Crab Bioturbation and Seasonality Control Nitrous Oxide Emissions in Semiarid Mangrove Forests (Ceará, Brazil)

Greenhouse gas fluxes of different land uses in mangrove ecosystem of East Kalimantan, Indonesia

BACKGROUND

Mangrove ecosystems exhibit significant carbon storage and sequestration. Its capacity to store and sequester significant amounts of carbon makes this ecosystem very important for climate change mitigation. Indonesia, owing to the largest mangrove cover in the world, has approximately 3.14 PgC stored in the mangroves, or about 33% of all carbon stored in coastal ecosystems globally. Unfortunately, our comprehensive understanding of carbon flux is hampered by the incomplete repertoire of field measurement data, especially from mangrove ecosystem-rich regions such as Indonesia and Asia Pacific. This study fills the gap in greenhouse gases (GHGs) flux studies in mangrove ecosystems in Indonesia by quantifying the soil CO2 and CH4 fluxes for different land use types in mangrove ecosystems, i.e., secondary mangrove (SM), restored mangrove (RM), pond embankment (PE) and active aquaculture pond (AP). Environmental parameters such as soil pore salinity, soil pore water pH, soil temperature, air temperature, air humidity and rainfall are also measured.

RESULTS

GHG fluxes characteristics varied between land use types and ecological conditions. Secondary mangrove and exposed pond embankment are potential GHG flux sources (68.9 ± 7.0 and 58.5 ± 6.2 MgCO2e ha- 1 yr- 1, respectively). Aquaculture pond exhibits the lowest GHG fluxes among other land use types due to constant inundation that serve as a barrier for the release of GHG fluxes to the atmosphere. We found weak relationships between soil CO2 and CH4 fluxes and environmental parameters.

CONCLUSIONS

The data and information on GHG fluxes from different land use types in the mangrove ecosystem will be of importance to accurately assess the potential of the mangrove ecosystem to sequester and emit GHGs. This will support the GHG emission reduction target and strategy that had been set up by the Indonesian Government in its Nationally Determined Contributions (NDC) and Indonesia's 2030 Forest and Other Land Use (FOLU) Net Sink.

Differences in organic carbon accumulation in mangrove soils due to foraging by herbivorous crabs

Crabs in mangroves could enhance the transfer of organic carbon (OC) from leaf litter to soils, whose variation with the difference in crab size is, however, not well known. A 32-day laboratory feeding experiment was conducted to explore the effects of different sizes of the crabs Parasesarma plicatum foraging on leaf litter of Kandelia obovata on OC accumulation in mangrove soils. Mean rates of soil OC accumulation due to leaf foraging by large, medium, and small crabs were 21.11, 16.11, and 0.77 mg C ind-1 d-1, corresponding to the rates of OC removal from leaf litter of 62.60%, 51.37%, and 2.19%, respectively. Large and medium crabs ingested larger amounts of leaf litter, and soil OC accumulation rates resulting from leaf foraging by large and medium crabs were approximately 8 times higher than those by leaf litter decomposition and triple those by non-leaf foraging. Small crabs ingested the smallest amount of leaf litter, which was almost used for their growth and metabolism. These results underline the key ecological roles of leaf foraging by crabs, especially those with large and medium sizes, in OC accumulation in mangrove soils, which is conducive to estimating carbon sequestration in mangrove soils.

The microbial landscape in bioturbated mangrove sediment: A resource for promoting nature-based solutions for mangroves

Globally, soils and sediments are affected by the bioturbation activities of benthic species. The consequences of these activities are particularly impactful in intertidal sediment, which is generally anoxic and nutrient-poor. Mangrove intertidal sediments are of particular interest because, as the most productive forests and one of the most important stores of blue carbon, they provide global-scale ecosystem services. The mangrove sediment microbiome is fundamental for ecosystem functioning, influencing the efficiency of nutrient cycling and the abundance and distribution of key biological elements. Redox reactions in bioturbated sediment can be extremely complex, with one reaction creating a cascade effect on the succession of respiration pathways. This facilitates the overlap of different respiratory metabolisms important in the element cycles of the mangrove sediment, including carbon, nitrogen, sulphur and iron cycles, among others. Considering that all ecological functions and services provided by mangrove environments involve microorganisms, this work reviews the microbial roles in nutrient cycling in relation to bioturbation by animals and plants, the main mangrove ecosystem engineers. We highlight the diversity of bioturbating organisms and explore the diversity, dynamics and functions of the sediment microbiome, considering both the impacts of bioturbation. Finally, we review the growing evidence that bioturbation, through altering the sediment microbiome and environment, determining a 'halo effect', can ameliorate conditions for plant growth, highlighting the potential of the mangrove microbiome as a nature-based solution to sustain mangrove development and support the role of this ecosystem to deliver essential ecological services.

