Tipping Points in the Mangrove March: Characterization of Biogeochemical Cycling Along the Mangrove-Salt Marsh Ecotone

Coastal wetland vegetation communities can respond to sea level rise via the encroachment of more salt- and inundation-tolerant species into existing vegetation communities. Black mangroves (Avicennia germinans L.) are encroaching on saltgrass (Distichlis spicata L.) within the Merritt Island National Wildlife Refuge in east central Florida (USA). Nine soil cores collected along three transects captured the transitions of both perceived abiotic drivers (salinity and inundation) and vegetation communities during both high- and low-water seasons to investigate patterns in soil biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P). Results showed no change in soil carbon dioxide production along the ecotone during either season, though changes in enzyme activity and mineralization rates of N and P could indicate changes in C quality and nutrient availability affecting C degradation along the ecotone. All parameters, excluding microbial biomass carbon, showed higher rates of activity or availability during the low-water season. Long-term soil nutrient stores (total C, N, P) were greatest in the saltgrass soils and similar between the mangrove and transition zone soils, indicating a 'tipping point' in biogeochemical function where the transition zone is functionally equivalent to the encroaching mangrove zone. Indicators of current biogeochemical cycling (that is, enzyme activity, potentially mineralizable N rates, and extractable ammonium concentrations) showed alterations in activity across the ecotone, with the transition zone often functioning with lower activity than within end members. These indicators of current biogeochemical cycling change in advance of full vegetation shifts. Increases in salinity and inundation were linked to mangrove encroachment.