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 kg C_org m^-2) at 3- to 4-fold higher rates (12.8 g C_org m^-2 yr^-1) compared to meadows closer to the deep limits of distribution (at 6-8m depth; 1.8 kg C_org m^-2 and 3.6 g C_org m^-2 yr^-1). In shallower meadows, C_org stocks were mostly derived from seagrass detritus (88% in average) compared to meadows closer to the deep limit of distribution (45% on average). In addition, soil accumulation rates and fine-grained sediment content (< 0.125 mm) in shallower meadows (2.0 mm yr^-1 and 9 %, respectively) were approximately 2-fold higher than in deeper meadows (1.2 mm yr^-1 and 5 %, respectively). The C_org stocks and accumulation rates accumulated over the last 500 years in bare sediments (0.6 kg C_org m^-2 and 1.2 g C_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 hypothesis 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 soil 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]