Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N <i>2</i> fixation and N balance using soil nitrate at sowing and chickpea yield

Abstract

Functions quantifying relationships between N2 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 N2 fixation are legume yield and soil nitrate and both should be included as independent variables in the functions. Data from 9 experiments in northern New South Wales on soil nitrate at sowing, yields of shoot and grain dry matter (DM) and N of chickpea, and d15N of shoots of chickpea and non N2-fixing reference crops (wheat, barley, and uninoculated chickpea) were used to determine the percentage of chickpea N derived from N2 fixation ( P fix), total N2 fixed, and N balance (fixed N2 minus grain N). Data were then subjected to simple and multivariate regression analyses with Pfix and total N2 fixed as the dependent variables and soil nitrate, shoot N, and grain yield as the independent variables. Simple regression coefficients (r2) for P fix were 0·26 with shoot N as the independent variable, 0·59 with soil nitrate, and 0·62 with grain yield. Coefficient values were increased in the 2-factor (multiple) regressions to 0·74 ( P &amp;lt;0·001) for soil nitrate plus shoot N, and 0·82 ( P &amp;lt; 0·001) for soil nitrate plus grain yield. For total N2 fixed, the regression coefficient was lower at 0·68 ( P &amp;lt; 0·001), using soil nitrate plus grain yield. We tested the functions against an independent data set with best prediction of P fix involving the nitrate and grain yield equation ( r 2 = 0·83; P &amp;lt; 0·001). Total N2 fixed was not well predicted. We concluded that further development of such functions is warranted to refine both accuracy and precision for chickpea and to extend the approach to other species.