Abstract Oxygen plays a central role in regulating biogeochemical processes across terrestrial and aquatic ecosystems. However, its dynamics in hydrologically and redox‐dynamic ecosystems remain poorly characterized due to high spatiotemporal variability and the lack of field‐ready instrumentation capable of long‐term, depth‐resolved deployments required to capture these dynamics. Here, we present a practical, modular system for measuring high‐frequency in situ dissolved oxygen time series across multiple soil depths in both unsaturated and saturated conditions. The platform integrates commercially available optical oxygen sensors with a portable logging and power system housed in a weatherproof enclosure. This system is cost‐effective and designed for rapid assembly, flexible deployment, and robust operation under harsh field conditions. We demonstrate system performance across contrasting environmental settings, including an experimental flooding event in an upland forest and king tide‐driven inundation in a coastal marsh, capturing rapid transitions between oxic and anoxic states and revealing depth‐dependent oxygen dynamics. To support broader adoption, we provide open‐source code for data processing and visualization, and outline pathways for future development, including integration with commercial dataloggers. By lowering technical and logistical barriers to depth‐resolved in situ soil oxygen monitoring, this approach facilitates improved characterization of short‐term oxygen dynamics and associated biogeochemical processes across a wide range of fundamental and applied research settings.
Abstract Marine oxygen minimum zones (OMZs) harbor a variety of nitrogen cycling processes and act as hotspots of fixed nitrogen loss and nitrous oxide (N 2 O) emissions, thereby influencing nitrogen availability and the Earth's climate system. Sulfide occasionally accumulates in OMZs due to in situ production or release from the sediments, but its impact on nitrogen cycling is not fully understood. Using nitrogen tracer incubations in the Eastern Tropical South Pacific OMZ and meta‐analysis of previous observations in other low oxygen environments, we found that sulfide addition generally inhibited nitrification and anammox while enhancing denitrification. These results illustrated the role of sulfide in determining the importance of different nitrogen cycling processes and changing the fate of nitrogen species, for example, increasing the contribution of denitrification to nitrogen loss compared to anammox and causing a stronger N 2 O accumulation in OMZs. Improved understanding of sulfide impacts on nitrogen cycling in marine OMZs strengthens our capacity to predict changes in nitrogen availability and N 2 O emissions during sulfidic events, whose frequency, duration and spatial extent are expected to change under future climate scenarios.
Abstract Land surface models (LSMs) such as the Community Land Model version 5 (CLM5) are central to climate–carbon assessments but exhibit persistent biases relative to site‐level observations. It remains unclear to what extent model‐data mismatches arise from uncertainty in meteorological forcing, soil properties, or vegetation traits, and to what extent they instead reflect structural deficiencies. We evaluated surface and root‐zone soil moisture (SM, SMr), evapotranspiration (ET), sensible heat flux (H), net ecosystem exchange (NEE), and gross primary production (GPP) at 14 European eddy‐covariance sites using 128‐member CLM5 ensembles in which (a) atmospheric inputs, (b) soil parameters, and (c) vegetation parameters were perturbed. Soil parameters dominate ensemble spread in SM/SMr, whereas vegetation parameters dominate spread in ET, H, NEE, and GPP. Despite this, substantial mean biases remained (0.18 cm 3 cm −3 for SM, −0.22 mm d −1 for ET, −19.7 W m −2 for H, −1.83 gC m −2 d −1 for GPP, and 1.16 gC m −2 d −1 for NEE), and the 99% ensemble envelopes failed to cover many observations. Coverage percentage (CP; fraction of observations within the 99% ensemble envelope) was 74.3% (SM), 74.6% (SMr), 68.8% (ET), 47.7% (H), 65.1% (NEE), and 70.1% (GPP). A companion CLM5‐SP (Satellite Phenology) experiment with prescribed ICOS leaf area index (LAI) confirmed that prognostic LAI errors in CLM5‐BGC (Biogeochemistry) are the primary structural driver of SM/SMr and GPP coverage deficits, while BGC's carbon–nitrogen coupling better captures ET and H variability. CP diagnostics delineate parametric from structural error regimes across variables and plant functional types.
