Land Use Type Affects SOM Molecular Composition in Forest Plantations by Altering Soil Nutrients and Enzyme Activities

Soil organic matter (SOM) molecular composition governs its stability and ecological functions in forest ecosystems. Nevertheless, how land-use changes (LUCs) regulate the SOM molecular composition remains poorly understood, particularly the underlying mechanisms mediated by soil properties. This study investigated the effects of LUCs on SOM molecular composition in a subtropical coastal region and examined the driving roles of soil nutrient availability and enzyme activities. The research was conducted in Huanghai National Forest Park, Jiangsu Province, China, focusing on four land-use types converted from historical wheat cropland (W, as control): monoculture plantations of Ginkgo biloba (G) and Metasequoia glyptostroboides (M), a ginkgo–metasequoia mixed forest (GM), and a ginkgo–wheat agroforestry system (GW). Soil samples were collected from 0 to 20 cm and 20–40 cm layers and analyzed for SOM molecular compositions using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Soil chemical properties and enzyme activity activities were also determined, with redundancy analysis (RDA) and correlation analysis applied to identify key influencing factors. Results demonstrated that LUCs significantly altered SOM molecular composition. The GW system exhibited the highest proportion of labile O-alkyl carbon (42.65%), while the M plantation accumulated greatest levels of stable aromatic carbon (up to 49.25%). During the initial decades following afforestation, soil nutrient availability and enzyme activities were confirmed as pivotal drivers of SOM molecular variation. Specifically, available potassium (AK), ammonium nitrogen (AN), and the carbon/phosphorus (C/P) ratio were significantly correlated with specific SOM components (p < 0.05). The elevated O-alkyl carbon proportion in GW was closely associated with its higher invertase activity. Notably, vertical differentiation in SOM stability was observed across land-use types, with the agroforestry system achieving the highest carbon pool management index in surface soil but showing a weakened capacity for subsoil C stabilization. RDA further confirmed that AK and AN were dominant factors shaping SOM molecular composition. In conclusion, LUCs modulate SOM chemical composition and stability primarily through altering soil nutrient availability and associated enzyme activities. Agroforestry system facilitates labile C accumulation in surface soil, whereas monoculture plantations are more conducive to stable C sequestration, especially in subsoil layers. These findings provide novel mechanistic insights into SOM dynamics following LUCs and offer a theoretical basis for formulating tailored management strategies to enhance C sequestration efficiency in subtropical coastal ecosystems.