Effect of rotation breaks on the diversity of bacteria in the rhizosphere of sugarcane and its potential impact on yield decline

A possible role of soil bacteria in yield decline was investigated by comparing the diversity of bacterial populations in the rhizosphere of sugarcane growing in soil from the Burdekin that had been in continuous cane with minimal breaks for more than 20 years and soil that had been taken out of cane and rotated to pasture and bare fallow for 3.5 years. Both the pasture and bare fallow breaks gave a significant increase in cane yield compared to that obtained in continuous cane soiL Isolates from 25 different bacterial genera were identified. The diversity of bacteria in the rhizosphere of cane growing in the continuous cane soil was greater than that in the rhizosphere of cane growing in soil that had been under pasture or bare fallow. Several bacterial genera including two, Acidovorax and Clavibacter, which contain known plant pathogens, were present in the continuous cane rhizosphere but absent from the rhizosphere of cane growing in the pasture and bare fallow soils. Compared to the rhizosphere of cane in the continuous cane soil, the rhizosphere of cane growing in the pasture and bare fallow soils contained higher numbers of bacteria (notably Pseudomonas spp.) that were inhibitory to the growth of Pachymetra chaunorhiza and Pythium graminicola in a laboratory bioassay. The results suggest that one possible mechanism by which rotation breaks reduce the impact of yield decline is by encouraging an increase in potentially beneficial bacteria in the rhizosphere.