Abstract Urban agriculture is widespread across tropical West African cities. Yet, the dynamics of phosphorus (P) pools in strongly weathered soils of these urban agroecosystems remain poorly constrained. We quantified, for the first time, the effect of urbanization on soil P stocks and partitioning in rainfed arable fields on Ferric Acrisols in Kumasi, Ghana. We hypothesized that inputs of P‐rich household and construction waste increase soil P stocks, and that accompanying calcium (Ca) enrichment weakens the soil organic matter (SOM) control on plant‐available P. We analyzed 225 topsoil samples (0–10 cm), classified into four groups representing a gradient of urbanization influence. We applied a tailored sequential P extraction, separating four operational fractions: plant‐available P (P Pa ), SOM‐bound P (P SOM ), Ca‐bound P (P Ca ), and P occluded in pedogenic oxides (P OCC ). Total P stocks increased from 28.0 ± 1.7 to 68.8 ± 6.7 g m −2 with urbanization influence. Stocks of all extracted soil P fractions consistently increased, with the greatest in P Ca (2.5 ± 0.6 to 14.2 ± 2.5 g m −2 ) and P Pa (1.4 ± 0.1 to 5.4 ± 0.8 g m −2 ). P OCC stocks also increased in absolute terms but declined in relative contribution from 79% to 53%. Urbanization did not weaken SOM control on P Pa as initially hypothesized. Instead, P Ca became increasingly relevant in explaining P Pa , suggesting an additional inorganic control on P Pa . We conclude that tropical urban arable soils can act as inadvertent sinks for P‐rich urban wastes, suggesting the potential for a deliberate waste management strategy that could enhance soil P supply.
Abstract Ethane and propane are short‐chain alkanes that are ubiquitous in marine environments, while the biological formation of ethane and propane remains largely unconstrained in coastal subsurface sediments. Here, we detected nanomolar concentrations of ethane and propane along 2.7–5.7 m sediment cores with different geochemical gradients at coastal sites of the East China Sea. We further conducted sediment incubation experiments and demonstrated that diethyl sulfide, bromoethane, ethanethiol, ethanol, and acetate were potential precursors for ethane biosynthesis, while propane could be produced from propanol and propanethiol. Ethane production was influenced by the change in temperature, substrate concentration, and H 2 partial pressure. Particularly, the addition of 80% H 2 in the headspace resulted in a 75‐fold increase in ethane production, suggesting the involvement of H 2 in ethanogenesis. The addition of methanogenesis inhibitor, 2‐bromoethanesulfonate, completely terminated the activity of methanogens and also inhibited ethane production. Taxonomic profiling of the ethane‐producing enrichment suggested that methanogens, primarily from the orders Methanomicrobiales , Methanosarcinales , and Methanococcales , might be involved in the formation of trace amounts of ethane, which was probably achieved via the proposed intermediate of CH 3 CH 2 ‐S‐CoM in a similar pathway for methane production. These results reveal the potential role of methanogens in ethane production and provide insights into the biogeochemical cycling of hydrocarbons in coastal sediments.
Abstract Monoalkyl glycerol ethers (MAGEs) are a suite of structurally stable lipid compounds ubiquitous in both biological and geological bodies; they are thought to derive mainly from bacteria such as sulfate‐reducing bacteria, but animal lipids such as monoalkyldiacylglycerols (MADAGs) are also raised as an important contributor. Thus, MAGEs in marine realms may originate from marine bacteria or animals or both, which in turn, complicate definite interpretations on their source recognition. Scleractinian corals, a distinctive type of shallow‐marine invertebrates thriving symbiotically with algae and bacteria, act as an ideal archive for assessing the animal and/or bacterial production of MAGEs. Here, 11 coral species randomly collected from tropical‐subtropical offshore reefs in the South China Sea were measured for molecular and δ 13 C compositions of MAGEs alongside symbiotic bacterial communities residing within host corals. The lipidomic and genomic analyses unveil that tissual MAGEs are mainly hydrolyzed from coral polyp MADAGs (and analogs); by contrast, skeletal MAGEs are sourced from outer tissues and/or other unknown producers/precursors, which vary by coral species. A roughly negative correlation between content and δ 13 C of tissual MAGEs is observed. This phenomenon is attributed to a crucial role of light in driving symbiotic algal photosynthesis in offshore oligotrophic reefs, such that more 12 C‐depleted low‐molecular precursors are preferentially incorporated into the biosynthesis of abundant MADAGs (and analogs) by coral host. Our findings signify that temporal profiles of MAGEs extracted from fossil coral skeletal sequences of certain species may offer a novel invaluable tool for high‐resolution reconstruction of past eco‐environmental changes in coral‐algal‐bacterial symbioses.
