Mycorrhizal hyphae, but not fine roots modulate drought effects on soil organic matter accumulation in a temperate forest

Extreme climatic events, such as drought, are projected to alter soil carbon (C) and nitrogen (N) cycling in forest ecosystems. However, how the effects of drought are modulated by tree roots and their associated mycorrhizal fungi remains poorly understood. Over 144 days of in-situ incubation, using mesocosms with different mesh sizes in an oak forest subjected to six consecutive years of throughfall rain reduction treatment, we distinguished the drought effects on soil organic C and N accumulations via root-pathway and mycorrhizal hypha-pathway. These effects were assessed within different stability fractions of soil organic matter, i.e., particulate organic matter (POM) and mineral-associated organic matter (MAOM). Drought led to greater accumulations of soil organic C and N via the hypha-pathway compared to the root-pathway. This outcome arose because the hypha-pathway drove greater accumulation in POM than losses in MAOM due to reduced decomposition rates, whereas the root-pathway led to greater POM losses relative to MAOM accumulation, primarily attributable to an enhanced root priming effect. Moreover, plants utilized more soil inorganic N relative to organic N through the hypha-pathway in response to drought, which may partly account for the inconsistent changes in C and N within different labile fractions. These findings emphasize the importance of distinguishing divergent roles of roots and mycorrhizal hyphae in modulating soil C and N processes in the context of future climate change scenarios.