Isotopic and Biomarker Constraints on Soil Carbon Dynamics Across Degraded, Restored, and Old-Growth Mangroves in the Sundarbans

Coastal mangrove habitats are among the most efficient natural carbon sinks on the planet: their upper soil typically contains about 3–5 times more soil organic carbon than the global average and can sequester carbon at rates 2–4 times greater than tropical terrestrial forests. Nevertheless, degradation and hydrological disruption driven by climate change and anthropogenic perturbations jeopardize these carbon reservoirs, potentially transforming mangroves from net carbon sinks into carbon sources. We hypothesized that restoring hydrological connectivity and vegetation would enhance soil organic carbon accumulation and reactivate blue carbon cycling. To test this, we examined the Sundarbans—the largest contiguous mangrove forest in the world, extending across India and Bangladesh. Three sites under distinct ecosystem conditions in the Indian part of the Sundarbans were selected: a degraded mangrove site—where vegetation loss and hydrological disruption have promoted oxidation and loss of soil organic carbon; a restored mangrove forest maintained by a local NGO; and a mature old-growth mangrove. Soil core samples collected up to 35 cm depth at 3–5 cm intervals were analyzed for total organic carbon (TOC) and nitrogen (N), stable carbon isotopes (δ¹³C), n-alkane biomarkers, and radiocarbon abundance. Soils from the degraded mangrove site contained low TOC, high δ¹³C values, a high ratio of aquatic-to-terrestrial n-alkanes (P_aq), and a consistent decrease in terrestrial plant wax n-alkanes (P_wax) with depth, along with variably lower C/N ratios, signifying heterogeneous carbon inputs dominated by aquatic sources. In contrast,  soils from the restored mangrove site showed TOC and δ¹³C values comparable to mature systems, higher P_wax, longer n-alkane average chain lengths (ACL), and consistently elevated C/N ratios, indicating stable, plant-dominated carbon inputs under restored vegetation and hydrological connectivity. The relationship between C/N ratios and δ¹³C values further confirmed a shift from aquatic-derived carbon in the degraded site to terrestrial C3 plant carbon in the restored site, with the old-growth mangrove site exhibiting mixed-source signatures. Radiocarbon profiles revealed a gradual decrease with depth, reflecting the aging and stabilization of organic matter. Our results highlight the dual role of mangrove restoration in rapidly rebuilding carbon stocks while enhancing carbon turnover, underscoring its importance for climate mitigation strategies and blue carbon credit frameworks.

[Keywords: mangrove restoration, blue carbon, carbon sequestration, n-alkane biomarker, carbon isotopes]