Soil Accretion and Mass Accumulation in a Scrub and Fringe Mangrove Forest in Biscayne Bay, Florida

This study estimated soil accretion and mass (organic carbon (OC), total nitrogen (TN), and total soil) accumulation in a scrub and fringe mangrove forest in Biscayne Bay, FL, to assess how forests of different morphologies (scrub vs. fringe) have kept pace with recently accelerating rates of sea-level rise. Accretion rates (AR) were estimated using the Constant Initial Concentration (CIC) model of lead-210 deposition and it was determined that the scrub forest has accreted at 1.3 ± 0.2 mm yr-1 over 75 years and the fringe forest at 2.8 ± 0.4 mm yr-1 over 92 years. The fringe forest estimate met the long-term (84-year, 1931-2015) relative sea-level rise (RSLR) rate of 2.7 ± 0.1 mm yr-1 (Virginia Key, FL; Station ID:8723214) while the scrub forest estimate did not. Long-term (scrub = 75 yrs; fringe = 92 yrs) OC, TN, and total soil accumulation rates were 61.8 ± 8.2, 3.2 ± 0.4, and 231.1 ± 30.4 g m-2 yr-1, respectively for the scrub forest and 153.8 ± 23.9, 9.2 ± 1.4, and 934.3 ± 145.3 g m-2 yr-1, respectively for the fringe forest. Organic carbon and soil accumulation rates in the fringe core were significantly greater (p \u3c 0.05) than those of the scrub core, supporting the hypothesis that forest type morphology affects soil accumulation. The two forest types displayed significant differences in both δ13C and δ15N values (p \u3c 0.0001), indicating that they were likely influenced by different environmental and physiological drivers, resulting in differences in forest stature and varying accumulation rates. Mangrove biomass is the primary source of soil accumulation in the fringe and the fringe site is likely to continue keeping pace with RSLR. Low accretion in the scrub is likely caused by environmental stressors such as nutrient limitation and salinity stress, and the acceleration of RSLR will likely inundate the scrub forest if current conditions continue. The soil inorganic matter (SIM) component in these forests was important as a lasting soil-building component. The fringe forest accumulated significantly more SIM than the scrub forest which likely enhanced accretion in the fringe over the scrub forest, which had less SIM. The high SIM intervals may be a potential signature of strong storm events. The bottom of the fringe core dates back to the 1920s; therefore, it is possible that the fringe core contains evidence of the Great Miami Hurricane (1926) and storms thereafter. Forests that accumulate a high proportion of recalcitrant SIM may be better equipped to respond to accelerating RSLR