Linking microbial community dynamics to rhizosphere carbon flow in a wetland rice soil

Photosynthesis by terrestrial vegetation is the driving force of carbon cycling between soil and the atmosphere. The soil microbiota, the decomposers of organic matter, is the second player carrying out carbon cycling. Numerous efforts have been made to quantify rhizodeposition and soil respiration to understand and predict the carbon cycling between the soil and atmosphere. However, there have been few attempts to link directly the soil microbial community to plant photosynthesis. We carried out a pulse-chase labeling experiment in a wetland rice system in which rice plants of various ages were labeled with (13)CO(2) for 6 h and the distribution of the assimilated (13)C to soil microorganisms was estimated by analyzing the (13)C profile of microbial phospholipid fatty acids (PLFAs). The results showed that total PLFA increased with plant growth, indicating an increase of microbial biomass. But the mono-unsaturated PLFAs increased faster than the branched chain fatty acids. The (13)C was incorporated into PLFAs immediately after the plant (13)CO(2) assimilation, proving the tight coupling of microbial activity to plant photosynthesis. In line with the finding of seasonal change in total PLFAs, more of (13)C was distributed to the straight chain fatty acids (16:0, 16:1omega7, 18:1omega7 and 18:1omega9) than to the branched chain fatty acids. The total plant carbon incorporation estimated from (13)C labeling roughly corresponded to the increase in total PLFAs over the growing season of plants. Our study suggests that microbial populations in rice soil differ greatly in their responses to plant photosynthate input.