Microbial utilization of rice root exudates: 13C labeling and PLFA composition

Abstract

The soluble components of rhizodeposition—root exudates—are the most important sources of readily available carbon (C) for rhizosphere microorganisms. The first steps of exudate utilization by microorganisms define all further flows of root C in the soil, including recycling and stabilization. Nevertheless, most studies have traced root exudates C much later after its initial uptake by microorganisms. To understand microbial uptake and utilization of rice root exudates, we traced 13C incorporated into microbial groups by 13C profiles of phospholipid fatty acids (PLFAs) within a short time (6 h) after 13CO2 pulse labeling. Labeling was conducted five times during three growth stages: active root growth (within the 21 days after transplanting), rapid shoot growth (37 and 45 days), and rapid reproduction (53 and 63 days). 13C was quickly assimilated throughout the rhizosphere microorganism, and the incorporation rate increased with rice maturity. Despite low redox conditions in paddy soil, fungi outcompeted bacteria in utilizing the root exudates. At all growth stages, fungal PLFAs (18:2 w6, 9c/18:0) showed the highest 13C levels, whereas actinomycete PLFAs (16:0 10-methyl) showed the lowest 13C incorporation. Principal component analysis revealed that the rhizosphere microbial community differed among rice growth stages, whereas the whole microbial community remained stable. In conclusion, the rapid incorporation of carbon from root exudates into microorganisms in paddy soils depends on the growth stage of the rice plant and is the first step of C utilization in rice rhizosphere, further defining C utilization and stabilization.