Root-microbe interactions affect decomposition of complex C more than simple C

Jessica A. M. Moore, Benjamin N. Sulman, Melanie A. Mayes, Courtney M. Patterson, and Aimée T. Classen

Sneha Patel measures soil respiration in root-ingrowth mesocosms at Marcell Experimental Forest. Photo credit: Jessica A. M. Moore
Sneha Patel measures soil respiration in root-ingrowth mesocosms at Marcell Experimental Forest. Photo credit: Jessica A. M. Moore

Roots stimulate microbes to decompose organic matter, but we know little about how roots influence decomposition of different organic matter substrates, especially those with simple versus complex structures. A model that can simulate root-microbe interactions in soil, named the Carbon, Organisms, Rhizosphere, and Protection in the Soil Environment (CORPSE) model, was previously used to simulate total soil carbon (C) decomposition in field experiments in temperate forest. We added to this previous work by forming alternative models to test how roots affect microbes decomposing simple and complex C. We paired this model investigation with a field experiment that manipulated root access to soil. We tested the hypothesis that decomposition of leaves would be more sensitive to root inputs than decomposition of starch, which corresponded to two alternative CORPSE models.  In the field study, leaf decomposition increased with roots present, while starch decomposition was unchanged by roots. Microbial biomass and enzyme activity consistently increased with root inputs in both CORPSE and the field study. Our field experiment supported the CORPSE simulations in the alternative model with high microbial-substrate affinity. Roots stimulated microbial growth and enzyme production, which increased degradation of more complex substrates such as leaf tissues. Substrates that were easily decomposed, such as starch, may already be degrading at a maximum rate in the absence of roots because their decomposition rate was unchanged by root inputs. We found that the degree to which roots stimulate microbial decomposition depends on the substrate being decomposed, and that root-microbe interactions influenced soil organic carbon stocks in both our model and field experiment. Environmental changes that alter root-microbe interactions could, therefore, alter soil C stocks and biogeochemical cycling, and models of these interactions should incorporate differential influence of rhizosphere inputs on different substrates.

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