Soil carbon storage in mangroves is primarily controlled by soil properties: A study at Dongzhai Bay, China

Highlights: • Spatial patterns of soil C concentration in mangroves were determined by soil properties. • Soil moisture was the major control of soil C concentration in the upper 60 cm soil zone. • Soil texture was the major control of soil C concentration in the 60–100 cm soil zone. • Mangrove soil C storage may be affected more by C decomposition rate than input rate. • Soil C concentration was a good predictor of soil C density. Coastal wetlands are well known for their considerable capacity to store carbon (C). However, the spatial patterns and major controls of soil C concentration and C density in coastal wetlands remain poorly known. We measured soil total C concentration up to one meter depth and assessed environmental and biological factors influencing soil C input and decomposition processes across various geomorphologic settings and mangrove forest types at Dongzhai Bay, China. Structural equation modeling (SEM) was used to determine the causal pathways of influencing factors on soil C concentration. We found that the variation pattern of soil C concentration across geomorphologic settings and forest types was mirrored by soil properties. From 68 to 94% (varying with soil depth) variations of soil C concentration were explained by the inter-related influencing factors included in SEM. In the upper 60 cm soil layers, soil moisture was the most important factor affecting soil C concentration. In the 60–100 cm subsoil zone, the proportion of finer soil particles was the primary control of soil C concentration variation. In contrast, aboveground biomass and nearness of sampling site to the open water, which affect autochthonous and allochthonous C inputs, had relatively weak effects on soil C concentration compared to soil properties, which affect C decomposition. Soil C concentration was a good predictor of soil C density at all soil depths. The results suggest that top- and subsoil C concentrations in mangroves are subjected to different environmental controls, but taken together, mangrove soil C storage may be primarily controlled by soil property-mediated C decomposition rate. Subsoil C deserves more attention since it may respond differently to environmental changes than the better-known topsoil C.