Carbon sequestration capacity of mangrove soils in micro tidal estuaries and lagoons: A case study from Sri Lanka
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Mangroves are supreme agents in building blue carbon pools by capturing atmospheric carbon and storing them in biomass and soil. Anaerobic mangrove soils favour accumulation of partially decomposed organic matter that builds the carbon stocks over time. This study is an attempt to quantify the stock of total organic carbon (TOC) and to determine its spatial as well as vertical distribution in soils of mangrove ecosystems in Sri Lanka. Seven mangrove areas representing the wet, dry and intermediate climatic zones of Sri Lanka were chosen for this purpose. Samples were taken from three depths, (0–15 cm, 16–30 cm and 31–45 cm) of mangrove soils and TOC was determined by dichromate-oxidation method followed by colorimetry. Surface layers of soils (0–15 cm depth) in general contained high amounts of carbon, particularly in mangrove soils of the wet and intermediate climatic zones. In majority of mangrove areas TOC stocks increased with depth and across the water-land gradient. Estimated magnitude of the carbon pools in Sri Lankan mangrove soils ranged from 316.29 to 580.84 Mg ha−1(1 Mg=1 t=103 kg) (total sampling area 13600m2). Mangrove soils of Rekawa lagoon, located in the intermediate climatic zone was found to be the largest soil carbon sink (580.84 Mg ha−1), while that of Batticaloa lagoon in the dry zone, was the smallest (316.29 Mg ha−1). A statistically significant (p < 0.05) relationship was observed between annual rainfall and total organic carbon storage in mangrove soils. The conservative estimate of the magnitude of total carbon pool in soils of all mangrove areas in Sri Lanka that extend over 15,670 ha therefore, is 5623.70×103 Mg. Mangrove soils in Sri Lanka, in their micro-tidal environment, preform a superior carbon sequestration function and therefore deserve better protection.