Sustaining productivity of a Vertisol at Warra, Queensland, with fertilisers, no-tillage or legumes 4. Nitrogen fixation, water use and yield of chickpea

Continuous cereal cropping in southern Queensland and northern New South Wales has depleted native soil nitrogen fertility to a level where corrective strategies are required to sustain grain yields and high protein content. The objective of this study was to examine the performance of chickpea in chickpea-wheat rotations in terms of yields, water use and N fixation. The effects of sowing time and tillage practice have been studied. Chickpea grain yields varied from 356 kg/ha in 1995 to 2361 kg/ha in 1988; these were significantly correlated with the total rainfall received during the preceding fallow period and crop growth. Almost 48% of total plant production and 30% of total plant nitrogen were below-ground as root biomass. Mean values of water-use efficiency for grain above-ground dry matter, and total dry matter were 5.9 14.2 and 29.2 kg/ha.mm, respectively. The water-use efficiency for grain was positively correlated with the total rainfall for the preceding fallow and crop growth period although cultural practices modified, water-use efficiency. The potential N fixation was estimated to be 0.6 kg nitrogen/ha.mm from 1992 total dry matter nitrogen yields assuming all of the nitrogen contained in chickpea was derived from the atmosphere. Sowing time had a much larger effect on grain yield and N fixation by chickpea than tillage practice (conventional tillage and zero tillage) although zero tillage generally increased grain yields. The late May-early June sowing time was found to be the best for chickpea grain yield and N fixation since it optimised solar energy use and water use, and minimiser frost damage. Nitrogen fixation by chickpea was low, less than 40% nitrogen was derived from atmosphere, representing less than 20 kg nitrogen/ha.year. The potential for N fixation was not attained during this period due to below-average rainfall and high soil NO-N accumulation because of poor utilisation by the preceding wheat crop. Increased soil NO-N due to residual from fertiliser N applied to the preceding wheat crop further reduced N fixation. A simple soil nitrogen balance indicated that at least 60% of crop nitrogen must be obtained from N fixation to avoid continued soil nitrogen loss. This did not occur in most years. The generally negative soil nitrogen balance needs to be reversed if chickpea is to be useful in sustainable cropping systems although it is an attractive cash crop. Sowing time and zero tillage practice, possibly combined with more appropriate cultivars, to enhance chickpea biomass, along with low initial soil NO-N levels, would provide maximum N fixation