Abstract
Mangroves play an important role in sequestering organic carbon in tropical and subtropical coastal areas, accounting up to 15% of the total carbon deposited in coastal sediments. Sequestered organic carbon occurs both in standing plant biomass, as well as in the below ground root biomass and mangrove soils. Unavailability of quantitative data on carbon retention capacity of Sri Lankan mangrove ecosystems compelled the authors to carry out the present study with the objective of estimating the total ecosystem carbon content in mangrove eco systems in the Batticaloa lagoon, Sri Lanka. This is the largest lagoon situated on the east coast and the third largest brackish water system in the country. Data on vegetation structure were gathered according to the standard procedures and biomass of mangrove trees was determined by the Allometric method. Total Organic Carbon (TOC) in three depths, (0-15 cm, 16-30 cm and 31-45 cm) of mangrove soils was determined by dichromate-oxidation method followed by colorimetry. Total mangrove plant biomass was found to be 298 Mg ha-1, of which 246 Mg ha-1 was in the above ground components of the plants while 52 Mg ha-1 was in the below ground components. TOC embedded in biomass was calculated to be 158 Mg C ha-1out of which 131 Mg C ha-1was found to occur in above ground and 27 Mg C ha-1in below ground components. TOC in mangrove soils (up to 45 cm depth) was revealed to be 348 Mg C ha-1. The total TOC of mangrove ecosystems in the Batticaloa lagoon was calculated to be 506 Mg C ha-1. Mangrove soils that sequester 68% of the organic carbon forms the largest fraction of the mangrove carbon sink. Below ground components account for only 5% of the total pool while the above ground biomass retains five times more (26%) carbon than the root biomass. These results assist pragmatic evaluation of ecological value of mangroves and justify their conservation and management.
Highlights
Mangroves rank among the most carbon rich ecotone-ecosystems that occur along tropical and subtropical coastlines
High stand density (4754 trees/ha) and low species composition were revealed in the mangrove ecosystems in the Batticaloa lagoon
The Present study revealed that Total Organic Carbon (TOC) values of the mangrove soils in the Batticaloa lagoon, 348 Mg C ha-1, which is twice higher than the TOC recorded for soils of tropical savanna forests, (146 – 198 Mg C ha-1) at Yap and Palau islands in Pacific Ocean (Donato et al, 2012) and Brazil, 90 – 160 Mg C ha-1(Moreno & Calderon, 2011)
Summary
Mangroves rank among the most carbon rich ecotone-ecosystems that occur along tropical and subtropical coastlines. Due to their relatively high primary productivity and anaerobic conditions in the inter-tidal soil/ sediment, they occupy an important role in carbon sequestration in intertidal environment (Komiyama et al, 2008; Donato et al, 2011; Hoque et al, 2011). In above ground plant biomass, below ground root biomass and in soil (Alongi, 2011; Kauffman et al, 2011; World Bank, 2011). It is estimated that carbon sequestration capacity of global mangroves is approximately 25.5 million tons of carbon per year (Ong, 1993)
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