Pneumatophores and crab burrows increase CO2 and CH4 emission from sediments in two Brazilian fringe mangrove forests
Erik Kristensen, Thomas Valdemarsen, PC Moraes, AZ Güth, PYG Sumida, Cintia O. Quintana
Abstract
We assessed the release of greenhouse gases (CO 2 and CH 4 ) from air-exposed sediments and dissolved inorganic carbon (DIC) from inundated sediments in 2 Brazilian mangrove forests. Our focus was on the impact of biogenic structures, i.e. pneumatophores and crab burrows, on greenhouse gas emissions. Emission of CO 2 from air-exposed bare sediment (111-156 and 57-148 mmol m -2 d -1 in darkness and in light, respectively) was comparable to DIC release from inundated sediment (122-158 and 52-62 mmol m -2 d -1 , respectively). Pneumatophores and crab burrows increased dark CO 2 emission during air exposure by 113-123 and 49-91%, respectively. CH 4 emission from air-exposed bare sediment (0.22-0.25 mmol m -2 d -1 ) was increased 92-137 and 288-607%, respectively, by pneumatophores and burrows. Carbon loss in the form of CO 2 and DIC from sediments with biogenic structures can at the two study locations be extrapolated to 64.1 and 71.0 mol C m -2 yr -1 . These values fit well with literature values of litterfall in the studied area, providing carbon accretion of 28.5 and 21.6 mol C m -2 yr -1 . However, the budget will be unbalanced if the role of biogenic structures is not considered. In the presence of biogenic structures, CH 4 emissions of 2.8 and 3.3 mol C m -2 yr -1 (when converted to CO 2 units) will partly (10-15%) counteract the climate mitigation effect of the accumulated carbon. Carbon budgets in mangrove sediments may therefore be flawed if the contribution of biogenic structures to greenhouse gas emissions is ignored.