Plant roots stimulate the decomposition of complex, but not simple, soil carbon

Abstract

Roots release carbon into soil and can alleviate energy limitation of microbial organic matter decomposition. We know little about the effects of roots on microbial decomposition of different organic matter substrates, despite the importance for soil carbon stocks and turnover. Through implementing root–microbe interactions, the Carbon, Organisms, Rhizosphere and Protection in the Soil Environment (CORPSE) model was previously shown to represent dynamics of total soil carbon in temperate forest field experiments. However, the model permits alternative hypotheses concerning microbial‐substrate affinity. We investigated how root inputs affect decomposition of soil organic carbon (SOC) with variable decomposability. We simulated SOC stocks in CORPSE and compared microbial degradation of two substrate types with varying root–microbe interactions under two alternative hypotheses that varied in microbial‐substrate affinity. We compared our modelled hypotheses to a forest field experiment where we quantified decomposition of isotopically labelled starch and leaf tissues in soils with manipulated root access to microbes. We tested the hypothesis that decomposition of leaves would be more sensitive to root inputs than decomposition of starch, corresponding to the alternative model hypotheses. In the field study, leaf decomposition increased with root density, whereas starch decomposition was unchanged by root density. Microbial biomass and enzyme activity consistently increased with root inputs in CORPSE and the field study. Our field experiment supported the CORPSE simulations with high microbial‐substrate affinity. Roots stimulated microbial growth and enzyme production, which increased the 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 rhizosphere influence 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 SOC 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. A free Plain Language Summary can be found within the Supporting Information of this article.

Causal hypotheses accounting for correlations between decomposition rates of different mass fractions of leaf litter

“…1a) and follows from the idea that the decay of cellulose, hemicellulose, and lignin requires the production and excretion of extracellular enzymes that are quite energy demanding (Deacon 2006). This also agrees with recent observations that labile root exudates stimulate the decomposition of complex, but not simple, soil carbon in the rhizosphere (Moore et al 2019). Based upon the values of the path coefficients, an increase in the rate of decomposition of the water‐soluble fraction ( k S ) increases the decomposition rate of the hemicellulose, cellulose, and lignin fractions about equally.…”

Different mechanisms underlying divergent responses of autotrophic and heterotrophic respiration to long-term throughfall reduction in a warm-temperate oak forest

“…On the other hand, soil C storage is also largely dependent on the C input (Riggs et al, 2015), thus the higher ne root biomass and production under TFR treatment may also facilitate soil C sequestration. However, recent studies have demonstrated that plant roots or belowground C allocation can drive the soil SOM decomposition (Moore et al, 2020;Street et al, 2020), indicating that the ne roots had a dual role in regulating soil C storage (Dijkstra et al, 2020). Our results suggest that how prolonged drought will ultimately in uence SR, and therefore soil C storage, will depend not only on soil microorganisms but also on plant belowground C allocation.…”

Genotypic variation in maize (Zea mays) influences rates of soil organic matter mineralization and gross nitrification

“…However, this relationship supports the positive impact of residue retention on N-supply to the total plant-available N pool. Moore et al (2020) showed that in soil environments dominated with leaf litter, even small amounts of root C inputs could significantly stimulate microbial decomposition of complex C compounds. Surey et al (2020) also demonstrated the importance of…”