Organic carbon accumulation and metabolic pathways in sediments of mangrove forests in southern Thailand

Abstract

Rates of organic carbon accumulation, mineralization and burial in sediments were examined during dry and wet seasons in four mangrove forests of a shallow-water embayment (Ao Sawi) in southern Thailand. Mass sediment accumulation (MAR), estimated from 210Pb and 137Cs profiles, was rapid at all forests, ranging from 2.9 to 7.6 kg m −2 yr −1; mixed layer sediment thicknesses ranged from 16 to 38 cm. Total inputs of organic carbon ranged from 26.4 to 40.9 mol C m −2 yr −1 and burial rates ranged from 15.3 to 23.4 mol C m −2 yr −1. Total rates of carbon mineralization, estimated from direct measurements of CO 2 gas flux from exposed sediments, from DIC flux from submerged sediments, and from core incubations for ΣCO 2, ranged from 7.0 to 16.4 mol C m −2 yr −1 with few consistent seasonal differences among the four forests. Rates of O 2 gas flux into exposed sediments ranged from 9.4 to 91.4 mmol O 2 m −2 d −1 and were significantly greater than rates of dissolved O 2 flux into submerged sediments (range: 21.9–38.3 mmol O 2 m −2 d −1). Oxic respiration was the largest decomposition pathway, with sulfate reduction (range: 0.6–16.9 mmol S m −2 d −1) the next most important pathway. Other metabolic pathways appeared to be minor, and methane was not detected in the porewater or in the gas flux chambers. The discrepancy between rates of O 2 and CO 2 fluxes implies involvement in geochemical processes (e.g. sulfide oxidation, authigenic mineral formation). Sediment budgets indicate that organic carbon preservation was greatest (71% burial efficiency) in the oldest forest with equivalent burial efficiency (57–59%) in the younger forests. Mineralization efficiency ranged from 27–40% with the lowest efficiency at the oldest forest. These tropical mangrove forests are storage sites for sediment and, on average, retain approximately 60% of total input of organic carbon to the sediment.