Contrasting root length, nutrient content and carbon sequestration of seagrass growing in offshore carbonate and onshore terrigenous sediments in the South China Sea

Due to distinct human disturbances and sediment type, seagrasses growing in offshore carbonate and onshore terrigenous sediments may show contrasting characteristics. A comparison of seagrass morphology, nutrient content and sediment carbon pools was taken for seagrass beds inhabiting offshore carbonate sediments in Xuande Atoll and onshore terrigenous sediments in Hainan Island, South China Sea. Lower nitrogen (N) content was observed in the aboveground (1.1%–2.8%) and belowground (0.4%–1.5%) tissue of seagrasses in Xuande Atoll than in the same species (aboveground: 2.7%–3.6%; belowground: 1.2%–2.8%) in Hainan Island. Greater depletion of leaf δ¹⁵N of Thalassia hemprichii (T. hemprichii) and Halodule pinifolia (H. pinifolia) in Xuande Atoll indicated nitrogen fixation might be the major source of nitrogen in oligotrophic reef environments. The root lengths of the seagrass species in Xuande Atoll were longer than the same species in Hainan Island. Sediment inorganic carbon (SIC) was considerably higher than sediment organic carbon (SOC) in the carbonate sediment, while the opposite trend was found in the terrigenous sediments. The SOC stock in the carbonate and terrigenous sediments was 2.41 ± 0.78 Mg C ha⁻¹ and 2.20 ± 0.34 Mg C ha⁻¹ in the top 5 cm, respectively, while the corresponding SIC was 84.38 ± 21.65 Mg C ha⁻¹ and 1.27 ± 0.51 Mg C ha⁻¹, respectively. The average CO₂ ₙₑₜ ₛₑqᵤₑₛₜₑᵣₑd in the carbonate sediment in Xuande Atoll and the terrigenous sediment in Hainan Island were −48.22 ± −12.21 Mg C ha⁻¹ and 1.44 ± 0.03 Mg C ha⁻¹, respectively. This suggested seagrass sediment was a source of CO₂ during sediment production in the carbonate sediment but a sink of CO₂ in the terrigenous sediment. Thus, the N concentration in seagrass leaf, root length, sediment carbon composition and pools were contrasted between offshore carbonate sediments and onshore terrigenous sediments.