Spatial distribution of root activity and nitrogen fixation in sorghum/pigeonpea intercropping on an Indian alfisol

A medium-duration pigeonpea cultivar (ICP 1-6) and a hybrid sorghum (CSH 5) were grown on a shallow Alfisol in monocropping and intercropping systems. Using a monolith method, spatial distribution of nodulation, acetylene reduction activity (ARA) and root respiration were measured. The number, mass and ARA of nodules decreased exponentially with distance from the plant base except at the late reproductive stage. Nodulation and ARA tended to be higher in the intercrop than in the monocrop. Respiration rate of roots increased with distance from the plant base and reached a maximum value at about 20-30 cm. The rate was higher in pigeonpea than in sorghum and also higher in intercrop than in monocrop. This study suggests that pigeonpea roots are physiologically more active than sorghum roots, implying that pigeonpea may become a strong competitor for nutrients in the soil when intercropped. The nitrogen-fixing ability of pigeonpea may be enhanced by intercropping because the sorghum rapidly absorbed inorganic N which would otherwise inhibit N₂ fixation.