Role of decomposition of mangrove and seagrass detritus in sediment carbon and nitrogen cycling in a tropical mangrove forest

Abstract

The elemental composition and in situ decomposition of detritus of Rhizophora apiculata and Enhalus acoroides were studied in mangrove and seagrass bed sediments in the Bangrong mangrove, Phuket, Thailand, and compared with controlled laboratory flux incubations. At the field site detritus was buried in litter-boxes in the sediments and changes in particulate organic carbon and nitrogen in detritus and sediments were followed for up to 2 mo. In the flux incubations the sediments were amended with mangrove and seagrass leaves and rhizomes, and the exchange of oxygen, CO2, dissolved organic carbon (DOC) and inorganic nitrogen was measured across the sediment-water interface for 43 d. The decomposition of seagrass leaves was rapid both in situ and in the laboratory, whereas about 40% of the mangrove leaves remained after 43 d, and no decomposition could be detected for the seagrass rhizome detritus. There was an immediate increase in sediment oxygen uptake (SOU) in the seagrass sediments followed by an increase in CO2 production after the addition of detritus, whereas the SOU remained low in the mangrove sediments. The C:N ratios decreased in the carbon-rich mangrove leaves and seagrass rhizome detritus, but increased in the less carbon-rich seagrass leaves during the decomposition. The decrease in C:N ratio was attributed to Colonization by bacteria which incorporated nitrogen from the overlying water column or from the indigenous particulate organic nitrogen pool in the sediments. The mineralised carbon from the plant detritus was recovered as CO2, whereas no accumulation or release of DOC was found. The effect of amendment with mangrove detritus on the nitrogen fluxes was minor, whereas the uptake of nitrate and release of ammonium was enhanced with the seagrass leaves. About 32 to 36% of the mineralised nitrogen from the seagrass leaves was released across the sediment-water interface, where it became available for primary producers or exported to adjacent ecosystems.