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Pneumatophores and crab burrows increase CO2 and CH4 emission from sediments in two Brazilian fringe mangrove forests

Erik Kristensen*, Thomas Valdemarsen, Paula C. de Moraes, Arthur Z. Güth, Paulo Y. G. Sumida, Cintia O. Quintana

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ABSTRACT: The present study assessed the release of greenhouse gases (CO2 and CH4) from air-exposed sediments and dissolved inorganic carbon (DIC) from inundated sediments in 2 Brazilian mangrove forests. Focus was on the impact of biogenic structures, i.e. pneumatophores and crab burrows, on greenhouse gas emissions. Emission of CO2 from air-exposed bare sediment, 111–156 mmol m–2 d–1 in darkness and 57–148 mmol m–2 d–1 in light, was comparable to DIC release from inundated sediment, 122–158 mmol m–2 d–1 in darkness and 52–62 mmol m–2 d–1 in light. Pneumatophores and crab burrows increased dark CO2 emission during air exposure by 113–123% and 49–91%, respectively. Methane emission from air-exposed bare sediment of 0.22–0.25 mmol m-2 d-1 was higher by 92–137% and 288–607% by pneumatophores and burrows, respectively. Carbon loss in the form of CO2 and DIC from sediments with biogenic structures can 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. Methane emissions of 2.8 and 3.3 mol C m–2 yr–1 (when converted to CO2 units) when biogenic structures are present will partly (10–15%) counteract the climate mitigation effect of the accumulated carbon. In conclusion, carbon budgets in mangrove sediments may be flawed if the contribution of biogenic structures to greenhouse gas emissions is ignored.