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.

Microbial dinitrogen (N2) fixation and intense bioturbation by macrofauna can contribute to reduce nitrogen (N) limitation in mangrove systems. In particular, crabs are important ecosystem engineers that rework sediments, redistribute organic matter, accelerate nutrient cycling and shape microbial communities in mangrove sediments. Hosting functionally diverse microbial communities, crabs form a discrete ecological unit (a holobiont). In this study, we report rates of respiration, inorganic N fluxes, net N2 fixation, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) measured in the fiddler crab Leptuca thayeri and sesarmid crab Armases rubripes holobionts, which are dominant in oligotrophic and eutrophic mangrove systems of southeast Brazil, respectively. We measured lower biomass-specific rates of respiration and ammonium (NH4+) production for the larger L. thayeri compared to A. rubripes, with very different molar ratios of O2 respiration to NH4+ production (152:1 vs. 20:1, respectively). This suggests a size-metabolism relationship, different food quality or different coupling of N excretion and assimilation by the crab holobionts in the 2 systems. Both crab holobionts contributed to net denitrification and DNRA, with faster N cycling in A. rubripes in the eutrophic system. Net N2 fixation was also detected, with nearly 4-fold higher rates in A. rubripes compared to L. thayeri. Overall, our results illustrate active and complex N cycling associated with the 2 dominant crab holobionts and highlight their potential and overlooked role as important conduits of fixed N, which may double N2 fixation rates in the mangrove’s rhizosphere.