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

Mangroves are blue carbon ecosystems that sequester significant carbon but release CO₂, and to a lesser extent CH_4, 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.