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1. Coastal inundation regime moderates the short-term effects of sediment and soil additions on seawater oxygen and greenhouse gas dynamics: a microcosm experiment

   https://doi.org/10.3389/fmars.2023.1308590  

   Regier, Peter; Ward, Nicholas D.; Izquierdo, Alex; Baldwin, Andrew H.; Day, Donnie; McElhinny, Julia; Patel, Kaizad; Vargas, Rodrigo; Zheng, Jianqiu; Myers-Pigg, Allison (December 2023, Frontiers in Marine Science)

   The frequency and persistence of tidal inundation varies along the coastal terrestrial-aquatic interface, from frequently inundated wetlands to rarely inundated upland forests. This inundation gradient controls soil and sediment biogeochemistry and influence the exchange of soils and sediments from terrestrial to aquatic domains. Although a rich literature exist on studies of the influence of tidal waters on the biogeochemistry of coastal ecosystem soils, few studies have experimentally addressed the reverse question: How do soils (or sediments) from different coastal ecosystems influence the biogeochemistry of the tidal waters that inundate them? To better understand initial responses of coastal waters that flood coastal wetlands and uplands, we conducted short-term laboratory experiments where seawater was amended with sediments and soils collected across regional gradients of inundation exposure (i.e., frequently to rarely inundated) for 14 sites across the Mid-Atlantic, USA. Measured changes in dissolved oxygen and greenhouse gas concentrations were used to calculate gas consumption or production rates occurring during seawater exposure to terrestrial materials. We also measured soil and water physical and chemical properties to explore potential drivers. We observed higher oxygen consumption rates for seawater incubated with soils/sediments from frequently inundated locations and higher carbon dioxide production for seawater incubated with soils from rarely inundated transect locations. Incubations with soil from rarely inundated sites produced the highest global warming potential, primarily driven by carbon dioxide and secondarily by nitrous oxide. We also found environmental drivers of gas rates varied notably between transect locations. Our findings indicate that seawater responses to soil and sediment inputs across coastal terrestrial-aquatic interfaces exhibit some consistent patterns and high intra- and inter-site variability, suggesting potential biogeochemical feedback loops as inundation regimes shift inland. 

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2. Epipelagic nitrous oxide production offsets carbon sequestration by the biological pump

   https://doi.org/10.1038/s41561-022-01090-2  

   Wan, Xianhui S; Sheng, Hua-Xia; Dai, Minhan; Casciotti, Karen L; Church, Matthew J; Zou, Wenbin; Liu, Li; Shen, Hui; Zhou, Kuanbo; Ward, Bess B; et al (January 2023, Nature Geoscience)

   Abstract The removal of carbon dioxide from the atmosphere by the marine biological pump is a key regulator of Earth’s climate; however, the ocean also serves as a large source of nitrous oxide, a potent greenhouse gas and ozone-depleting substance. Although biological carbon sequestration and nitrous oxide production have been individually studied in the ocean, their combined impacts on net greenhouse forcing remain uncertain. Here we show that the magnitude of nitrous oxide production in the epipelagic zone of the subtropical ocean covaries with remineralization processes and thus acts antagonistically to weaken the radiative benefit of carbon removal by the marine biological pump. Carbon and nitrogen isotope tracer incubation experiments and nitrogen isotope natural abundance data indicate enhanced biological activity promotes nitrogen recycling, leading to substantial nitrous oxide production via both oxidative and reductive pathways. These shallow-water nitrous oxide sources account for nearly half of the air–sea flux and counteract 6–27% (median 9%) of the greenhouse warming mitigation achieved by carbon export via the biological pump. 

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3. Instability in the geological regulation of Earth’s climate

   https://doi.org/10.1126/science.adh7730  

   Hülse, Dominik; Ridgwell, Andy (September 2025, Science)

   Negative feedback between climate and atmospheric carbon dioxide (CO₂), mediated by the weathering of silicate minerals on land, is thought to provide the primary regulation of Earth’s climate on geological timescales. By contrast, we found that faster feedbacks involving organic matter are not only critical to Earth system recovery but can also create unexpected instability. Our Earth system model experiments show how sedimentary organic carbon burial, amplified by redox-sensitive phosphorus regeneration, can outweigh silicate weathering and paradoxically drive climate overcooling in response to massive CO₂release. This instability depends on the initial balance between silicate weathering and organic carbon burial in addition to the state of global phosphorus cycling. It is most strongly expressed at intermediate ocean redox states, which may help us understand the timing of past ice ages. 

