Key biogeochemical factors affecting soil carbon storage in Posidonia meadows

Abstract. 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 2–4 m 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 6–8 m 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.