Plant-specific effects of sunn hemp (Crotalaria juncea) and sudex (Sorghum bicolor × Sorghum bicolor var. sudanense) on the abundance and composition of soil microbial community

We investigated the effect of sunn hemp (leguminous) and sudex (non-leguminous) on the abundance and composition of microbial community in soil with a low nutrient content for two years. Abundance of microbial groups was determined by phospholipid fatty acid (PLFA) analysis. Pyrosequencing analysis was used to compare the composition of bacterial communities in the rhizosphere with that in the bulk soil of the unplanted control. The concentration of microbial PLFA increased during cultivation period and decreased after incorporation of plant residues, which implies that the responses depended on living plants. Overall, the bacterial PLFA concentration was lower with sunn hemp than sudex, despite the above-ground biomass was about five times greater with the former than with the latter crop. This indicates that the increase in bacterial PLFA with sudex is plant-specific. Pyrosequencing analysis revealed that the abundance of Arthrobacter, Bacillus, and Sphingomonas were the highest in the bulk soil and decreased in the rhizospheres of the cover plants. The proportions of Gaiella and the nitrogen fixing bacteria Mesorhizobium increased with sunn hemp, while those of cyanobacteria, including Calothrix, Leptolyngbya, and Nostoc, increased greatly with sudex. Our results indicated that the high level of bacterial PLFA with sudex could be primarily explained by the colonization of cyanobacteria. We show that the impacts of cover plants on the abundance and composition of the microbial communities depend more on the plant species than on the aboveground plant biomass, both during growth and incorporation stages. Our results also suggest that non-leguminous as well as leguminous plants harbor beneficial microbes such as nitrogen-fixing bacteria to cope with nutrient-poor environments.