BG - recent papers

Point-scale organic-matter decomposition in streambeds is weakly associated with reach-scale respiration

2026-06-17T18:28:55+02:00

Point-scale organic-matter decomposition in streambeds is weakly associated with reach-scale respiration
James C. Stegen, Morgan Barnes, Dillman Delgado, Brieanne Forbes, Vanessa A. Garayburu-Caruso, Amy E. Goldman, Maggi Laan, Sophia McKever, Peter Regier, Lupita Renteria, and Scott D. Tiegs
Biogeosciences, 23, 3981–3993, https://doi.org/10.5194/bg-23-3981-2026, 2026

Stream and river ecosystems play a central role in the movement and decomposition of particulate organic matter, serving as a conduit between terrestrial hillslopes and coastal environments. Microbe-catalyzed decomposition generates simpler organic molecules that fuel respiration, often in the sediments of these ecosystems. However, the degree of connection between sediment-associated respiration (ER_sed) and organic-matter-decomposition potential remains poorly understood. It is also unclear whether organic-matter-decomposition potential is more closely associated with ER_sed, whole-ecosystem respiration (ER_tot), or water-column respiration (ER_wc). We examined the link between particulate organic-matter-decomposition potential – using cellulose-based cotton strips as a standardized substrate – and all three components of respiration across 48 sites in the environmentally diverse Yakima River Basin (Washington State, USA). We hypothesized that decomposition within sediments would be most strongly related to ER_sed, but decomposition rates were more closely associated with ER_tot, less so with ER_sed and not at all with ER_wc. This suggests that point-scale particulate organic-matter-decomposition potential within stream/river sediments is more closely associated with integrated system respiration rather than with processes confined to sediments or the water column alone though these relationships were weak overall. Further, across the basin, decomposition rates nearly spanned the previously reported global range for streams and rivers and were best explained by total dissolved nitrogen (TDN), sediment grain size, and aridity of the upstream drainage area. These results highlight the strong influence of land cover and basin-scale biophysical variation on sediment-associated decomposition processes and indicate that mechanistic models of organic-matter decomposition in streams and rivers should account for coupled sediment–water–land interactions.

Surface area and Ω-aragonite oversaturation as controls of the runaway precipitation process in ocean alkalinity enhancement

2026-06-16T18:28:55+02:00

Surface area and Ω-aragonite oversaturation as controls of the runaway precipitation process in ocean alkalinity enhancement
Niels Suitner, Jens Hartmann, Selene Varliero, Giulia Faucher, Philipp Suessle, and Charly A. Moras
Biogeosciences, 23, 3965–3980, https://doi.org/10.5194/bg-23-3965-2026, 2026

Ocean alkalinity enhancement (OAE) is a strategy for marine carbon dioxide removal that aims to increase the total alkalinity (TA) of seawater to sequester atmospheric CO₂ in the form of dissolved inorganic carbon (DIC). An intense alkalinization of seawater resulting from OAE treatment could trigger a significant runaway carbonate precipitation process, which may lead to a loss of initially added TA, thereby limiting its efficiency. Even under natural background aragonite saturation states, a continuous yet barely detectable loss of TA is theoretically expected to occur in seawater. With the additional increase through OAE, time ranges to initiate an appreciable TA-loss process could be reduced significantly. Therefore, predicting the TA stability ranges might be a necessity for application scenarios. The main drivers of the precipitation process are (i) the aragonite saturation state of seawater and (ii) the available surface area for heterogeneous precipitation.

In this study, we refined the use of logistic functions to describe the temporal evolution of both drivers, with experimental datasets using natural seawater from the Raunefjorden (Bergen, Norway; Temp.: ∼11 °C, Sal.: ∼32.6). The observed patterns were then used to derive a process-based model for calculating TA-loss rates, focusing on the accelerated precipitation phase of the runaway process while considering saturation levels and available particle surface area. The formation of carbonate phases reduces seawater TA concentrations, inducing a delay or halting the TA-loss process. In addition, the sinking of precipitated particles decreases the potential for further precipitation by reducing the available surface area in the system. To assess the impact of particle sinking on TA-loss, their shape and size distribution were determined. Under the environmental conditions presented here, TA-loss rates could be reduced by up to 30 %–40 % due to the sinking of particles, after just one day.

Integrating the proposed concepts into ocean models could enhance the accuracy of predictions regarding the fate of added TA. Gaining insights into the evolution of the identified, seemingly stable TA levels can help prevent accelerated precipitation phases. Additionally, an understanding of particle sinking or dilution processes, reducing the available reactive particle surface area, is relevant to assess the efficacy and durability of OAE.

