Biofilm and temperature controls on greenhouse gas (CO and CH) emissions from a Rhizophora mangrove soil (New Caledonia).

Seasonal variations of CO and CH fluxes were investigated in a Rhizophora mangrove forest that develops under a semi-arid climate, in New Caledonia. Fluxes were measured using closed incubation chambers connected to a CRDS analyzer. They were performed during low tide at light, in the dark, and in the dark after having removed the top 1-2 mm of soil, which may contain biofilm. CO and CH fluxes ranged from 31.34 to 187.48 mmol m day and from 39.36 to 428.09 μmol m day, respectively. Both CO and CH emissions showed a strong seasonal variability with higher fluxes measured during the warm season, due to an enhanced production of these two gases within the soil. Furthermore, CO fluxes were higher in the dark than at light, evidencing photosynthetic processes at the soil surface and thus the role of biofilm in the regulation of greenhouse gas emissions from mangrove soils. The mean δC-CO value of the CO fluxes measured was -19.76 ± 1.19‰, which was depleted compared to the one emitted by root respiration (-22.32 ± 1.06‰), leaf litter decomposition (-21.43 ± 1.89‰) and organic matter degradation (-22.33 ± 1.82‰). This result confirmed the use of the CO produced within the soil by the biofilm developing at its surface. After removing the top 1-2 mm of soil, both CO and CH fluxes increased. Enhancement of CH fluxes suggests that biofilm may act as a physical barrier to the transfer of GHG from the soil to the atmosphere. However, the δC-CO became more enriched, evidencing that the biofilm was not integrally removed, and that its partial removal resulted in physical disturbance that stimulated CO production. Therefore, this study provides useful information to understand the global implication of mangroves in climate change mitigation.