Vertical redistribution of iron-bound organic carbon and microbial functional shifts across a mangrove restoration chronosequence: redox-mediated constraints on blue carbon stabilization

Abstract

Mangrove soils are vital blue carbon reservoirs; however, the mechanisms governing organic carbon stabilization and vertical distribution dynamics during restoration remain poorly constrained. We systematically sampled soil profiles (0–100 cm) from Kandelia obovata plantations aged 5, 10, 15, and 20 years to analyze spatiotemporal dynamics of organic carbon fractions, iron-bound organic carbon (Fe-OC), iron oxides, and microbial communities. Results revealed that all carbon fractions followed a unimodal accumulation pattern, peaking at the 15th year before declining, challenging the traditional linear accumulation paradigm. The vertical distribution of Fe-OC exhibited a fundamental shift, with the center of Fe-OC enrichment shifting progressively from surface soils (0–20 cm) in early-stage stands to deep soil layers (80–100 cm) in mature stands, reflecting a reorganization of iron-mediated carbon stabilization along the soil profile. In 20-year stands, a notable increase in the Fe³⁺/Fe²⁺ ratio signified a critical redox transition, mechanistically linked to enhanced nitrification and reduced methanogenesis. Variance partitioning analysis demonstrated that soil depth accounted for 18.78% of the variation in carbon fractions—nearly four times the 4.85% attributable to stand age—underscoring the critical importance of deep soil carbon. Structural equation modeling identified a negative correlation between SOC and the Mineral Associated Organic Carbon (MAOC)/SOC ratio (β = −0.282), suggesting progressive constraints on mineral protection capacity. We identify an apparent intermediate-stage peak (10–15 years) in carbon accumulation and stabilization within the studied chronosequence, providing a scientific basis for evaluating carbon storage and stabilization potential and optimizing management strategies for restored mangrove ecosystems.