Profile-wide desalinization is associated with increased deep-soil microbial biomass and reduced 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, with salinity decreasing by 43 %–53 %. It also redistributed microbial biomass, increasing microbial biomass C in 30 %–100 cm soils from approximately 25 % at 30–50 cm to more than 100 % at 50–100 cm. 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). These coupled changes suggest a potential weakening of Fe-mediated protection of subsoil C under rapid desalinization. Depth-resolved partial least squares path modeling suggested contrasting linkages by horizon: surface bacterial community attributes were associated with SOC retention, whereas deep-soil MBC covaried with reactive Fe decline and potential C stock loss. Integrated across 0–100 cm, PM was associated with a potential soil C stock loss 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.

The authors' abstract, as published at the source. Biogeosciences, 2026 · DOI ↗