In situ production of hybrid N₂O in dust-rich Antarctic ice

2026-06-15T18:28:55+02:00

In situ production of hybrid N2O in dust-rich Antarctic ice
Lison Soussaintjean, Jochen Schmitt, Joël Savarino, J. Andy Menking, Edward J. Brook, Barbara Seth, Vladimir Lipenkov, Thomas Röckmann, and Hubertus Fischer
Biogeosciences, 23, 3939–3963, https://doi.org/10.5194/bg-23-3939-2026, 2026

Nitrous oxide (N₂O) is a potent greenhouse gas involved in the destruction of stratospheric ozone. Past atmospheric mixing ratios of N₂O are archived in ice cores; however, the presence of in situ N₂O production in dust-rich Antarctic ice complicates their accurate reconstruction, especially during glacial periods. This production occurs in extremely cold ice and without sunlight. This study aims to understand the reaction producing N₂O in Antarctic ice by identifying the precursors and the reaction pathway. We compared the oxygen and nitrogen bulk and position-specific isotope composition of in situ N₂O in ice cores to the isotopic composition of nitrate ( ${NO}_{3}^{-}$ ), a possible precursor of N₂O. The ¹⁵N signature of ${NO}_{3}^{-}$ is fully transferred into the central N atom (N^α) of in situ N₂O, but it is not transferred into the terminal N atom (N^β), resulting in a 50 % transfer of the ¹⁵N signature of ${NO}_{3}^{-}$ into the bulk ¹⁵N isotopic composition. These findings suggest that the in situ N₂O production involves two different nitrogen precursors present in ice: the central N atom (N^α) originates from ${NO}_{3}^{-}$ and the terminal N atom (N^β) from a different precursor not yet identified. Oxygen isotope analysis shows that ${NO}_{3}^{-}$ cannot be the only reservoir for the O atom of in situ N₂O. Temperature, pH, and absence of sunlight in Antarctic ice point to an abiotic N-nitrosation reaction. The limiting factor of the reaction is probably associated with mineral dust and might be Fe²⁺, reducing ${NO}_{3}^{-}$ to ${NO}_{2}^{-}$ or the precursor of the N^β atom. The site preference (SP) values of in situ N₂O are highly variable between different ice cores and depend on the bulk ¹⁵N isotopic composition of N₂O, itself depending on the ¹⁵N isotopic composition of the ${NO}_{3}^{-}$ precursor. This finding is unexpected because SP is usually determined by the production pathway through symmetric reaction intermediates that mix the N atoms in α and β positions and average out their isotopic difference. In contrast, our results provide the first evidence of a hybrid N₂O production pathway involving an asymmetric intermediate that preserves the distinct ¹⁵N signatures of two different precursors – one contributing to the N^α atom and the other to the N^β atom.

Rapid soil degradation following deforestation in Eastern Africa

2026-06-15T18:28:55+02:00

Rapid soil degradation following deforestation in Eastern Africa
Laura Summerauer, Fernando Bamba, Bendicto Akoraebirungi, Ahurra Wobusobozi, Marijn Bauters, Travis William Drake, Negar Haghipour, Clovis Kabaseke, Daniel Muhindo, Landry Cizungu Ntaboba, Leonardo Ramirez-Lopez, Johan Six, Daniel Wasner, and Sebastian Doetterl
Biogeosciences, 23, 3907–3938, https://doi.org/10.5194/bg-23-3907-2026, 2026

Deforestation for cropland expansion in tropical sloping landscapes causes severe soil erosion and thus the loss of fertile, organic rich topsoil. Whether there is variation in the effect of land degradation on tropical soils developed from different parent materials, which may influence soil fertility is still largely unknown. Here, we compared SOC and other soil fertility indicators in undisturbed tropical forest topsoils with cleared hillslope topsoils (cropland, abandoned cropland, and reforestation with Eucalyptus monocultures) along the East African rift system using soil chronosequences after deforestation on both mafic and felsic parent material. In the mafic region, we found a consistent decrease of SOC, nitrogen, and phosphorus content with time after deforestation (relative changes of contents up to −69 % SOC, −72 % nitrogen, and −92 % phosphorus). SOC was strongly stabilized by reactive metal phases with little to no benefits to general soil fertility. Consequently, cropland was frequently abandoned by farmers due to the combination of low pH, high Al³⁺ mobility, and low available nutrient status at a relatively high average SOC content of 14–29 g kg⁻¹ in topsoils. In the felsic region, the ameliorating effect of mid-Holocene carbonate volcanism mitigated soil degradation to some extent. In both geochemical regions, SOC content did not or only weakly positively correlate with clay content and cation exchange capacity. These results emphasize that soil organic matter, as well as clay content, appears to be unreliable indicators for soil fertility in degraded tropical cropland soils. Additionally, no significant improvement of soil fertility or SOC stocks was observed after replanting degraded fields with Eucalyptus monocultures. The estimated lifespan of croplands on hillslopes in our study area, approximately 145±56 years, underscores the severity of soil degradation for food production and forest protection in the upcoming decades, especially considering that many soils are already approaching the end of this estimated lifespan.

Denitrification as the dominant process in nitrous oxide production in the water column of two eutrophic reservoirs

2026-06-12T18:28:55+02:00

Denitrification as the dominant process in nitrous oxide production in the water column of two eutrophic reservoirs
Elizabeth Leon-Palmero, Claudia Frey, Bess B. Ward, Rafael Morales-Baquero, and Isabel Reche
Biogeosciences, 23, 3887–3905, https://doi.org/10.5194/bg-23-3887-2026, 2026

