Soil moisture mediates the effect of plant below‐ground carbon allocation on the decomposition of root litter in a subtropical forest

Abstract Trees allocate carbon below‐ground to fuel the functioning of roots and mycorrhizal fungi, which affect litter decomposition, but the direction and magnitude of this effect are variable. While tree mycorrhizal type is often suggested to mediate this above‐ground–below‐ground linkage, previous studies yield mixed results. In this study, we investigated how absorptive root traits, soil conditions and litter type influence the response of litter decomposition to altered below‐ground carbon allocation, both within and across mycorrhizal types. We girdled transport roots of seven subtropical tree species to eliminate carbon allocation to distal absorptive roots. We monitored leaf and root litter decomposition surrounding girdled and un‐girdled root branches. We found that girdling generally slowed leaf litter decomposition. However, the effect of girdling on root litter decomposition depended on soil moisture, stimulating decomposition in dry soil but suppressing it in moist soil. Absorptive root traits did not influence the girdling effect on either leaf or root litter decomposition. These findings suggest that disturbance in carbon allocation can impact litter decomposition, with the outcome largely contingent on litter type and soil moisture. Synthesis . Our findings highlight the importance of accounting for local soil variability in understanding the relationship between above‐ground and below‐ground carbon dynamics. This study underscores a critical need for comprehensive assessment of below‐ground ecosystem responses to above‐ground disturbances, as it is essential for accurately predicting future forest carbon and nutrient cycles.