Desalinization-driven deep microbial reactivation destabilizes iron-bound carbon in coastal wetland restoration

Abstract. Tillage- and mulching-based interventions are increasingly used to control invasive plants and modify soil hydro-salinity, but their effects on subsoil carbon (C) stabilization are poorly quantified. We conducted an 18-month field experiment in a Spartina alterniflora–invaded estuarine wetland to compare plastic mulching (PM) and deep tillage (DT) and to resolve microbial–mineral controls on C across the 0–100 cm profile. PM induced pronounced, profile-wide desalinization (43–53 % lower salinity) and a redistribution of microbial activity, increasing microbial biomass C in 30–100 cm soils by 25–61 % while reducing activity in surface horizons. Relative to DT, PM was associated with much larger C depletion, with total C declining by 19–35 % and the strongest SOC losses occurring at depth (up to ~65 %). Carbon losses co-varied with weakened mineral protection, including 30–50 % decreases in poorly crystalline Fe oxides (Feo) and 35–50 % reductions in iron-bound organic carbon (Fe–OC). Amino-sugar biomarkers indicated coherent shifts in microbial necromass C with Fe–OC dynamics, suggesting vulnerability of long-lived, microbially derived subsoil C under rapid desalinization. Depth-resolved partial least squares path modeling showed contrasting dominant linkages by horizon: surface microbial communities aligned with SOC retention, whereas deep microbial properties covaried with iron mobilization and net C loss. Integrated across 0–100 cm, PM resulted in a net soil C decline of 65 ± 12 Mg C ha⁻¹ over 18 months. These results highlight that mulching and tillage practices can have divergent subsoil C outcomes and that reactive Fe–C metrics are valuable for evaluating management impacts beyond the plough layer.