A comparison of soil carbon pools across a mangrove-salt marsh ecotone at the southern African warm-temperate range limit

Abstract

Mangroves and salt marshes are globally recognized for their ability to efficiently sequester and store carbon in biomass and soil carbon pools. An increase in soil carbon has been found at range limits where mangroves are expanding along ecotones into salt marsh, as mangroves store more carbon per unit area. However, the magnitude of this trend is variable at different mangrove range limits depending on the rates of expansion and the local climatic and geomorphological factors that influence current mangrove distribution. This study presents the first account of soil carbon storage across a mangrove-salt marsh ecotone at the Nahoon Estuary, a warm-temperate range limit for the east coast of the African continent. Soil carbon (to 50 cm depth) was compared between mangrove, salt marsh, and a transitional ecotone habitat. We found that soil carbon was highest at 30–50 cm below the surface across all habitats. The average soil carbon in the upper 50 cm was similar in mangrove (110.14 MgC.ha−1), ecotone (114.50 MgC.ha−1), and salt marsh habitats (109.62 MgC.ha−1). These results show that although mangroves have expanded at the Nahoon Estuary over the past 50 years, the soil carbon content is still similar across the mangrove-salt marsh ecotone. This is in contrast to other studies from southern hemisphere mangrove range limits in Australia. However, these trends have been reported at northern hemisphere range limits in the USA. Further studies in this region are needed to identify environmental factors influencing soil carbon variability.