Patterns of soil organic carbon in mangrove forests reflect spatial and temporal drivers
Wetlands are potentially important carbon sinks, but the spatial and temporal factors that affect accumulation of organic carbon in tropical coastal systems have not been extensively examined. Mangrove forests, which occur across a wide range of geomorphic and hydrologic settings, provide an opportunity to study drivers of soil carbon storage. This study examined carbon stores and rates of soil organic carbon (SOC) accumulation across a global network representing thirty geographic locations and six forest types (total of 230 study plots). SOC density (mass of organic C in a unit volume of soil) was determined at all sites using the same standard methods; at a subset of sites, C sequestration rates were estimated based on surface accretion of organic C plus subsurface accumulation of root C.
SOC density varied from 0.002 to 0.1 g cm-3, with an overall average of 0.019 and 0.058 g cm-3 in mineral and organic soils, respectively. Sites spanned a latitudinal range from 37° S to 29° N, and carbon density was positively correlated with average annual temperature. However, high variation in SOC density within latitude indicated additional influences. At a regional scale, SOC density varied with forest type and generally increased with hydrologic energy. Carbon sequestration rates, estimated from surface accretion of organic C above marker horizons, were similar in organic (216 g C m-2 yr-1) and mineral (145 g C m-2 yr-1) soil types, but varied across geographic locations (41 to 591 g C m-2 yr-1). Subsurface inputs of carbon, estimated using measured rates of root matter accumulation and root carbon content, averaged 121 g m-2 yr-1, but exceeded 400 g m-2 yr-1 at several sites. Mangrove peat deposits varied in thickness across sites from < 1 m to over 10 m, indicating the potentially large carbon stores that can develop under certain conditions in these intertidal forests. Rates of carbon accretion varied from 90 to 300 g C m-2 yr-1 over an 8,000-year record at a site in Belize with peat deposits up to ten meters thick. This temporal pattern matched variation in rates of sea-level rise over the same time period. These patterns indicate spatial and temporal variability in SOC and suggest multiple controls on rates of carbon accumulation in mangrove ecosystems.