Abstract
Aims The allocation of recent plant photosynthates to soil via arbuscular mycorrhizal (AM) fungi is a critical process driving multiple ecosystem functions in grasslands. Yet, our understanding of how defoliation modifies below-ground allocation of recent plant photosynthate and its response to drought, which is becoming more intense and frequent, remains unresolved. Methods Here we undertook a 13 C pulse-labelling experiment in a mesotrophic temperate grassland to evaluate in situ how defoliation intensity modifies the transfer of recently assimilated 13 C from plant shoots to roots, extraradical AM fungal hyphae, soil, and 13 C-CO 2 efflux (soil respiration) in response to simulated drought. Results We found that, individually, both defoliation and drought reduced initial plant 13 C uptake, but when defoliation and drought were combined, we detected a significant reduction in below-ground 13 C allocation to soil. Furthermore, while defoliation stimulated 13 C transfer to plant roots and soil, high intensity defoliation amplified 13 C-CO 2 efflux relative to the amount of 13 C taken up by plants. Drought stimulated 13 C transfer to fungal hyphae relative to initial plant uptake. High intensity defoliation, however, suppressed both 13 C enrichment of extraradical AM fungal hyphae and 13 C transfer to fungal hyphae relative to initial uptake. Conclusions Our findings suggest that defoliation can reduce the transfer of recent photosynthate below-ground under simulated drought and provide new insights into how defoliation may influence grassland C allocation dynamics and cycling between plants and AM fungi in grasslands facing drought.