Coastal carbon processing rates increase with mangrove cover following a hurricane in Texas, USA

Changes in species distributions and disturbances have complex impacts on ecosystem functioning. In many subtropical coastal wetlands, plant identity and cover are changing, as salt marshes dominated by low-stature herbaceous species transition to woody mangroves. These systems are also subject to frequent and potentially more intense hurricanes, which can alter a range of ecosystem structures and functions. We examined how changes in dominant plant species affected carbon processing in coastal wetlands following a hurricane. We experimentally manipulated cell-scale (3 × 3 m) cover of black mangroves (Avicennia germinans) and salt marsh plants (e.g., Spartina alterniflora and Batis maritima) in fringe and interior locations of 10 plots (24 × 42 m) to create a gradient in mangrove cover in coastal Texas, USA. Hurricane Harvey made direct landfall over our site on 25 August 2017, uniformly decreasing soil nutrients and impacting the spatial patterns of total soil sulfide accumulation (δ34S). To test how mangrove cover affected carbon processing and retention after the hurricane, we measured litter breakdown rates (k) of A. germinans and S. alterniflora in surface soils and associated microbial respiration rates, and fast- and slow-decomposing standard litter substrates (green and red tea, respectively) in subsurface soils (15 cm depth). Soil temperatures were lower in mangrove than marsh cells, and A. germinans litter k increased linearly with plot-level mangrove cover. Breakdown rates of S. alterniflora and fast-decomposing green tea litter increased nonlinearly with mangrove cover (highest k at intermediate % cover). Slow-decomposing red tea had similar k in all plots and was highest where soil δ34S was greatest. Microbial respiration rates (R) did not change with plot-level mangrove cover for either S. alterniflora or A. germinans litter. Respiration associated with S. alterniflora litter was highest in interior marsh cells, and R associated with A. germinans litter was similar between fringe and interior mangrove cells. Despite widespread declines in soil nutrient concentrations and increases in δ34S, all predicted to decrease R and k, post-hurricane carbon processing increased with mangrove cover in coastal wetlands. Our results indicate that disturbances can reduce local plant identity effects and may enhance carbon loss.