Microbial community dynamics associated with rhizosphere carbon flow

Abstract

Root-deposited photosynthate (rhizodeposition) is an important source of readily available carbon (C) for microbes in the vicinity of growing roots. Plant nutrient availability is controlled, to a large extent, by the cycling of this and other organic materials through the soil microbial community. Currently, our understanding of microbial community dynamics associated with rhizodeposition is limited. We used a (13)C pulse-chase labeling procedure to examine the incorporation of rhizodeposition into individual phospholipid fatty acids (PLFAs) in the bulk and rhizosphere soils of greenhouse-grown annual ryegrass (Lolium multiflorum Lam. var. Gulf). Labeling took place during a growth stage in transition between active root growth and rapid shoot growth on one set of plants (labeling period 1) and 9 days later during the rapid shoot growth stage on another set of plants (labeling period 2). Temporal differences in microbial community composition were more apparent than spatial differences, with a greater relative abundance of PLFAs from gram-positive organisms (i15:0 and a15:0) in the second labeling period. Although more abundant, gram-positive organisms appeared to be less actively utilizing rhizodeposited C in labeling period 2 than in labeling period 1. Gram-negative bacteria associated with the 16:1omega5 PLFA were more active in utilizing (13)C-labeled rhizodeposits in the second labeling period than in the first labeling period. In both labeling periods, however, the fungal PLFA 18:2omega6,9 was the most highly labeled. These results demonstrate the effectiveness of using (13)C labeling and PLFA analysis to examine the microbial dynamics associated with rhizosphere C cycling by focusing on the members actively involved.

FIG. 1.

Nonmetric multidimensional scaling plots of: mole percentages of PLFAs of rhizosphere and bulk soils sampled days 1 and 8 of each labeling period (a) and weighting factors of individual PLFAs (b). The proportion of variance explained by each axis is indicated in parentheses.

FIG. 2.

Mean δ¹³C values (with standard errors, n = 4) of individual PLFAs extracted from rhizosphere and bulk soils. “r” (rhizosphere) and “b” (bulk) indicate that there is no significant difference from the unlabeled control soils (n = 10). *, significant difference between rhizosphere and bulk δ¹³C values on that day. Within each labeling period, significant time differences (day 1 versus day 8) are designated by “a” for the rhizosphere data and “y” for the bulk soil data.