Abstract Coastal wetlands play a crucial role in global carbon dynamics, acting as carbon dioxide (CO 2 ) sinks while also emitting methane (CH 4 ), a potent greenhouse gas. This study investigates CH 4 fluxes over a six‐month period in a sub‐tropical coastal mangrove‐saltmarsh wetland in southern Moreton Bay, Queensland, Australia, using eddy covariance. The site functioned predominantly as a net CH 4 source, with a mean (±SD) half‐hourly flux of 0.054 ± 0.943 μmol m −2 s −1 over the full six‐month period, exceeding previous mean CH 4 estimates from coastal wetlands and reflecting high variability. Hydrological factors, particularly the extent and duration of tidal inundation and rainfall, strongly influenced coastal wetland CH 4 dynamics. Spring tides amplified emissions by promoting waterlogged anoxic conditions ideal for methanogenesis. In contrast, rainfall events disrupted typical flux patterns, enhancing both emissions and uptake, overall leading to a net CH 4 sink. Diel and seasonal variations were also evident, with elevated night‐time emissions and seasonal fluxes modulated by hydrological events. These findings underscore the importance of short‐term hydrological disturbances in shaping CH 4 fluxes and highlight the need for region‐specific long‐term monitoring in coastal wetlands. As climate change intensifies precipitation, storm frequency, and associated inundation patterns, capturing these transient, high‐impact events is essential for accurate GHG accounting and climate mitigation strategies.
Abstract Pollen is a terrigenous particulate regarded as recalcitrant organic matter (OM) in aquatic ecosystems, seasonally introducing undetermined quantities of terrigenous OM to inland lakes. The role of pollen as a carbon and nutrient delivery mechanism for large aquatic ecosystems remains an understudied component of biogeochemical cycling. Here, we examined the effects of pollen mineralization on carbon and nutrient availability to evaluate its potential as an unquantified seasonal source and driver of microbial community succession in Laurentian Great Lake (LGL) Superior. Pollen was added to water collected from LGL Superior and incubated in the dark at 4°C for 60 days. Water samples were measured for dissolved and particulate fractions of carbon (C), nitrogen (N), and phosphorus (P) alongside ultraviolet‐visible spectroscopy. Microbial community succession was assessed through metagenomic sequencing and abundance estimates at selected times. Results show immediate, significant increases in dissolved chemical fractions, with 21.0% of the initial weight of pollen C, 28.1% N, and 40.1% P chemically leaching. Extrapolated lake‐wide, pollen leaching resembles a potential loading of up to 0.30 Tg C yr −1 , 17.3 Gg N yr −1 , and 1.64 Gg P yr −1 . Subsequent increases in estimated prokaryotic abundance and decreases in dissolved nutrients were accompanied by shifts in microbial community composition expressing increases of 5% amino acid and 10% carbohydrate metabolism over the experimental duration. These results suggest pollen contains labile OM fractions that are a substantial source of bioavailable carbon and nutrients that can fuel microbial succession and seasonal dynamics in large lakes.
Abstract Forests are expected to continue to expand into alpine ecosystems as temperatures increase. While forest expansion typically increases aboveground carbon storage, the consequences for belowground soil carbon storage and stability remain unclear. To address this, we combined a field‐based study in the eastern Himalaya of Bhutan with a meta‐analysis examining how forest expansion influences alpine soil organic carbon (SOC) and its fractions. We sampled 120 surface soils (0–15 cm) and excavated 20 soil pits (0–100 cm) across a forest‐ecotone‐alpine gradient on Chelela Mountain in western Bhutan. We quantified total SOC, particulate organic carbon (POC), mineral‐associated organic carbon (MAOC), and MAOC:POC ratio. Locally, forest expansion was associated with higher total SOC, especially in the ecotonal transition zone and in deeper soil layers. Ecotone soils had the greatest SOC, driven largely by MAOC, linked to greater root biomass and higher clay and silt content, whereas POC peaked in the mature forest soils. Although alpine soils stored less total SOC because of shallower soils, they contained a higher proportion of stable carbon, reflected in the highest MAOC:POC ratios. Globally, forest expansion did not significantly change SOC or POC, but significantly reduced MAOC (−28.4%) and the MAOC:POC ratio (−18.3%). These responses varied across ecosystems, where broadleaf expansion decreased SOC via reduced POC, while coniferous and shrub expansion reduced MAOC:POC ratios. Collectively, these findings suggest that forest expansion may shift soil carbon storage from stable mineral‐associated forms to more labile particulate fractions, potentially reducing the long‐term stability of carbon stored in alpine ecosystems.
