Enhanced stabilisation of soil carbon via arbuscular mycorrhizal fungi and biochar.

The fate of carbon (C) in soil is governed by a complex interplay of chemical, physical, and biological processes that determine whether C inputs are rapidly respired or stabilised within more permanent pools. The latter is critical for climate change mitigation, as soil can act as both a source and sink of atmospheric CO₂. Optimising stabilisation pathways is therefore essential to increasing soil C persistence and ensuring meaningful sequestration outcomes. Despite growing recognition of the role of fungi in soil C cycling, the biological mechanisms governing long-term soil C storage remain poorly understood. Here, we introduce the Hyphal Carbon Transfer Theory, which proposes that fungal hyphae can transport plant-derived C from the plant sphere to more stable soil environments. We test this theory by examining the effects of arbuscular mycorrhizal fungi (AMF) inoculation on C partitioning across plant and soil pools using stable isotope ¹³C pulse labelling. We also evaluated biochar as a potential facilitator of this process, given its role in enhancing carbon retention through physical protection, adsorption capacity, and modulation of microbial activity. Biochar did not alter AMF colonisation; however, elevated ¹³C concentrations in both the stable soil fraction and biochar-associated extracts under AMF colonisation indicate that AMF facilitate C transfer away from the highly respiratory root zone toward physically protected microsites. These results point to a previously underappreciated pathway of soil C stabilisation, in which fungal-mediated transport and biochar interactions contribute to C accrual through physical protection and sorption mechanisms.