Edinburgh Napier University

  Understanding the effects of seedling density on sediment accretion, biogeochemistry and 3 belowground biomass in mangrove systems can help explain ecological functioning and inform 4 appropriate planting densities during restoration or climate change mitigation programs. The 5 objectives of this study were to examine: 1) impacts of mangrove seedling density on surface 6 sediment accretion, texture, belowground biomass and biogeochemistry, and 2) origins of the 7 carbon (C) supplied to the mangroves in Palakuda, Puttalam Lagoon, Sri Lanka. Rhizophora 8 mucronata propagules were planted at densities of 6.96, 3.26, 1.93 and 0.95 seedlings m-2 along 9 with an unplanted control (0 seedlings m-2). The highest seedling density generally had higher 10 sediment accretion rates, finer sediments, higher belowground biomass, greatest number of fine 11 roots and highest concentrations of C and N (and the lowest C/N ratio). Sediment accretion rates,
12 belowground biomass (over 1370 days), and C and N concentrations differed significantly 13 between seedling densities. Fine roots were significantly greater compared to medium and coarse 14 roots across all plantation densities. Sulphur and carbon stable isotopes did not vary significantly 15 between different density treatments. Isotope signatures suggest surface sediment C (to a depth 16 of 1 cm) is not derived predominantly from the trees, but from seagrass adjacent to the site.