Effect of rotation and inoculation with Bradyrhizobium on nitrogen fixation and yield of peanut (Arachis hypogaea L., cv. Virginia Bunch)

Abstract

The effect of different rotational systems on symbiotic N2 fixation by, and yield of, peanut (Arachis hypogaea L. cv. Virginia Bunch) was determined over two growing seasons in southern Queensland. Estimates of N2fixation were obtained with the 15N natural abundance procedure by using a non-nodulating peanut genotype as a non-N2-fixing reference plant. Three rotational treatments were compared in both years. Crops were rainfed and dependent solely upon indigenous rhizobia for nodulation in the first season, but in the second, supplementary irrigation was supplied and portions of the trial were inoculated with three different rhizobial strains. Dry matter, crop N and pod yield were greatest in both years following a period of grass ley and lowest in a continuous peanut system. However, the proportion of the crop N derived from N2 fixation (PN2) was the same across all rotational and inoculation treatments, despite pre-sowing differences in the levels of soil nitrate and numbers of native rhizobia, and despite seasonal differences in the amounts of N mineralized. Proportions and amounts of N2 fixed were lower (22-31% and 32-57 kg N/ha, respectively) during the first year than under the better moisture conditions and lower concentrations of soil nitrate that prevailed in following season (44-48s and 82-120 kg N/ha). A greater proportion of fixed N was located in the pods and kernels (PN2 62-70%) than in the supporting shoot (PN2 13-21%) in the second year. When estimates of N2 fixation were compared with N removal in the pods at harvest, the net N balance suggested that soil N reserves had been depleted by 27-37 kg N/ha as a result of that season's peanut cropping.