Structural equation modelling reveals factors regulating surface sediment organic carbon content and CO2 efflux in a subtropical mangrove.

Mangroves are blue carbon ecosystems that sequester significant carbon but release CO2, and to a lesser extent CH4, from the sediment through oxidation of organic carbon or from overlying water when flooded. Previous studies, e.g. Leopold et al. (2015), have investigated sediment organic carbon (SOC) content and CO2 flux separately, but could not provide a holistic perspective for both components of blue carbon. Based on field data from a mangrove in southeast Queensland, Australia, we used a structural equation model to elucidate (1) the biotic and abiotic drivers of surface SOC (10cm) and sediment CO2 flux; (2) the effect of SOC on sediment CO2 flux; and (3) the covariation among the environmental drivers assessed. Sediment water content, the percentage of fine-grained sediment (<63μm), surface sediment chlorophyll and light condition collectively drive sediment CO2 flux, explaining 41% of their variation. Sediment water content, the percentage of fine sediment, season, landform setting, mangrove species, sediment salinity and chlorophyll collectively drive surface SOC, explaining 93% of its variance. Sediment water content and the percentage of fine sediment have a negative impact on sediment CO2 flux but a positive effect on surface SOC content, while sediment chlorophyll is a positive driver of both. Surface SOC was significantly higher in Avicennia marina (2994±186gm-2, mean±SD) than in Rhizophora stylosa (2383±209gm-2). SOC was significantly higher in winter (2771±192gm-2) than in summer (2599±211gm-2). SOC significantly increased from creek-side (865±89gm-2) through mid (3298±137gm-2) to landward (3933±138gm-2) locations. Sediment salinity was a positive driver of SOC. Sediment CO2 flux without the influence of biogenic structures (crab burrows, aerial roots) averaged 15.4mmolm-2d-1 in A. marina stands under dark conditions, lower than the global average dark flux (61mmolm-2d-1) for mangroves.