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

Abstract

We 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. Our 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 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 CO2 emission during air exposure by 113-123 and 49-91%, respectively. CH4 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 CO2 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, CH4 emissions of 2.8 and 3.3 mol C m-2 yr-1 (when converted to CO2 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.

Anaerobic carbon oxidation in sediment of two Brazilian mangrove forests: the influence of tree roots and crab burrows

“…The rates of anaerobic C oxidation in sediments of the subtropical Olaria and Nobrega forests during the warm wet season (Table 3) are within the range of those observed in tropical mangrove environments (Alongi et al, 2000;Kristensen et al, 2000;Alongi, 2020). Litterfall and associated benthic metabolism are considerably lower during the cold dry season, when the average temperature is 8OC lower than during the warm wet season, as argued by Rovai et al (2021) and Kristensen et al (2022). In any case, the depth integrated anaerobic DIC production in the upper 18 cm of the Olaria and Nobrega creek flat (OC and NC) sediments during the warm season is not significantly affected by biogenic structures (Fig.…”

“…6). The rates were only 5-22 % lower than the daily average sediment/air CO 2 and sediment/water DIC fluxes measured at the same locations during the warm season by Kristensen et al (2022). This is surprising because SR certainly must be active below 18 cm and drive anaerobic C oxidation deep in these sediments.…”

“…Finally, SR and FeR are given as the % of DIC production. (Kristensen et al, 2022). In these forested and bioturbated sediments, the remainder must be partly derived from deep (> 18 cm) microbial C oxidation as well as root and fauna respiration.…”

Amazonian vegetated estuarine ecosystem as a global hotspot in carbon sequestration

“…This means that our results (111 to 3,870 g m −2 y −1 ) imply that local mangroves accumulate up to 21.7% and 23.6% of the annual litterfall as soil C. The implications are variable, this may indicate that, as proposed in the 1960s that over 70% of the mangrove litterfall is exported. It should also be noted that crabs and other secondary producers in the soils consume a large portion of the litterfall 55 , and are likely to in uence soil parameters and carbon sequestration, as has been noted for other South American coastal wetlands 29,56,57 . However, the mangroves of Marajó may potentially retain a comparatively small fraction of the litterfall due to the enormous hydraulic force of the Amazon river.…”

Tidally driven porewater exchange and diel cycles control CO2 fluxes in mangroves

“…Mangrove sediments are partly permeable due to irrigation by abundant crab burrows and pneumatophores which allows seawater to infiltrate into deep layers and supply electron acceptors for organic matter respiration (Xiao et al 2021;Kristensen et al 2022). Tidal variations in mangroves drive seawater inflow into sediments during flood tide and the discharge of porewater at ebb tides (Chen et al 2021).…”