Agroforestry hedges either increase or decrease soil organic carbon stocks depending on the use of plastic mulch in Southern Quebec (Canada).

Agroforestry hedges (AFH), including windbreaks and riparian buffers, can mitigate climate change and improve agroecosystem resilience. Many studies report positive effects of AFH on soil organic carbon (SOC) stocks, yet outcomes remain variable. The factors controlling SOC dynamics under AFH, particularly management practices, therefore must be better understood. We quantified C stocks in soils (0-20 and 20-50 cm), tree biomass, litter and woody debris across 36 AFH located in Southern Quebec (Canada), and compared them with adjacent croplands. These AFH varied in age (5-46 years), tree composition, soil texture and mulch type (none or limited vs. continuous plastic strip). SOC stability was assessed through physical size fractionation into mineral-associated organic matter (MAOM) and particulate organic matter (POM). Contrary to expectations, SOC stocks in the whole profile (0-50 cm) were slightly lower (-6.4 ± 3.9%; mean ± SE) under AFH compared to adjacent fields, due to losses (-13 ± 4%) in the subsoil. These losses were not observed in more developed AFH, suggesting a gradual recovery over time. In the topsoil, SOC increased (+24 ± 5%) in the absence of continuous plastic strips, but decreased (-12 ± 5%) when strips were present. The labile POM fraction accounted for most SOC accrual under AFH (+67 ± 14% relative to controls without plastic strip in the topsoil), while MAOM declined across all layers. Despite these soil responses, total C stocks increased by 21 ± 7 Mg C ha-1 in AFH, mainly due to tree biomass. Biomass C sequestration peaked at 3.7 Mg C ha-1 yr-1 23 years after plantation. Our results highlight that management practices during AFH establishment can strongly influence SOC dynamics and that plastic mulch may offset potential SOC gains, even though overall C sequestration remains positive due to tree biomass accumulation.