Seasonal variability of CO2 emissions from sediments in planted mangroves (Northern Viet Nam)

Mangrove sediment stores a large quantity of organic carbon. However, its decomposition produces a significant amount of CO2, which can be further emitted to the atmosphere. The objective of the present study was to determine CO2 fluxes at the sediment-air interface of planted mangrove forests and adjacent bare sediment and determine how sediment characteristics, seasonal and tidal variations impact CO2 emissions. The results suggest sediment organic carbon content (TOC) was a key parameter influencing sediment properties beside others sediment characteristics such as grain-size and sediment water content. At the sediment-air interface, CO2 fluxes measured in the wet season were significantly higher (P < 0.001) than in dry season in mangroves, with mean values of 140.55 ± 109.66 and 50.50 ± 31.64 mmol CO2 m−2 d−1, respectively. In the bare sediment, the fluxes were significantly lower than in mangroves (P < 0.01), with 66.23 ± 38.16 mmol CO2 m−2 d−1 in the wet season and 18.61 ± 10.61 mmol CO2 m−2 d−1 in the dry season. The higher fluxes measured in the mangroves were associated with higher organic carbon content of the mangrove sediment. The study determined the CO2 emission rate in mangrove sediment was 1.75 ± 0.76 MgC ha−1 yr−1. After having removed 2 mm depth of sediment surface, containing microbes and biofilms, CO2 fluxes increased from 1.27 to 3.33 times compared to dark conditions. There was also evidence that temperature greatly influenced CO2 emissions with Q10 values of 2.63 for mangrove sediment and of 2.75 for bare sediment. This study suggests that TOC, temperature, tidal cycles and biofilms developing at the sediment surface were among the main factors driving CO2 emissions from the sediment of the planted mangroves. In a future research effort, it is recommended that CO2 fluxes are partitioned to identify CO2 origin, specifically the amount produced by root respiration. We also suggest that in mangrove planting programmes, a whole carbon budget be performed to determine the exact amount of carbon (or CO2eq) that can be fixed.