Effects of roots and mycorrhizal fungi on soil carbon decomposition depend on soil mineralogy
Microbial communities are integral to decomposition and carbon release from soil to atmosphere, yet we still have a poor understanding of what drives microbial processes. Because soils are the largest terrestrial carbon sink, a change in soil decomposition rates can influence the global carbon budget and climate-carbon feedbacks. The relative influence of roots and mycorrhizal fungi on microbes remains uncertain. For example, roots could facilitate microbial decomposition by exuding easily degraded substrates, while mycorrhizal fungi could compete with microbes for moisture and nutrients, thereby slowing microbial activity and thus decomposition. Furthermore, clay-rich soils slow decomposition by physically protecting carbon, which may overwhelm biological processes. We investigated effects of roots and mycorrhizal fungi on ecosystem processes mediated by microbes by assessing rates of microbial enzyme activity, soil respiration, and stocks of soil carbon. We predicted that roots would stimulate microbial decomposition rates due to exudation while mycorrhizae would slow decomposition due to competition for nutrients, and these processes would be more apparent in soils with low clay content. Using a progressively exclusive in situ study design, we performed regression analyses between biomass (root, mycorrhizal, microbial) and processes involved in decomposition of soil carbon.
We found that root biomass was positively correlated with microbial extra-cellular enzymes involved in cellulose (alpha-glucosidase, p < 0.001, r2 = 0.24; beta-glucosidase, p < 0.001, r2 = 0.32; cellobiohydrolase, p < 0.001, r2 = 0.21), but not lignin decomposition (peroxidase, p > 0.05; phenol oxidase, p > 0.05) at our low soil clay content site. These results indicate roots stimulate microbial activity, consistent with the positive correlation we found between root biomass and soil respiration at the low soil clay content site (p = 0.04, r2 = 0.04). However, the effect of roots on microbial activity is negligible (p > 0.05) at our site with high soil clay content. This is likely because exudates from roots become bound to soil particles before microbes can metabolize the exudates and benefit from the pulse of labile carbon. Interestingly, mycorrhizal biomass was not correlated with any measured enzyme activities or soil respiration, indicating that roots have a stronger effect on microbes than mycorrhizal fungi do via either positive or negative interactions. Since roots heterogeneously occupy the soil matrix, the stimulating effect roots have on microbial activity could explain some of the observed spatial variation in decomposition of soil carbon.