Reservoirs are important sites for nitrogen cycling and a significant global source of the potent greenhouse gas nitrous oxide (N₂O). They receive nitrogen inputs from agriculture and urban sources, fueling N₂O production via nitrification, denitrification, and photochemodenitrification. However, existing estimates of N₂O production in reservoirs remain uncertain because most studies have focused on N₂O in rivers or lake sediments, often overlooking the water column of lentic systems. Here, we present the first integrated assessment of N₂O production pathways in reservoir water columns using stable isotope tracer incubations alongside analyses of in situ natural abundance of nitrogen pools and functional genes involved in nitrification (amoA) and denitrification (nirS), across two eutrophic reservoirs with contrasting morphometries. We used ¹⁵N- ${NH}_{4}^{+}$ and ¹⁵N- ${NO}_{3}^{-}$ tracers to quantify rates of N₂O production, nitrification, and nitrate reduction at the beginning and the end of the stratification period. Notably, nitrate concentration decreased by up to 49 % over the two months. N₂O production from ammonium ranged from 0.02 to 48.6 nmol N L⁻¹ d⁻¹, while N₂O production from nitrate varied from 0.2 to 61.0 $nmol N L^{- 1} d^{- 1}$ . High rates of nitrification, nitrate reduction to nitrite, and rapid nitrite turnover were observed, with total N₂O production significantly correlated with nirS gene abundance. A strong positive correlation was found between δ¹⁵N- ${NO}_{2}^{-}$ and both N₂O concentration and nirS abundance. These findings reveal that denitrification and nitrite dynamics play a central role in N₂O formation within reservoir water columns, advancing understanding of nitrogen loss and greenhouse gas emissions from lentic systems.

Understanding the resilient carbon cycle response to the 2014–2015 Blob event in the Gulf of Alaska using a regional ocean biogeochemical model

2026-06-12T18:28:55+02:00

Understanding the resilient carbon cycle response to the 2014–2015 Blob event in the Gulf of Alaska using a regional ocean biogeochemical model
Yumi Abe, Takamitsu Ito, Amanda H. V. Timmerman, Christopher T. Reinhard, and Joseph P. Montoya
Biogeosciences, 23, 3871–3885, https://doi.org/10.5194/bg-23-3871-2026, 2026

Marine heatwaves (MHWs), characterized by anomalously high sea surface temperatures, are increasing in frequency and intensity and strongly impact ocean circulation, biogeochemistry, and marine ecosystems. During the 2014–2015 MHW (commonly called the Blob) in the NE subarctic Pacific, moored observations at Ocean Station Papa (OSP; 145° W, 50° N) showed a moderate decrease in oceanic pCO₂, contrary to the increase expected from warming-induced solubility reduction alone. Using a regional model that reproduces the observed pCO₂ variability and trend at OSP, we show that this decline resulted from a decrease in dissolved inorganic carbon (DIC) supply that outweighed the warming-driven increase in pCO₂. The DIC reduction was primarily caused by weakened vertical transport associated with enhanced upper-ocean stratification and reduced Ekman pumping prior to the onset of the Blob, which suppressed the upwelling of DIC-rich subsurface waters. Horizontal transport also contributed locally, particularly at OSP. These results demonstrate that anomalous physical circulation, rather than biological processes, was the primary driver of the enhanced CO₂ uptake during the Blob and highlight the importance of resolving physical transport mechanisms when assessing carbon cycle responses to extreme warming events.

Global quantification of the eco-hydrological co-benefits of soil carbon sequestration

2026-06-11T18:28:55+02:00

Global quantification of the eco-hydrological co-benefits of soil carbon sequestration
Inne Vanderkelen, Marie-Estelle Demory, Sean Swenson, David M. Lawrence, Benjamin D. Stocker, Myke Koopmans, and Édouard L. Davin
Biogeosciences, 23, 3829–3854, https://doi.org/10.5194/bg-23-3829-2026, 2026

Soil carbon sequestration is an important strategy for climate change mitigation with several co-benefits, including increased water holding capacity and infiltration. However, a global-scale quantification of hydrological co-benefits for water availability to plants is still lacking. In this study, we investigate the effect of soil carbon sequestration on hydrology and water resources by conducting experiments with the Community Terrestrial Systems Model (CTSM). Using global experiments with spatially explicit soil organic carbon (SOC), we apply various soil carbon sequestration scenarios, including one aligned with the “4 per 1000” initiative, to investigate the effect on soil moisture and soil water balance variables with a focus on cropland regions. Our results show that soil organic carbon redistributes water within the soil profile, retaining moisture in the rooting zone and limiting percolation into deeper layers, which is particularly pronounced in relatively arid regions with sandy soils. Global average soil water content increases by 4 mm in the first 30 cm under a scenario with a uniform SOC increase of 5.5 gC kg⁻¹ soil. Carbon sequestration also redistributes the mean annual soil water balance, with global mean reductions in surface runoff (−1 mm yr⁻¹), subsurface runoff (−0.6 mm yr⁻¹), and an increase in evapotranspiration (+2 mm yr⁻¹), contributing to improved vegetation productivity. Water stress is overall reduced across most regions. Although the hydrological impacts of soil carbon sequestration are generally small in magnitude, they are consistent and systematic. The relative changes following realistic and policy-relevant SOC enhancement scenarios, such as those under the 4 per 1000 initiative, are limited due to the modest carbon additions involved. Nevertheless, these changes offer measurable eco-hydrological co-benefits that may support both climate mitigation and ecosystem resilience, particularly in water-limited environments.

