CHICKPEA INCREASES SOIL-N FERTILITY IN CEREAL SYSTEMS THROUGH NITRATESPARING AND N-2 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 whea t, production. Annual crop legumes, grown in rotation with cereal crop s, can contribute to the total pool of N in the soil and improve yield s of the cereals. Experiments, in 1989 and 1990 at two sites near Nort h Star, N.S.W., are described which aim to (i) assess the effects of c hickpea on concentrations of soil nitrate, both in terms of ''N sparin g'' during legume growth and release of N bound in the crop residues; (ii) quantify N-2 fixation by chickpea using the natural N-15 abundanc e and modified N different techniques; and (iii) explore the concept o f water use efficiency (WUE) of N-2 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 tr eatment. There was general agreement between the natural N-15 abundanc e and modified N difference methods. Soil nitrate spared by chickpea r anged 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 Ma y sampling in the following year, i.e. after the summer and autumn fal low, differences in soil nitrate-N between the chickpea and wheat plot s 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 N-2 fi xation varied between 0.14 and 0.24 kg ha(-1) mm(-1), with the higher values associated with higher N-2 fixation.