Geostatistical Analysis of Mangrove Ecosystem Health: Mapping and Modelling of Sampling Uncertainty Using Kriging

This study assessed the health of the mangrove ecosystem and mapped the spatial variation in selected variables sampled across the Matang Mangrove Forest Reserve (MMFR) by using a geostatistical technique. A total of 556 samples were collected from 56 sampling points representing mangrove biotic and abiotic variables. All variables were used to generate the semivariogram model. The predicted variables over the entire MMFR have an overall prediction accuracy of 85.16% (AGB), 90.78% (crab abundance), 97.3% (soil C), 99.91% (soil N), 89.23% (number of phytoplankton species), 95.62% (number of diatom species), 99.36% (DO), and 87.33% (turbidity). Via linear weight combination, the prediction map shows that mangrove ecosystem health in Kuala Trong throughout the Sungai Kerang is excellent (5: MQI > 1.5). Some landward areas of Kuala Trong were predicted to have moderate health (3: −0.5 ≤ MQI ≤ 0.5), while Kuala Sepetang was predicted to have the bad ecosystem health (2: -1.5 ≤ MQI ≤ −0.5), with active timber harvesting operations and anthropogenic activities in the landward areas. The results of this method can be utilised to carry out the preferred restoration, through appropriate management and facilities distribution, for improving the ecosystem health of mangroves.

Mine tailings in a redox-active environment: Iron geochemistry and potential environmental consequences

Iron (Fe) oxyhydroxides provide many functions in soils, mainly owing to their large surface area and high surface charge density. The reactivity of Fe oxyhydroxides is function of their mineralogical characteristics (e.g., crystallinity degree and crystal size). Detailed studies of these features are essential for predicting the stability and reactivity of these minerals within soil and sediments. The present study aimed to evaluate geochemical changes in Fe-rich tailings after the world's largest mining disaster in SE Brazil (in 2015) and to predict the potential environmental implications for the estuary. The mineralogical characteristics of the tailings were studied at three different times (2015, 2107, and 2019) to assess how an active redox environment affects Fe oxyhydroxides and to estimate the time frame within which significant changes occur. The study findings indicate a large decrease in the Fe oxyhydroxides crystallinity, which were initially composed (93%) of highly crystalline Fe oxyhydroxides (i.e., goethite and hematite) and 6.7% of poorly crystalline Fe oxyhydroxides (i.e., lepidocrocite and ferrihydrite). Within 4 years the mineralogical features of Fe oxyhydroxides had shifted, and in 2019 poorly crystalline Fe oxyhydroxides represented 47% of the Fe forms. Scanning electron microscope micrographs and the mean crystal size evidenced a decrease in particle size from 109 nm to 49 nm for goethite in the d₁₁₁ direction. The changes in mean crystal size increased the reactivity of Fe oxyhydroxides, resulting in a greater number of interactions with cationic and anionic species. The decreased crystallinity and increased reactivity led to the compounds being more susceptible to reductive dissolution. Overall, the findings show that the decrease in crystallinity along with higher susceptibility to reductive dissolution of Fe oxyhydroxides can affect the fate of environmentally detrimental elements (e.g., phosphorus and trace metals) thereby increasing the concentration of these pollutants in estuarine soils and waters.

Crab bioturbation alters nitrogen cycling and promotes nitrous oxide emission in intertidal wetlands: Influence and microbial mechanism

Intertidal wetlands provide important ecosystem functions by acting as nitrogen (N) cycling hotspots, which can reduce anthropogenic N loading from land to coastal waters. Benthic bioturbations are thought to play an important role in mediating N cycling in intertidal marshes. However, how the burrowing activity of benthos and their microbial symbionts affect N transformation and greenhouse gas nitrous oxide (N₂O) emission remains unclear in these environments. Here, we show that bioturbation of crabs reshaped the structure of intertidal microbial communities and their N cycling function. Molecular analyses suggested that the microbially-driven N cycling might be accelerated by crab bioturbation, as the abundances of most of the N related functional genes were higher on the burrow wall than those in the surrounding bulk sediments, except for genes involved in N fixation, dissimilatory nitrate reduction to ammonium (DNRA), and N₂O reduction, which were further confirmed by isotope-tracing experiments. Especially, the potential rates of the main N₂O production pathways, nitrification and denitrification, were 2-3 times higher in the burrow wall sediments. However, even higher N₂O emission rates (approximately 6 times higher) were observed in this unique microhabitat, which was due to a disproportionate increase in N₂O production over N₂O consumption driven by burrowing activity. In addition, the sources of N₂O were also significantly affected by crab bioturbation, which increased the contribution of hydroxylamine oxidation pathway. This study reveals the mechanism through which benthic bioturbations mediate N cycling and highlights the importance of considering burrowing activity when evaluating the ecological function of intertidal wetlands.

Benthic bioturbation: A canary in the mine for the retention and release of metals from estuarine sediments

After the largest mining tailings spill in Brazil, the Rio Doce estuarine ecosystem was severely impacted by metal contamination. In a 28-day laboratory experiment, we examined the effects of the polychaeta Laeonereis sp. on fluxes of oxygen and metal across the sediment-water interface. The density-dependent effect of Laeonereis sp. in the oxygen and metal fluxes was tested at low and high (74 and 222 ind m^-2, respectively) densities, and compared with defaunated controls. The higher worm density had an amplified effect on the oxygen flux, sediment uptake of Al and Mn, and Fe oxidation compared with the control, but no significant effects on other metals (Ba, Cd, Co, Cr, Cu, Ni, and Zn). Higher worm density increased the oxidation of Fe phases, but no effect in the solid phase of other metals. Consequently, Laeonereis sp. bioturbation prevents the reduction of Fe phases and the release of metal-bound-contaminants to estuarine systems.