Significant loss of organic carbon storage and accumulation rate in mangrove sediments due to deforestation: Insights from the Beibu Gulf, China

Mangrove ecosystems in the Beibu Gulf, China, underwent severe deforestation before the 2000s. However, the influence of the deforestation event on sedimentary organic carbon (OC) biogeochemistry has not been reported in this region yet. Based on the ²¹⁰Pb chronology of four sediment cores sampled in the Beilun River Estuary National Nature Reserve, this study analyzed soil texture, bulk OC, stable and radiocarbon isotopes, and lignin biomarkers to reconstruct historical changes in OC storage and accumulation rate over the past half-century. Before the National Nature Reserve establishment in 2000, the stage of pre-establishment I (Pre-EST I, before 1970) was characterized by an average TOC of 1.24 ± 0.14 % with finer particles (76.2 ± 6.2 %), older (6406 ± 362 BP) terrestrial sources (72.2 ± 6.6 %). After significant transitional changes of all the parameters in Pre-EST II (1970–2000), the average TOC became lower (0.68 ± 0.15 %) with mainly younger (1357 ± 147 BP) marine-derived OC (52.8 ± 11.0 %) with coarser particles (59.4 ± 4.7 %) in Post-EST stage after 2000. The lignin concentration Σ₈ also decreased from 0.61 ± 0.12 mg 100 mg⁻¹ sediment in Pre-EST I to a half value of 0.32 ± 0.11 mg 100 mg⁻¹ sediment in Post-EST. The three-stage changes were attributed to deforestation before the National Nature Reserve establishment in 2000. As a result, the remnant sedimentary OC in mangrove sediments indicated not only the source change of OC, but also a decrease in carbon accumulation rate by 14.5 %, a decrease in carbon density by 30.2 % and a decrease in carbon storage by 54.0 % in Beibu Gulf, China. If the mangrove recovery rate was kept the same as in the study, the global mangrove carbon storage may increase by 1.2 Tg C till the end of this century under good protection.