Seasonal variability of CO2 fluxes at different interfaces and vertical CO2 concentration profiles within a Rhizophora mangrove forest (Can Gio, Viet Nam)

Abstract

Despite being organic-rich, mangrove soils are usually characterized by low rates of CO2 emissions due to waterlogged conditions, mangroves being thus considered as valuable blue carbon sinks. However, these emissions are highly variable, notably depending on climatic parameters. Can Gio mangrove is a tropical one, being the largest in Vietnam, having being replanted after the war in the late 70's. The main objectives of this study were to quantify the seasonal variability of CO2 emissions at different interfaces (soil-air, water-air, and trunk-air) within a mature Rhizophora stand, and to assess the influence of these emissions on the vertical CO2 concentration profiles in the canopy. CO2 emissions from the soil in the dark were the highest of the different interfaces studied, and were among the highest measured for a mangrove soil, with a mean yearly value reaching more than 270 mmol CO2 m−2 d−1. We suggest that the very high temperatures characterizing Southern Vietnam all year round were responsible for these elevated CO2 emissions. In addition to temperature, the first rainfall pulse of the monsoon resulted in the highest CO2 fluxes both from the soil, reaching up to 500 mmol CO2 m−2 d−1, and from the trunks, reaching almost 230 mmol CO2 m−2 d−1. Because rainfall pulses are usually followed by a period of increasing rates of ecosystem photosynthesis, and a decrease of ecosystem respiration, we measured lower CO2 fluxes during the middle of the wet season. Additionally, it is possible that, during the rainy season, part of the CO2 produced within the soil was exported trough pore-water seepage, which was enhanced because of the higher level of the river. Concerning CO2 emissions at high tide within the Rhizophora stand, the seasonal trend followed the one of precipitation. During the rainy season, estuarine waters are enriched in organic matter and CO2 due to increased runoff in watersheds, and consequently CO2 fluxes from the water column were the highest. However, these emissions were much lower than those from the soil at low tide or from the trunks. Eventually, CO2 concentrations profiles in the canopy varied with the season, the tide, and the night-day alternation. Elevated CO2 concentrations at the root level may imply that prop roots can produce high amount of CO2 but also that they may trap gas emitted from the soil or the water column, possibly acting as physical barriers to air movement. Further studies using eddy-covariance technique should be developed to confirm the hypothesis suggested in this preliminary study.