Chickpea increases soil-N fertility in cereal systems through nitrate sparing and N2 fixation

There is a need to introduce cropping practices in the northern N.S.W. cereal belt that increase N supply for cereal, and in particular wheat, production. Annual crop legumes, grown in rotation with cereal crops, can contribute to the total pool of N in the soil and improve yields of the cereals. Experiments, in 1989 and 1990 at two sites near North Star, N.S.W., are described which aim to (i) assess the effects of chickpea on concentrations of soil nitrate, both in terms of “N sparing” during legume growth and release of N bound in the crop residues; (ii) quantify N2 fixation by chickpea using the natural15N abundance and modified N different techniques; and (iii) explore the concept of water use efficiency (WUE) of N2 fixation for use in the management of N in cropping systems. The two sites and three rates of fertilizer N (0, 50 and 100 kg ha−1) provided six combinations of site, season and nitrate-N fertility. Nitrogen fixed ranged from 29 to 85 kg ha−1, with amounts influenced by method of assessment and fertilizer N treatment. There was general agreement between the natural 15N abundance and modified N difference methods. Soil nitrate spared by chickpea ranged from 6 to 31 kg N ha−1, over sites and treatments, and averaged 11 kg N ha−1 at Windridge and 26 kg N ha−1 at Glenhoma. In the May sampling in the following year, i.e. after the summer and autumn fallow, differences in soil nitrate-N between the chickpea and wheat plots ranged from 29 to 51 kg ha−1, and averaged 44 kg ha−1 (Windridge) and 43 kg ha−1 (Glenhoma). Water use efficiency of chickpea N2 fixation varied between 0.14 and 0.24 kg ha−1 mm−1, with the higher values associated with higher N2 fixation.