Chickpea in wheat-based cropping systems of northern New South Wales. III. Prediction of N2 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 δ¹⁵N 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 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 increased in the 2-factor (multiple) regressions to 0·74 (P<0·001) for soil nitrate plus shoot N, and 0·82 (P < 0·001) for soil nitrate plus grain yield. For total N₂ fixed, the regression coefficient was lower at 0·68 (P < 0·001), using soil nitrate plus grain yield. We tested the functions against an independent data set with best prediction of Pfix involving the nitrate and grain yield equation (r² = 0·83; P < 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.