Soil greenhouse gas fluxes partially reduce the net gains in carbon sequestration in mangroves of the Brazilian Amazon.

There is interest in assessing the potential climate mitigation benefit of coastal wetlands based on the balance between their greenhouse gas (GHG) emissions and carbon sequestration. Here we investigated soil GHG fluxes (CO2 and CH4) on mangroves of the Brazilian Amazon coast, and across common land use impacts including shrimp farms and a pasture. We found greater methane fluxes near the Amazon River mouth (1439 to 3312 µg C m-2 h-1), which on average are equivalent to 37% of mangrove C sequestration in the region. Soil CO2 fluxes were predominant in mangrove forests to the East of the Amazon Delta. Land use change shifted mangroves from C sinks (mean sequestration of 12.2 ± 1.4 Mg CO2e ha-1 yr-1) to net GHG sources (mean loss of 8.0 ± 3.3 Mg CO2e ha-1 yr-1). Our data suggests that mangrove forests in the Amazon can aid decreasing the net annual emissions in the Brazilian forest sector in 9.7 ± 0.8 Tg CO2e yr-1 through forest conservation and avoided deforestation.

The inclusion of Amazon mangroves in Brazil's REDD+ program.

The Legal Amazon of Brazil holds vast mangrove forests, but a lack of awareness of their value has prevented their inclusion into results-based payments established by the United Nations Framework Convention on Climate Change. Based on an inventory from over 190 forest plots in Amazon mangroves, we estimate total ecosystem carbon stocks of 468 ± 67 Megagrams (Mg) ha-1; which are significantly higher than Brazilian upland biomes currently included into national carbon offset financing. Conversion of mangroves results in potential emissions of 1228 Mg CO2e ha-1, which are 3-fold higher than land use emissions from conversion of the Amazon rainforest. Our work provides the foundation for the inclusion of mangroves in Brazil's intended Nationally Determined Contribution, and here we show that halting mangrove deforestation in the Legal Amazon would generate avoided emissions of 0.9 ± 0.3 Teragrams (Tg) CO2e yr-1; which is equivalent to the annual carbon accumulation in 82,400 ha of secondary forests.

The novel mangrove environment and composition of the Amazon Delta.

Both freshwater floodplain (várzeas and igapós) forests and brackish-saline mangroves are abundant and well-described ecosystems in Brazil.1 However, an interesting and unique wetland forest exists in the Amazon Delta where extensive mangroves occur in essentially freshwater tidal environments. Unlike the floodplain forests found upriver, the hydrology of these ecosystems is driven largely by large macro-tides of 4-8 m coupled with the significant freshwater discharge from the Amazon River. We explored these mangroves on the Amazon Delta (00°52' N to 01°41' N) and found surface water salinity to be consistently <5; soil pore water salinity in these mangrove forests ranged from 0 nearest the Amazon mouth to only 5-11 at the coastal margins to the north (01°41' N, 49°55' W). We also recorded a unique mix of mangrove-obligate (Avicennia sp., Rhizophora mangle) and facultative-wetland species (Mauritia flexuosa, Pterocarpus sp.) dominating these forests. This unique mix of plant species and soil porewater chemistry exists even along the coastal strands and active coastlines of the Atlantic Ocean. Part of these unique mangroves have escaped current global satellite mapping efforts, and we estimate that they may add over 180 km2 (20% increase in mangrove area) within the Amazon Delta. Despite having a unique structure and function, these freshwater-brackish ecosystems likely provide similar ecosystem services to most mangroves worldwide, such as sequestering large quantities of organic carbon, protection of shoreline ecosystems from erosion, and habitats to many terrestrial and aquatic species (monkeys, birds, crabs, and fish).