Bomb-radiocarbon signal suggests that soil carbon contributes to chlorophyll a in archival oak leaves

2026-06-11T18:28:55+02:00

Bomb-radiocarbon signal suggests that soil carbon contributes to chlorophyll a in archival oak leaves
Naoto F. Ishikawa, Hisami Suga, Tessa S. van der Voort, Reto Nyffeler, Nanako O. Ogawa, Negar Haghipour, Lukas Wacker, Timothy I. Eglinton, and Naohiko Ohkouchi
Biogeosciences, 23, 3855–3869, https://doi.org/10.5194/bg-23-3855-2026, 2026

Carbon exchange between biosphere and rhizosphere is an important component of the global carbon cycle. Photosynthetic products being sequestered into soils have been intensively studied, yet the reverse pathway from rhizosphere to biosphere is poorly known. In the present study, we determined the radiocarbon content (Δ¹⁴C) of the bulk leaves of the deciduous Quercus oak and of chlorophyll a (Chl a) extracted from the same leaves collected in Switzerland during the 1950s and 2000s. Our results demonstrate that old soil-derived carbon significantly contributes to the synthesis of Chl a, an essential molecule for photoautotrophs. The Δ¹⁴C values of Chl a were consistently lower than those of bulk leaves which closely tracked bomb-derived Δ¹⁴C signals in the atmosphere. The results cannot be explained without invoking an additional carbon source with a turnover time exceeding 100 years. A two-pool mixing model assuming atmosphere and rhizosphere as two endmembers indicates that contributions of the soil carbon to Chl a are 17 ± 2 % (n=4), and turnover time of such soil carbon is no shorter than 1000 years. We suggest that hydrophilic compounds such as amino acids or phytol are transferred into plant roots from soils through mycorrhizal symbionts, and Chl a is one of the destinations of such ¹⁴C-depleted carbon in vascular plants.

Diatom–environment relationships and limnological variability: an updated quantitative tool for palaeoclimatology on sub-Antarctic Macquarie Island

2026-06-11T18:28:55+02:00

Diatom–environment relationships and limnological variability: an updated quantitative tool for palaeoclimatology on sub-Antarctic Macquarie Island
Caitlin A. Selfe, Karina Meredith, Liza McDonough, Justine Shaw, Stephen J. Roberts, and Krystyna M. Saunders
Biogeosciences, 23, 3807–3827, https://doi.org/10.5194/bg-23-3807-2026, 2026

Sub-Antarctic Macquarie Island is ideally located for reconstructing past variations in Southern Hemisphere westerly wind strength. Diatoms are a valuable palaeolimnological tool on sub-Antarctic islands, providing a means to reconstruct past climate and environmental changes. Diatom communities are sensitive to changes in lake electrical conductivity (EC) linked to westerly wind–driven sea-spray inputs on Macquarie Island, and diatom–conductivity models have previously been used to infer past westerly wind variability. Here we present new diatom data from 52 lakes to assess diatom–environment relationships and develop an updated diatom–conductivity model for Macquarie Island. Seasonal and multi-year water chemistry and isotope data were analysed to assess temporal variability in hydrochemical processes and the influence of evaporation, ensuring the resulting diatom-conductivity model reflects external climatic drivers rather than local dynamics. Statistically robust transfer functions were developed for EC (bootstrapped r² = 0.80, RMSEP = 0.40), while pH had weaker predictive performance. For EC, weighted averaging and maximum-likelihood approaches performed comparably, although the former showed reduced predictive power at high EC where low species turnover and nutrient collinearity affected accuracy. This quantitative-diatom model combined with understanding of hydrogeochemical processes provides an improved basis for reconstructing past Southern Hemisphere westerly wind variability, which can be applied in future palaeoclimate studies on Macquarie Island.

Addition of brackish water to tundra soils does not inhibit methane production: implications for Arctic coastal methane production

2026-06-10T18:28:55+02:00

Addition of brackish water to tundra soils does not inhibit methane production: implications for Arctic coastal methane production
Alexie Roy-Lafontaine, Rebecca Lee, Peter M. J. Douglas, Dustin Whalen, and André Pellerin
Biogeosciences, 23, 3777–3792, https://doi.org/10.5194/bg-23-3777-2026, 2026

In Arctic regions where coastal sediments contain permafrost, global climate change drives processes such as erosion and subsidence. The contribution of these processes to carbon emissions, especially from ground subsidence, are still uncertain. Relative sea level rise can lead to more waterlogged environments, promoting anoxic degradation of organic matter but it can also lead to a greater exposure of coastal sediments to seawater. This could alter methane (CH₄) production dynamics, although the controls remain poorly understood. For instance, sulfates contained in seawater may have a tampering effect on methanogenesis through competitive inhibition but the increase in microbial abundance could enhance methanogenesis. In this study, we present CH₄ production rates alongside geochemical analyses in a rapidly evolving coastal landscape near the community of Tuktoyaktuk, NWT, Canada, which is located in the continuous permafrost zone. To better constrain CH₄ production dynamics along the land to ocean continuum, sediment cores were collected from nearshore marine sediments and soil profiles were collected from the active layer of the coastal (intertidal) zone and inland soils. Anoxic incubations were performed, amended with brackish water to simulate the effect of seawater on the breakdown of organic matter and the production of CH₄. We found marine sediments expectedly led to negligible CH₄ production rates, while the inland sites showed variable rates between null and 35 nmol cm⁻³ d⁻¹. The coastal (intertidal) zone had the highest rates reaching 415 nmol cm⁻³ d⁻¹. Interestingly, sulfate present in brackish water and sediments did not suppress methanogenesis in the incubations of the coastal and inland zones. Analyses of stable carbon isotopes from CH₄ produced in the incubation experiment indicated greater acetotrophy and higher organic matter lability in the coastal zone, possibly contributing to higher CH₄ production rates. This study highlights the potential for significant CH₄ emissions even with high sulfate concentrations which are classically thought to inhibit methanogenesis. This suggests that Arctic coastal microbial CH₄ production might be an understudied source to the atmosphere.

