N2 fixation by faba bean (Vicia faba L.) in a gypsum-amended sodic soil

The response of faba bean to the application of four rates of gypsum (0, 2.5, 5.0, 10.0 t ha(-1)) to a non-saline, alkaline sodic soil was measured in terms of grain yield, dry matter (DM) production, N accumulation and the proportional dependence of the legume on symbiotic N-2 fixation (P (atm)). A yield-independent, time-integrated N-15-dilution model was used to estimate symbiotic dependence. A significant decrease in the exchangeable sodium percentage and significant increases in exchangeable Ca++ and the Ca++:Mg++ ratio in the 0-10-cm soil layer were measured 30 months after application of 10 t ha(-1) gypsum. Despite low and erratic rainfall during crop growth, faba bean DM and N uptake responded positively to gypsum application. The symbiotic dependence of the legume at physiological maturity was little affected by sodicity (P (atm) = 0.74 at zero gypsum and 0.81-0.82 at 2.5-10 t ha(-1) gypsum). The increase in fixed N due to gypsum application was mainly due to increases in legume DM and total N uptake. At 10 t ha(-1) of gypsum, faba bean fixed more than 200 kg N ha(-1) in above-ground biomass.