Key biogeochemical factors affecting soil carbon storage in Posidonia meadows.

Biotic and abiotic factors influence the accumulation of organic carbon (C_org) in seagrass ecosystems. We surveyed Posidonia sinuosa meadows growing in different water depths to assess the variability in the sources, stocks and accumulation rates of C_org. We show that over the last 500 years, P. sinuosa meadows closer to the upper limit of distribution (at 2-4m depth) accumulated 3 to 4-fold higher C_org stocks (averaging 6.3 kgC_org m^-2) at 3 to 4-fold higher rates (12.8 gC_org m^-2 yr^-1) compared to meadows closer to the deep limits of distribution (at 6-8m depth; 1.8 kgC_org m^-2 and 3.6 gC_org m^-2 yr^-1). In shallower meadows, C_org stores were mostly derived from sea-grass detritus (88% in average) compared to meadows closer to the deep limit of distribution (45%on average). Also, sediment accumulation rates and fine-grained sediment content (< 0.125 mm) in shallower meadows (2.0mmyr^-1 and 9%, respectively) were approximately 2-fold higher than in deeper meadows (1.2mmyr^-1 and 5%, respectively). The C_org stocks and accumulation rates accumulated over the last 500 years in bare sediments (0.6 kgC_org m^-2 and 1.2 gC_org m^-2 yr^-1) were 3 to 11-fold lower than in P. sinuosa meadows, while fine-grained sediment content (1%) and seagrass detritus contribution to the C_org pool (20%) were 8 and 3-fold lower than in Posidonia meadows, respectively. The patterns found support the hypotheses that C_org storage in seagrass soils is influenced by interactions of biological (e.g. meadow productivity, cover and density), chemical (e.g. recalcitrance of C_org stocks) and physical (e.g. hydrodynamic energy and sediment accumulation rates) factors within the meadow. We conclude that there is a need to improve global estimates of seagrass carbon storage accounting for biogeochemical factors driving variability within habitats. [ABSTRACT FROM AUTHOR]