Snowmelt timing constrains green-up but not peak productivity in alpine grasslands of the western Pyrenees

2026-06-10T18:28:55+02:00

Snowmelt timing constrains green-up but not peak productivity in alpine grasslands of the western Pyrenees
Pablo Domínguez-Aguilar, Jesús Revuelto, Simon Gascoin, and Juan I. López-Moreno
Biogeosciences, 23, 3793–3806, https://doi.org/10.5194/bg-23-3793-2026, 2026

It is now well established that climate change modifies the snow cover regime in European mountains. However, the impact of snow cover changes on alpine ecosystems is not well understood. In this study, we assess how snowmelt timing affects alpine grassland growth (onset and evolution) in the western Spanish Pyrenees using eight years (2018–2025) of Sentinel-2 imagery at 10 m resolution. We combined satellite-derived snow melt-out dates (SMOD) with NDVI time series, meteorological data, and fractional snow-covered area (fSCA) to evaluate the temporal and spatial relationships between snowmelt timing and vegetation greening. Our results confirm that snowmelt consistently influences the onset of greening and regulates the timing of peak NDVI (annual maximum). However, short-term variations in melt-out timing had limited influence on the intensity of peak NDVI, which was more strongly linked to post-melt meteorological conditions. The spatial pattern of peak NDVI remained stable across years despite variable melt timing. Our observations suggest that site-specific characteristics – such as soil properties, microclimate, and vegetation composition – that can be linked to the long-term legacy of snow presence can exert a stronger influence on productivity than year-to-year snowmelt dynamics.

Ideas and perspectives: Max MACS – constraining the potential global scale of Marine Anoxic Carbon Storage for CO₂ removal

2026-06-08T18:28:55+02:00

Ideas and perspectives: Max MACS – constraining the potential global scale of Marine Anoxic Carbon Storage for CO2 removal
Morgan Reed Raven, Nitai Amiel, Dror L. Angel, James P. Barry, Thomas M. Blattmann, Laura Boicenco, Antoine Crémière, Natalya Evans, Nora Gallarotti, Sebastian Haas, Jan-Hendrik Hehemann, Peter Krost, Pranay Lal, David Lordkipanidze, Tiia Luostarinen, Aaron M. Martinez, Allison J. Matzelle, Selma Menabit, Mihaela Muresan, Andreas Neumann, Jean-Daniel Paris, Christopher R. Pearce, Nick Reynard, Daniel L. Sanchez, Florence Schubotz, Violeta Slabakova, Adrian Stanica, Elena Stoica, Andrew K. Sweetman, Tina Treude, Yoana G. Voynova, and Nikolaos D. Zarokanellos
Biogeosciences, 23, 3755–3776, https://doi.org/10.5194/bg-23-3755-2026, 2026

Marine Anoxic Carbon Storage (MACS) is a potential strategy for enhancing atmospheric CO₂ removal (CDR) by sequestering organic carbon produced by terrestrial plants in stable, anoxic marine reservoirs. Initial results suggest that MACS could, in theory, operate at the gigatonne scale that would be required to impact global climate, with limited environmental risk and promising opportunities for co-benefits. However, several outstanding knowledge gaps make it challenging to quantify the actual potential global scale of MACS with confidence. To inform decisions about climate mitigation and trade-offs in the future, it is essential that we know how MACS implementation at scale would impact critical environmental and economic systems in the context of likely future scenarios.

Building on the results of a workshop in Bucharest, Romania in 2025, we discuss the potential impacts of MACS activities on the ecology, biogeochemistry, economy, and community around the Black Sea, seafloor brines, and other anoxic marine sites. Quantifiable limits to the potential maximum feasible scale of MACS for CDR are organized into five criteria: (1) Durable storage site capacity; (2) Biomass sources and logistics; (3) Greenhouse gas balance; (4) Oxygen and sulfide impacts at the redoxcline; and (5) Impacts on dissolved organic matter or nutrients in the oxic zone. For each criterion, we evaluate the factors that could limit scale, our current state of knowledge, and the priority knowledge gaps that, if addressed, would improve our ability to estimate the potential global scale of MACS for CDR. Research is needed to understand its potential impacts at scale, but MACS is nonetheless worthy of serious consideration as a potential pathway for climate mitigation in coming decades.

