Dynamics of soil organic carbon pools following afforestation in coastal saline–alkali soils: A model–data fusion study

Coastal saline–alkali lands are important targets for ecological restoration and coastal carbon management, yet assessing durable soil carbon sequestration requires considering not only SOC recovery but also how carbon is distributed between particulate and mineral-associated pools. Whether and when net SOC accumulation occurs after afforestation, and how POC and MAOC recover across stand ages and soil depths, remain unresolved. Here, we integrated field observations from a 21-year chronosequence of Taxodium hybrid ‘Zhongshanshan’ plantations with Biome-BGC simulations to reconstruct SOC stock dynamics in coastal saline–alkali soils of eastern China. The model-based reconstruction suggested a non-monotonic decline–recovery–accumulation trajectory, characterized by an early SOC deficit followed by delayed recovery and subsequent accumulation. The prolonged early decline suggested that new carbon inputs were initially insufficient to offset SOC losses. POC and MAOC showed distinct stand-age- and depth-dependent responses. POC recovered rapidly in the 0–20 and 20–40 cm layers at later stages of afforestation, whereas MAOC recovered more slowly and remained below the mudflat level in the 40–60 cm layer after 21 years. In mixed-effects models accounting for plot-level non-independence, land-use stage and soil depth, POC remained positively associated with sucrase activity, whereas MAOC and SOC were primarily associated with electrical conductivity. These findings indicate that SOC recovery following afforestation is characterized by asynchronous and depth-dependent changes in particulate and mineral-associated carbon pools, with a rapid but non-monotonic POC response and a more gradual, depth-constrained recovery of MAOC. Assessments of soil carbon sequestration in coastal saline–alkali plantations should therefore distinguish the rapid but non-monotonic response of POC from the slower and more depth-constrained recovery of MAOC, while explicitly accounting for subsoil carbon pools.