Roots and mycorrhizae have different effects on microbial decomposition of soil carbon

Background/Question/Methods

Microbes are key players in movement of carbon from soil to atmosphere, one of the most important and least understood fluxes of terrestrial carbon. Understanding drivers of microbial decomposition of organic matter is critical for understanding carbon stabilization and release from soil, and ultimately making predictions about carbon-climate feedbacks. We know that roots increase microbial activity through priming effects, exuding simple sugars that provide microbes with energy to build complex enzymes used to break down organic matter. Microbes are also closely associated with mycorrhizal fungi because they occupy the same space and use similar resources. However, we know little about the effects of mycorrhizae on microbial activity. Mycorrhizae may also prime microbial activity through exudation. In this study we ask how roots and mycorrhizae affect microbial decomposition of organic matter. We expected mycorrhizae to amplify priming effects of roots on microbial activity. To address this question, we experimentally manipulated root and mycorrhizal access to soil cores using mesh of varying opening size. We used a ¹³C-labelled substrate to trace carbon from soil organic matter into microbial biomass and respiration and we measured extracellular enzyme activity.

Results/Conclusions

Roots and mycorrhizae affect microbial activity differently, which has implications for the way we model carbon dynamics. Enzyme activity is two to three times higher when roots and mycorrhizae are either simultaneously excluded or present compared to when roots are excluded and mycorrhizae are present. This finding suggests mycorrhizae may compete with microbes for nutrients or space in the soil matrix and thus inhibit their activity. According to respiration and ¹³CO₂ measurements, we found that microbes would prefer to utilize root and mycorrhizal exudates rather than easily degradable soil organic matter when both (roots and mycorrhizae) are included. Our findings suggest that, in contrast to our expectations, mycorrhizae have different effects on microbes depending on proximity to roots. Mycorrhizae seem to inhibit microbial enzyme activity when roots are not present. In conclusion, roots and mycorrhizae affect microbial activity thereby altering fluxes of carbon from soil to atmosphere and they may be an important biological component currently missing from widely used earth system models.