The impact of large-scale macroalgae cultivation and harvesting strategies on the marine carbon dioxide removal efficacy and marine biogeochemistry

2026-06-05T18:28:55+02:00

The impact of large-scale macroalgae cultivation and harvesting strategies on the marine carbon dioxide removal efficacy and marine biogeochemistry
Prima Anugerahanti, Julien Palmiéri, Chelsey A. Baker, Ekaterina Popova, and Andrew Yool
Biogeosciences, 23, 3735–3754, https://doi.org/10.5194/bg-23-3735-2026, 2026

The large-scale cultivation of macroalgae has been proposed as a marine carbon dioxide removal (mCDR) strategy, yet its efficiency and consequences for ocean biogeochemistry remain uncertain. Using a new macroalgae aquaculture module within an ocean biogeochemistry model, NEMO-MEDUSA, we investigate carbon removal potential and biogeochemical feedbacks under hypothetical global-scale macroalgae cultivation with varying harvest strategies, loss rates, and iron availability. Overall cultivation enhances air–sea CO₂ uptake by 11.0 Pg C yr⁻¹, but only ∼ 27 % of macroalgal production results in additional CO₂ uptake. Furthermore, phytoplankton and zooplankton biomass is suppressed by almost 50 % and is geographically displaced by significant surface nutrient changes. Sinking of harvested biomass increases oxygen demand during remineralisation, leading to widespread oxygen depletion and the emergence of suboxic conditions at the seafloor in deposition regions. When macroalgal growth is not supplemented with iron micronutrient, its production declines sharply (−74 %), revealing a significant limitation for large-scale feasibility. Collectively, our results reveal that large-scale macroalgal cultivation offers low mCDR potential, that it is both spatially extensive and locally intensive, and its unintended biogeochemical consequences can be substantial. Our findings highlight the urgent need to assess nutrient constraints and ecological trade-offs before considering this method as a viable large-scale mCDR strategy.

Temperature dependence of the contribution of soil water content to soil respiration in a monsoon influenced temperate deciduous forest

2026-06-04T18:28:55+02:00

Temperature dependence of the contribution of soil water content to soil respiration in a monsoon influenced temperate deciduous forest
Dongmin Seo, Minyoung Lee, Jaeho Lee, and Jaeseok Lee
Biogeosciences, 23, 3723–3734, https://doi.org/10.5194/bg-23-3723-2026, 2026

Soil respiration (R_s) in forest soils is a key flux governing forest carbon balance and the global carbon cycle. Because this flux is expected to respond rapidly to climate warming, understanding the controls on R_s is essential for predicting changes in forest carbon balance induced by warming. In natural field conditions, soil temperature (T_s) and soil water content (SWC) often covary seasonally, which tends to limit our ability to isolate and quantify the independent contribution of SWC and to evaluate how its contribution varies with temperature. Although changes in the R_s response to temperature have been reported, few studies have quantitatively identified such patterns from field observations and examined whether the relative contribution of SWC becomes more evident across temperature conditions. Here, we used two years of continuous automated chamber measurements in a temperate deciduous forest to assess how the relative contribution of SWC varies with T_s by comparing models across temperature ranges, and to evaluate whether a breakpoint occurs in the R_s response to T_s and whether that breakpoint is observed near the temperature range where the contribution of SWC increases. At the annual scale, the explanatory power of SWC alone was limited, but the relationship between SWC and R_s was significant. In contrast, above 15 °C, the relationship between SWC and R_s strengthened consistently, indicating that the relative contribution of SWC became more evident under warm soil conditions. Piecewise regression of the relationship between R_s and T_s identified a breakpoint near 17 °C, and models including this breakpoint improved fit relative to an exponential model. These results suggest a possible difference in the relative importance of controls on R_s across temperature conditions. Therefore, projections of forest R_s may benefit from considering temperature dependent changes in the contribution of SWC, particularly near the temperature range where this contribution becomes more evident.

Sedimentary insights into organic matter alteration in Arctic Alaska's saline permafrost

2026-06-03T18:28:55+02:00

Sedimentary insights into organic matter alteration in Arctic Alaska's saline permafrost
Fabian Seemann, Michael Zech, Maren Jenrich, Guido Grosse, Benjamin M. Jones, Claire Treat, Lutz Schirrmeister, Susanne Liebner, and Jens Strauss
Biogeosciences, 23, 3675–3695, https://doi.org/10.5194/bg-23-3675-2026, 2026

In Arctic coastal lowland regions such as northernmost Alaska, thermokarst landscapes are often underlain by saline deposits, a factor frequently overlooked when assessing permafrost thaw risks. To evaluate the influence of thaw and salinity on organic matter degradation and landscape dynamics, we analyzed six sediment cores from representative landforms near Utqiaġvik (Alaska) using a multiproxy, carbon-focused approach, with emphasis on n-alkane biomarkers. Undisturbed tundra uplands contained well-preserved, organic-rich Holocene sediments (∼140 cm thick) overlying brackish late Pleistocene deposits, indicating the presence of saline permafrost. Thermokarst lake subsidence into these substrates led to enhanced organic matter degradation, as reflected by lower n-alkane carbon preference index (CPI) values. While West Twin Lake talik sediments exhibited brackish porewater, East Twin Lake sediments were characterized by predominantly saline porewater, indicating the presence of a cryopeg driven by salt-induced thaw-point depression. Lagoonal environments, receiving both terrestrial and lacustrine inputs, accumulate sediments under unfrozen hypersaline conditions, presenting a high potential for organic matter decomposition. Carbon proxy signatures statistically distinguish perennially frozen uplands, unfrozen lake sediments, refrozen drained lake basins, and lagoonal settings. Our results demonstrate that salt-bearing deposits, as found in all investigated sites, are vulnerable to active layer deepening, talik and cryopeg formation, and lake/coastal shoreline erosion. These processes accelerate organic matter degradation and alter landscape trajectories. Our study underscores the need to better understand the role of saline permafrost in Arctic coastal lowlands and its broader implications under ongoing climate change.