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4. Arctic Permafrost Thawing Enhances Sulfide Oxidation

   https://doi.org/10.1029/2022GB007644  

   Kemeny, Preston Cosslett; Li, Gen K.; Douglas, Madison; Berelson, William; Chadwick, Austin J.; Dalleska, Nathan F.; Lamb, Michael P.; Larsen, William; Magyar, John S.; Rollins, Nick E.; et al (November 2023, Global Biogeochemical Cycles)

   Abstract Permafrost degradation is altering biogeochemical processes throughout the Arctic. Thaw‐induced changes in organic matter transformations and mineral weathering reactions are impacting fluxes of inorganic carbon (IC) and alkalinity (ALK) in Arctic rivers. However, the net impact of these changing fluxes on the concentration of carbon dioxide in the atmosphere (pCO₂) is relatively unconstrained. Resolving this uncertainty is important as thaw‐driven changes in the fluxes of IC and ALK could produce feedbacks in the global carbon cycle. Enhanced production of sulfuric acid through sulfide oxidation is particularly poorly quantified despite its potential to remove ALK from the ocean‐atmosphere system and increasepCO₂, producing a positive feedback leading to more warming and permafrost degradation. In this work, we quantified weathering in the Koyukuk River, a major tributary of the Yukon River draining discontinuous permafrost in central Alaska, based on water and sediment samples collected near the village of Huslia in summer 2018. Using measurements of major ion abundances and sulfate () sulfur (³⁴S/³²S) and oxygen (¹⁸O/¹⁶O) isotope ratios, we employed the MEANDIR inversion model to quantify the relative importance of a suite of weathering processes and their net impact onpCO₂. Calculations found that approximately 80% of in mainstem samples derived from sulfide oxidation with the remainder from evaporite dissolution. Moreover,³⁴S/³²S ratios,¹³C/¹²C ratios of dissolved IC, and sulfur X‐ray absorption spectra of mainstem, secondary channel, and floodplain pore fluid and sediment samples revealed modest degrees of microbial sulfate reduction within the floodplain. Weathering fluxes of ALK and IC result in lower values ofpCO₂over timescales shorter than carbonate compensation (∼10⁴ yr) and, for mainstem samples, higher values ofpCO₂over timescales longer than carbonate compensation but shorter than the residence time of marine (∼10⁷ yr). Furthermore, the absolute concentrations of and Mg²⁺in the Koyukuk River, as well as the ratios of and Mg²⁺to other dissolved weathering products, have increased over the past 50 years. Through analogy to similar trends in the Yukon River, we interpret these changes as reflecting enhanced sulfide oxidation due to ongoing exposure of previously frozen sediment and changes in the contributions of shallow and deep flow paths to the active channel. Overall, these findings confirm that sulfide oxidation is a substantial outcome of permafrost degradation and that the sulfur cycle responds to permafrost thaw with a timescale‐dependent feedback on warming. 

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5. Principal role of fungi in soil carbon stabilization during early pedogenesis in the high Arctic

   https://doi.org/10.1073/pnas.2402689121  

   Trejos-Espeleta, Juan Carlos; Marin-Jaramillo, Juan P; Schmidt, Steven K; Sommers, Pacifica; Bradley, James A; Orsi, William D (July 2024, Proceedings of the National Academy of Sciences)

   Climate warming is causing widespread deglaciation and pioneer soil formation over glacial deposits. Melting glaciers expose rocky terrain and glacial till sediment that is relatively low in biomass, oligotrophic, and depleted in nutrients. Following initial colonization by microorganisms, glacial till sediments accumulate organic carbon and nutrients over time. However, the mechanisms driving soil nutrient stabilization during early pedogenesis after glacial retreat remain unclear. Here, we traced amino acid uptake by microorganisms in recently deglaciated high-Arctic soils and show that fungi play a critical role in the initial stabilization of the assimilated carbon. Pioneer basidiomycete yeasts were among the predominant taxa responsible for carbon assimilation, which were associated with overall high amino acid use efficiency and reduced respiration. In intermediate- and late-stage soils, lichenized ascomycete fungi were prevalent, but bacteria increasingly dominated amino acid assimilation, with substantially decreased fungal:bacterial amino acid assimilation ratios and increased respiration. Together, these findings demonstrate that fungi are important drivers of pedogenesis in high-Arctic ecosystems that are currently subject to widespread deglaciation from global warming. 

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