Compositional differences in simulated root exudates elicit a limited functional and compositional response in soil microbial communities.

Michael S Strickland,Rebecca L Mcculley,Jim A Nelson,Mark A Bradford

doi:10.3389/fmicb.2015.00817

Michael S Strickland, Rebecca L Mcculley + Show 2 more

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https://doi.org/10.3389/fmicb.2015.00817

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Abstract

Inputs of low molecular weight carbon (LMW-C) to soil – primarily via root exudates– are expected to be a major driver of microbial activity and source of stable soil organic carbon. It is expected that variation in the type and composition of LMW-C entering soil will influence microbial community composition and function. If this is the case then short-term changes in LMW-C inputs may alter processes regulated by these communities. To determine if change in the composition of LMW-C inputs influences microbial community function and composition, we conducted a 90 day microcosm experiment whereby soils sourced from three different land covers (meadows, deciduous forests, and white pine stands) were amended, at low concentrations, with one of eight simulated root exudate treatments. Treatments included no addition of LMW-C, and the full factorial combination of glucose, glycine, and oxalic acid. After 90 days, we conducted a functional response assay and determined microbial composition via phospholipid fatty acid analysis. Whereas we noted a statistically significant effect of exudate treatments, this only accounted for ∼3% of the variation observed in function. In comparison, land cover and site explained ∼46 and ∼41% of the variation, respectively. This suggests that exudate composition has little influence on function compared to site/land cover specific factors. Supporting the finding that exudate effects were minor, we found that an absence of LMW-C elicited the greatest difference in function compared to those treatments receiving any LMW-C. Additionally, exudate treatments did not alter microbial community composition and observable differences were instead due to land cover. These results confirm the strong effects of land cover/site legacies on soil microbial communities. In contrast, short-term changes in exudate composition, at meaningful concentrations, may have little impact on microbial function and composition.

Highlights

One of the major ways that plants interact with soil microbial communities is via the exudation of low molecular weight carbon (LMW-C) compounds, composed primarily of sugars, amino acids, and organic acids (Grayston et al, 1997; Yang and Janssen, 2002; van Hees et al, 2005; Boddy et al, 2007; Phillips et al, 2011)
We address two questions: (1) What is the relative effect of contemporary additions of combinations of specific LMW-C compounds versus land-use legacies on soil microbial community composition and structure; and (2) Do specific land-use legacies, if present, mediate the effect of LMW-C compounds on soil microbial community composition and structure?
For significant land cover effects (F2,42 = 3.4; P < 0.05), we found that the functional differences tended to be driven by greater proportional mineralization of glucose for the forest soils, glycine for the meadow soils, and oxalic acid for the pine soils (Figure 2A)

Summary

Introduction

One of the major ways that plants interact with soil microbial communities is via the exudation of low molecular weight carbon (LMW-C) compounds, composed primarily of sugars, amino acids, and organic acids (Grayston et al, 1997; Yang and Janssen, 2002; van Hees et al, 2005; Boddy et al, 2007; Phillips et al, 2011). The exudation of sugars may increase microbial activity in soil leading to increased plant available nitrogen (N; Drake et al, 2011; Phillips et al, 2011), organic acid exudation may solubilise phosphorus or chelate heavy metals (Strom et al, 2002; Bais et al, 2006; Nguyen, 2009; Marschner et al, 2011; Keiluweit et al, 2015), and exudation may counteract the mineral protection of soil C (Keiluweit et al, 2015) While all of these compounds play a role from the plant perspective, they influence ecosystem processes, the surrounding soil, and soil communities (Bertin et al, 2003; Bronick and Lal, 2005; Dennis et al, 2010; Rukshana et al, 2011; Bais, 2012). Root exudates have been shown to alter the composition of soil microbial communities and may alter the function of these communities as well (Hanson et al, 2008; Chapin et al, 2009; Eilers et al, 2010; Shi et al, 2011)

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