Colored and fluorescent DOM in the sea-surface microlayer: response to a phytoplankton bloom and photodegradation in a mesocosm study

2026-06-03T18:28:55+02:00

Colored and fluorescent DOM in the sea-surface microlayer: response to a phytoplankton bloom and photodegradation in a mesocosm study
Claudia Thölen, Jochen Wollschläger, Michael G. Novak, Rüdiger Röttgers, and Oliver Zielinski
Biogeosciences, 23, 3697–3721, https://doi.org/10.5194/bg-23-3697-2026, 2026

A month-long mesocosm study at the Institute for Chemistry and Biology of the Marine Environment (Wilhelmshaven, Germany) examined how a phytoplankton bloom and photodegradation influence the composition of colored and fluorescent dissolved organic matter (CDOM and FDOM, respectively) in the sea-surface microlayer (SML) and underlying water (ULW). The SML, a thin (<1000 µm) interface between ocean and atmosphere, plays a key role in air-sea exchange processes, but temporal mechanisms behind organic matter enrichment remain unclear. To isolate biogeochemical processes from environmental variability, daily SML and ULW samples were analyzed using spectral fluorometric and photometric methods, with supporting data e.g. on irradiance, temperature, and chlorophyll-a. The study covered bloom onset, peak, and decay of two partially overlying phytoplankton blooms. Samples were taken alternatively in the morning and in the afternoon, varying the exposure time to UV-light. Changes in composition and quality of organic matter were tracked using CDOM/FDOM derived metrics. Changes in the FDOM component composition were investigated using PERMANOVA. The significant influence of the bloom phases and the layer (SML or ULW) on the component composition was confirmed, however, their interaction was not significant. Protein-like FDOM components increased in both layers during bloom progression, while humic-like FDOM components decreased throughout the study. It is likely that the change in FDOM component composition is a joint result of the influences of the phytoplankton bloom and photodegradation effects. Based on the slope ratio (SR) of CDOM absorption slopes S_275–295 and S_350–400, photodegradation was identified as the dominant sink of organic matter over microbial activity. While some CDOM/FDOM derived metrics indicated stronger photodegradation effects in the SML, a consistently enhanced photodegradation signal could not be conclusively confirmed due to co-occurring enrichment, passive accumulation, and degradation processes.

The impact of essential climate variables on respiration rates in subpolar and polar planktonic foraminifera

2026-05-29T18:28:55+02:00

The impact of essential climate variables on respiration rates in subpolar and polar planktonic foraminifera
Diane V. Armitage, Nicolaas Glock, Thomas L. Weiss, Mohamed M. Ezat, Adele Westgård, Freya E. Sykes, Julie Meilland, Elwyn de la Vega, Alessio Fabbrini, Tali L. Babila, and Audrey Morley
Biogeosciences, 23, 3655–3673, https://doi.org/10.5194/bg-23-3655-2026, 2026

This study investigates the impact of Essential Climate Variables (ECVs) on the respiration rate of polar planktonic foraminifera Neogloboquadrina pachyderma and subpolar Turborotalita quinqueloba and Neogloboquadrina incompta to advance our understanding of foraminifera physiology and geochemical proxy interpretation for species living in understudied subpolar and polar environments. Respiration rates were measured on a total of 158 specimens collected during two field campaigns to the Nordic Seas. To size-normalise respiration rates, we measured cavity volume and maximum diameter using x-ray microcomputed tomography (micro-CT) ( $\sqrt[3]{c} avity volume$ = (0.56 (max Ø) − 0.38)). Our results show that the physiological response of foraminifera sharing overlapping environments is diverse, with N. pachyderma exhibiting remarkable stability over large gradients in temperature, salinity, carbonate chemistry, dissolved oxygen and nutrients. Conversely, N. incompta and T. quinqueloba have a much stronger thermal response. The difference between species is best described by their respective Q₁₀ (the factor by which the rate of respiration changes with a 10 °C increase in temperature) values of 1.48 for N. pachyderma and 3.69 and 4.43 for N. incompta and T. quinqueloba, respectively. We also find a significant relationship between biovolume and respiration rate when rates are normalized to 4 °C (log₁₀ R_biovolume=0.40 (log₁₀ biovolume) − 0.80)) for all three species analysed here, which is consistent with marine protists globally. We conclude that respiration is unlikely to influence geochemical proxies and therefore past climate reconstructions derived from N. pachyderma, however, this may not apply to N. incompta and T. quinqueloba.

Shoreline exposure controls teal carbon accumulation in boreal lakes

2026-05-28T18:28:55+02:00

Shoreline exposure controls teal carbon accumulation in boreal lakes
Ana Lúcia Lindroth Dauner, Max O. A. Kankainen, Sakari Väkevä, Eero Asmala, Marko Järvinen, Karoliina Koho, and Tom Jilbert
Biogeosciences, 23, 3637–3653, https://doi.org/10.5194/bg-23-3637-2026, 2026

