpea that had a plentiful supply of soil mineral N for growth accumulated significantly more plant N than plants dependent on N2 fixation. Chickpea grain yields were, however, unaffected by the source or quantity of plant N accumulated. Chickpea provided a positive soil N balance at high fixation rates and a negative balance at low fixation rates.