SUSTAINING PRODUCTIVITY OF A VERTISOL AT WARRA, QUEENSLAND, WITH FERTILIZERS, NO-TILLAGE OR LEGUMES .4. NITROGEN-FIXATION, WATER-USE AND YIELD OF CHICKPEA

Continuous cereal cropping in southern Queensland and northern New Sou th Wales has depleted native soil nitrogen fertility to a level where corrective strategies are required to sustain grain yields and high pr otein content. The objective of this study was to examine the performa nce of chickpea in chickpea-wheat rotations in terms of yields, water use and N-2 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 t otal rainfall received during the preceding fallow period and crop gro wth. Almost 48% of total plant production and 30% of total plant nitro gen were below-ground as root biomass. Mean values of water-use effici ency 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 g rain was positively correlated with the total rainfall for the precedi ng fallow and crop growth period although cultural practices modified water-use efficiency. The potential N-2 fixation was estimated to be 0 .6 kg nitrogen/ha.mm from 1992 total dry matter nitrogen yields assumi ng all of the nitrogen contained in chickpea was derived from the atmo sphere. Sowing time had a much larger effect on grain yield and N-2 fi xation by chickpea than tillage practice (conventional tillage and zer o 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-2 fixation since it optimised solar energy use and water use, and minimised frost damage. Nitrogen fixation by chickpea w as low, less than 40% nitrogen was derived from atmosphere, representi ng less than 20 kg nitrogen/ha.year. The potential for N-2 fixation wa s not attained during this period due to below-average rainfall and hi gh soil NO3-N accumulation because of poor utilisation by the precedin g wheat crop. Increased soil NO3-N due to residual from fertiliser N a pplied to the preceding wheat crop further reduced N-2 fixation. A sim ple soil nitrogen balance indicated that at least 60% of crop nitrogen must be obtained from N-2 fixation to avoid continued soil nitrogen l oss. This did not occur in most years. The generally negative soil nit rogen balance needs to be reversed if chickpea is to be useful in sust ainable cropping systems although it is an attractive cash crop. Sowin g time and zero tillage practice, possibly combined with more appropri ate cultivars, to enhance chickpea biomass, along with low initial soi l NO3-N levels, would provide maximum N-2 fixation.