Aquatic vegetated ecosystems play an important role in global carbon sequestration. While research on coastal marine environments has expanded in recent decades, freshwater vegetated shorelines remain understudied despite their potential for significant carbon burial. This is especially relevant in boreal landscapes with high numbers of small, shallow lakes. In this study, we quantify organic carbon stocks (mass of carbon per area) in boreal lacustrine vegetated shorelines, so-called teal carbon environments. Moreover, we identified the main environmental drivers of carbon storage in these areas. We took 27 sediment cores from three large lakes in Finland with available satellite data of macrophyte coverage. At each site, sediment cores were sampled along a depth transect through macrophyte zones, from the landside towards the waterside. Sedimentary organic carbon (SOC) stocks ranged from 0 to 40.8 kg m⁻², and showed a large spatial variability among lakes, zones and type of vegetation. We identified grain size as the most significant parameter explaining variability in the size of SOC stocks. Sites dominated by silts and with large SOC stocks were found in sheltered embayments, independent of proximity to rivers, density of vegetation or slope of the shoreline, implying a strong control of exposure on SOC accumulation. In more exposed areas, vegetation density might play an additional controlling role in SOC accumulation. Accounting for shoreline exposure is crucial for improving regional carbon budget estimates. This study highlights the central role of teal carbon ecosystems in carbon cycling in the boreal zone, often characterized by very high densities of lakes.

Limited iron isotope variation among tissues of a marine fish: a case study of wild chub mackerel (Scomber japonicus)

2026-05-27T18:28:55+02:00

Limited iron isotope variation among tissues of a marine fish: a case study of wild chub mackerel (Scomber japonicus)
Nanako Hasegawa, Yoshio Takahashi, and Takaaki Itai
Biogeosciences, 23, 3605–3614, https://doi.org/10.5194/bg-23-3605-2026, 2026

Iron homeostasis in marine organisms operates under chronically low iron bioavailability, which may shape the strategies of iron uptake and storage in fish. Stable iron isotope ratios (δ⁵⁶Fe) have emerged as tracers of iron storage and uptake in terrestrial mammals, yet the physiological drivers of isotope fractionation in marine organisms remain poorly understood. Here, we investigated δ⁵⁶Fe variation and iron speciation across eight tissues of wild chub mackerel (Scomber japonicus), along with pool size estimation of key Fe species, including ferritin-bound (ferric) and heme-bound (mainly ferrous) Fe, using Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy. In all the specimens, the liver δ⁵⁶Fe values were higher than the average value of all tissues, with an apparent isotopic shift between ferritin- and heme-bound Fe (Δ⁵⁶Fe) in the liver averaging 2.72 ‰ ± 3.03 ‰ (2 S.D.). In contrast to the liver, no enrichment of heavy Fe isotope was observed in the ovary and red muscle despite their high ferritin-Fe contribution, suggesting a high interconversion rate between ferritin- and heme-bound Fe pools in these tissues. The overall range of δ⁵⁶Fe variation among tissues was smaller than that reported in mammals. Our results suggested that muscular δ⁵⁶Fe in marine teleost is primarily governed by source signatures and intestinal uptake efficiency, while tissue heterogeneity due to heavy Fe storage by ferritin exerts only a minor influence. These findings highlight the potential of δ⁵⁶Fe as a proxy for intestinal iron acquisition in fish and provide new geochemical perspectives on iron cycling through marine food webs.

Thawing Siberian permafrost stabilizes organic carbon from recent plant litter inputs

2026-05-27T18:28:55+02:00

Thawing Siberian permafrost stabilizes organic carbon from recent plant litter inputs
Christian Knoblauch, Christian Beer, and Carolina Voigt
Biogeosciences, 23, 3615–3635, https://doi.org/10.5194/bg-23-3615-2026, 2026

Greenhouse gas release due to microbial decomposition of thawing permafrost organic matter receives ample attention but the other side of the permafrost soil carbon budget, the stabilization of organic matter due to rising plant litter input in a greening Arctic has hardly been addressed. Here we explore whether thawing permafrost material may act as a long-term sink of fresh plant litter carbon. To identify the magnitude and drivers of litter carbon stabilization in thawing permafrost material, we incubated samples from the permafrost layer under oxic and anoxic conditions with ¹³C-labelled plant litter. Subsequently, we used the microbial CO₂ and CH₄ production from the added litter carbon (litter-C) and from the carbon in the thawed permafrost material (permafrost-C) to calibrate a carbon decomposition model with a fast and a slow carbon pool. Beside the size of the different pools, their mean residence times (MRT) were calculated as an indicator for carbon stabilization in these soils. Finally, we fractionated the remaining organic matter into a dissolved, a mineral-associated and a particulate fraction. At the end of the experiment, after nine years, on average 40 % to 60 % of the added litter-C persisted in the thawed permafrost material. The MRT of the slow litter-C pool of 18 years (oxic) and 52 years (anoxic) indicate a substantial stabilization of fresh litter-C over the course of the experiment. More than 80 % of the remaining litter-C was part of the mineral-associated fraction, but in contrast to current understanding, litter decomposability was positively correlated with the size of the mineral bound litter-C pool. Although the fraction of mineral-bound permafrost-C (64 % to 68 %) was significantly smaller than of litter-C, the MRT of the slow permafrost-C pools was more than 10-fold higher. Hence, the size of the mineral bound carbon pool alone may not be a suitable measure of carbon stabilization. We furthermore identified interactions between new litter carbon and pre-existing mineral-bound carbon from the thawed permafrost material as an important driver of litter-C stabilization. Such interactions could reduce net carbon emissions from thawing permafrost and add complexity to the permafrost carbon climate feedback.