Keywords:Carbon cycle, Dissolved inorganic carbon, Dissolved organic carbon, Net ecosystem production, Mangrove
Carbon (C) sequestration of a forest ecosystem is the biological process of removing CO2 from the atmosphere and storing it in carbon pools, such as biomass and soil organic matter (SOM). Biometric based NEP (net ecosystem production) is described as the balance between net primary production (NPP) of autotrophs and respiration of heterotrophs (HR) in an ecosystem, and is conceptually equivalent to the rate of C sequestration in terrestrial ecosystems. Mangrove forests had a higher NEP compared to the other terrestrial forests, because of high NPP under tropical conditions and low HR due to anaerobic soil condition, and thus mangroves are well known for most C-rich ecosystems among the forests. For example, the NEP of Thailand’s mangroves was estimated to range from 7.3 to 11.3 t C ha-1 yr-1 (Poungparn et al. 2012), which is more than three times higher than the mean of temperate forests (2.5 t C ha-1 yr-1, Kato & Tang 2008). However, the previous NEP estimates in mangroves did not consider the process of C fluxes with hydrological regimes. Thus, we conducted the study of C cycling not only using biometric method, but also including the dynamics of dissolved organic C (DOC) and inorganic C (DIC). The study was conducted in a sub-tropical mangrove forest of the Fukido River, Ishigaki Island, Okinawa, Japan (24°29′N, 124°13′E). The C pools and fluxes in the mangrove forest were estimated using a 0.64 ha permanent plot. The carbon pools in the permanent plot (164.6 t C ha-1 as aboveground biomass, and 261.5 t C ha-1 as SOM) were high despite the sub-tropical high latitude among mangroves. The aboveground NPP of the mangrove forest was 1.6 ± 0.15 t C ha-1 yr-1 as woody production, and 3.8 ± 0.28 t C ha-1 yr-1 as foliage production. Soil and water surface CO2 efflux (as indicator of HR) in the permanent plot was continuously monitored using AOCC (automatic open-close chamber) method. The mean soil CO2 efflux during low tide was 164 mgCO2 m-2 h-1 (ranged from 84 to 392 mgCO2 m-2 h-1) which was lower than terrestrial forest ecosystems. The CO2 efflux from soil surface during low tide was correlated significantly with exponential soil temperature. The mean water surface CO2 efflux was 84 mgCO2 m-2 h-1 (ranged from 7 to 353 mgCO2 m-2 h-1) during high tide using continuous chamber measurements in the field. DIC concentrations in the river mouth changed with tidal lever. During high tide, the concentration was the same as sea water level, however, the concentration was gradually increased during low tide. This indicated that the part of mangrove-derived CO2 from soil surface dissolved in the sea water as DIC, and were flow out to sea with tide. The trend of DOC concentrations in the river mouth was the same as DIC. Therefore, it seems inappropriate to apply biometric-based NEP to mangrove ecosystems. I will renovate the C cycle in mangroves including dissolved C fluxes such as DIC and DOC, and calculate NEP using biometric method with water flow model.