[A-CG43] Coastal Ecosystems - 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG43-P04] Carbon cycle and net ecosystem production of a mature sub-tropical mangrove forest on Ishigaki Island

*Ohtsuka Toshiyuki¹, Takeo Onishi², Shinpei YOSHITAKE¹, Hideshi Arai¹, Mitsutoshi Tomotsune³, Nobuhide Fujitake⁴, Morimaru Kida⁴, Miyuki Kondo⁵, Yasuo Iimura⁶, Kazutoshi Kinjo⁷ (1.River Basin Research Center, Gifu University, 2.Faculty of Applied Biological Sciences, Gifu University, 3.Faculty of Education and Integrated Arts and Sciences, Waseda University, 4.Graduate School of Agricultural Science, Kobe University, 5.National Institute for Environmental Studies, 6.School of Environmental Sciences, The University of Shiga Prefecture, 7.Faculty of Agriculture, University of the Ryukyus)

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.