Response of tree legumes to rhizobial inoculation in relation to the population density of indigenous rhizobia

Abstract

To determine the relationship between leguminous tree yield response to inoculation and indigenous rhizobial population density, an inoculation experiment was conducted in pots using four soils and six tree species: Acacia auriculiformis A. Cunn. ex Benth., A. mangium Willd., A. mearnsii De Wild., Leucaena diversifolia (Schlecht.) Benth., Robinia pseudoacacia L. and Sesbania grandiflora Poir. Densities of indigenous rhizobia were determined by most probable number (MPN) plant-infection assays. Statistically significant increases in shoot N due to inoculation were observed most frequently in soils with <50 rhizobia g −1 soil. Inoculation resulted in significant increases in shoot N ( P < 0.05) for R. pseudoacacia and A. meamsii in three and one soils, respectively, despite the presence of >1000 rhizobia g −1 soil. A hyperbolic model best described the relationship between response to inoculation and the density of indigenous rhizobia. Incorporating an index of available soil N into the hyperbolic model reduced residual mean square values, indicating that available mineral N attenuates the response to inoculation.