Soil organic carbon stocks and sources in a tropical mangrove-associated Halophila beccarii meadow

Seagrass meadows have a high organic carbon (OC) storage capacity, and the long-lived recalcitrant OC (ROC) pool is an important component of the soil OC pool. Halophila beccarii is a widespread species that develops intertidally and is typically associated with mangroves; however, the soil OC stocks and the contribution of mangrove-derived OC to the soil OC pool are still poorly studied. Here we investigated the soil OC and ROC pools, as well as the carbon sources, in an H. beccarii meadow located in Huachang Bay, a tropical lagoon in Hainan, South China. The results showed that the meadow had a low seagrass biomass, and the soil OC contents and stocks down to 90 cm depths were low in the H. beccarii meadow, ranging from 0.51 to 6.27 mg g -1 and from 17.19 to 48.64 t C ha -1 , respectively. However, the seagrass soils had high ROC:OC ratios (69.6-100%), and the soil OC content showed a significant linear increase with soil ROC content. Isotopic analyses revealed that the soil δ 13 C values were closer to the δ 13 C values of mangrove litter than to those of seagrass plant tissues. The results suggest that mangrove-derived OC contributes more to soil OC than seagrass-derived OC in intertidal H. beccarii meadows and that the soil OC contents change as a function of the mangrove ROC accumulation in the soils.