CHICKPEA IN WHEAT-BASED CROPPING SYSTEMS OF NORTHERN NEW-SOUTH-WALES - III - PREDICITON OF N-2 FIXATION AND N BALANCE USING SOIL NITRATE ATSOWING AND CHICKPEA YIELD

Functions quantifying relationships between Na fixation by legumes and other factors would be useful to farmers in the management of legumes and nitrogen in their production systems. The 2 most critical factors regulating N-2 fixation are legume yield and soil nitrate and both sh ould be included as independent variables in the functions. Data from 9 experiments in northern New South Wales on soil nitrate at sowing, y ields of shoot and grain dry matter (DM) and N of chickpea, and delta( 15)N of shoots of chickpea and non N-2-fixing reference crops (wheat, barley, and uninoculated chickpea) were used to determine the percenta ge of chickpea N derived from N-2 fixation (Pfix), total N-2 fixed, an d N balance (fixed N-2 minus grain N). Data were then subjected to sim ple and multivariate regression analyses with Pfix and total N-2 fixed as the dependent variables and soil nitrate, shoot N, and grain yield as the independent variables. Simple regression coefficients (r(2)) f or Pfix were 0.26 with shoot N as the independent variable, 0.59 with soil nitrate, and 0.62 with grain yield. Coefficient values were incre ased in the 2-factor (multiple) regressions to 0.74 (P < 0.001) for so il nitrate plus shoot N, and 0.82 (P < 0.001) for soil nitrate plus gr ain yield. For total N-2 fixed, the regression coefficient was lower a t 0.68 (P < 0.001), using soil nitrate plus grain yield. We tested the functions against an independent data set with best prediction of Pfi x involving the nitrate and grain yield equation (r(2) = 0.83; P < 0.0 01). Total N-2 fixed was not well predicted. We concluded that further development of such functions is warranted to refine both accuracy an d precision for chickpea and to extend the approach to other species.