Pore-water methane dynamics as an indicator of ecosystem functioning following Ecological Mangrove Rehabilitation (EMR) in South Sulawesi, Indonesia

Abstract

Increasing carbon sequestration in mangrove ecosystems has several benefits, including a potential role in climate change mitigation. However, mangroves are also known to emit methane (CH4), a potent greenhouse gas, and the net climate change benefit of planting mangroves is not yet fully understood. In this research, I investigated pore-water CH4, a proxy for CH4 production and export, in a mangrove rehabilitation area on an Indonesian tropical island. Initially, I established a new pore-water extraction method that is simple, cheap and reliable. This sampler was used to measure pore-water CH4 in several mangrove rehabilitation sites on a tropical island in Indonesia. The pore-water CH4 concentrations were very high (20 to 30,000 times the saturated value). However, my estimates of the CH4 fluxes at the soil surface were very low, which agreed with previous studies. Therefore, I surmised that the exceptionally high levels of CH4 in the pore-water would be exported not only through the sediment-air interface, but also by lateral tidal flow and especially via mangrove stem (sediment-root-stem-lenticel pathways). Temperature and pore-water chemistry were found to be ideal for CH4 production, thereby implying that the major constraint was substrate supply. This was confirmed by the finding that the pore-water CH4 concentration was associated with the stage of the mangrove regrowth, and roughly followed mangrove forest productivity, with the highest pore-water CH4 concentration at intermediate stand ages. This was also confirmed by the dominant pathway of CH4 production that was through the degradation of methylated compounds (supplied by mangrove vegetation), rather than via acetate or CO2/H2. Thus, this study indicates that mangrove vegetation plays a critical role in CH4 production.