The subsoil horizons are the preferential location for organic carbon stabilization in chestnut forests

Forest soils are recognized to be important organic carbon storage, but the role of surface and subsurface soil horizons on such function and its drivers are still field of debate. In this context, we examined the dynamics of soil organic carbon (SOC) for a chestnut forestry system in a temperate area of northern part of Apennine mountain range in Italy. Specifically, we questioned: what are the main i) SOC forms both in mineral surface and subsurface soil horizons? ii) factors affecting SOC stabilization?. Soil samples were collected by horizon and SOC was separated into organic C of the particulate organic matter (POM_C), sand–size aggregates (sand_C) and the mineral–associated organic C (MAOM_C). The easily oxidizable C (EOC), water–soluble organic C (WSOC), the microbial biomass–C and its respiration, and the total and easily extractable glomalin–related soil protein (T–GRSP and E–GRSP, respectively) were also estimated. Then, the E–GRSP–to–T–GRSP and E-GRSP–to–SOC ratios, the metabolic (qCO₂) and microbial (qMIC) quotients were calculated. The POM_C, sand_C and MAOM_C showed the highest concentrations in A horizon (26.5, 14.6 and 13.9 g kg^–1, respectively) highlighting the importance of the litter floor on the organic matter pools quantity in the topsoil. Further, the A horizon was enriched of the most labile organic C forms (i.e., EOC and WSOC) indicating the key role of A horizon for the development and growth of chestnut forest ecosystems. In fact, the labile organic C forms provide several ecosystem services, such as plant growth and yield. Unlike A horizon, the subsurface horizons preserved SOC mostly in the most stable form (63.8 %, on average). Because of the role of fungal biomass and its exudates to increase SOC capture and stabilization, the great potential of the subsurface horizons to store MAOM_C can be attributed both to the higher release of exogenous GRSP (higher E–GRSP–to–T–GRSP ratio) by mycorrhizal fungi and fungal mycelium expansion (higher E-GRSP–to–SOC ratio) within such horizons (0.504  and 0.061, respectively) compared to the A horizon (0.244 and 0.034, respectively). Therefore, the subsurface soil horizons seemed to have more favourable conditions for microorganisms compared to surface one as shown by the lower qCO₂ and the higher qMIC values found in the former than in the latter. Overall, the present investigation highlighted the importance of subsurface soil horizons of chestnut forests on C stabilization processes compared to the A horizon likely due to the better edaphic conditions for the microbial communities. Thus, our results pointed out the key role that the subsurface soil horizons of chestnut forests could have for mitigating the current climate change.