Abstract Droughts have been extensively studied at small to large scales, yet limited work has integrated the mountain‐range and tree‐level perspectives to explain tree drought response at the intra‐catchment scale where management decisions are made. Here, we investigated tree response to drought in terms of resistance—the ability of a forest to continue transpiring during drought—and resilience—the ability to rebound post‐drought. We estimated resistance and resilience using Landsat‐derived normalized difference vegetation index (NDVI) over a 0.5 km 2 catchment of the Southern Sierra Critical Zone Observatory. At the catchment‐wide scale, we fitted generalized additive models with eight remotely sensed predictors to explain 51% of the variance in resistance and 59% in resilience. Topography and baseline greenness were the strongest predictors and exhibited opposite effects on resistance versus resilience, underscoring the need to distinguish their drivers. Aspect and snow depth were significant for resilience only, further highlighting that resistance and resilience are governed using partially distinct processes. Slope and elevation effects contradicted regional‐scale patterns, whereas canopy height effects were consistent across scales. Remote sensing revealed spatial patterns of drought response, while in situ ecohydrological, meteorological, and geophysical (electrical resistivity) data from six stations offered process‐based insights into the conditions underlying them: valley‐bottom hydrologic refugia, inferred reliance on internal sapwood water stores, and consistently low atmospheric demand were all associated with locations that experienced greater drought resistance. This work demonstrates that forest vulnerability emerges from coupled, scale‐dependent interactions among hydrology, vegetation structure, and topography.
Abstract River discharge considerably influences marine ecosystems through hydrological and biogeochemical drivers. Measuring baseline variability of riverine inputs, and their relation to anthropogenic land‐use impacts and global climate stressors affecting watersheds is therefore crucial for managing coastal marine ecosystems. This is particularly relevant in the relatively undisturbed Northwest Patagonia region (NWP; 41°S–46.5°S), where an extensive estuarine system influenced by high river runoff supports much of Chile's aquaculture economy (est. $6+ billion USD). Combining an unprecedented weekly citizen‐monitoring effort and daily streamflow data with a weighted‐regression modeling approach, we estimated inputs of macronutrients, dissolved iron, dissolved silica, and total suspended solids as well as characterized dissolved carbon concentrations from the five largest watersheds to the NWP marine environment. We used these estimates to evaluate export magnitude, seasonality, synchrony, and potential influence on marine ecosystems. Our results suggest that large rivers have a net dilution effect on total nitrogen and phosphorus, with freshwater concentrations substantially lower than those reported for NWP fjords and comparable coastal temperate rainforest rivers globally. Conversely, freshwater concentrations of dissolved silica and iron were markedly higher than those in marine waters and other coastal temperate rainforest rivers. Collectively, these results suggest a potential regulating function of large watersheds to marine ecosystems in Northwest Patagonia, with implications for harmful algal blooms and for managing external inputs from aquaculture and changing watershed land‐use. Our findings provide one of the first comprehensive regional baselines for major watersheds in reference conditions linked to temperate fjord ecosystems.
Abstract Evapotranspiration (ET) links the terrestrial water and carbon cycles, yet its accurate estimation remains limited by simplified representations of plant‐atmosphere interactions. Solar‐induced chlorophyll fluorescence (SIF) has emerged as a promising physiological constraint on ET through its link to stomatal conductance, but current SIF‐based semi‐mechanistic models assume fixed, linear relationships that limit their applicability under variable environmental conditions. Here, we evaluate a widely used SIF‐driven semi‐mechanistic ET model against eddy covariance observations from 31 flux tower sites spanning eight plant functional types, and employ an interpretable machine learning (ML) framework with SHAP (SHapley Additive exPlanations) analysis to identify where these formulations become limiting. The performance gap widens markedly under environmental stress, with the semi‐mechanistic model showing reduced skill under low soil moisture ( = 0.53 vs. 0.70 for ML) and high vapor pressure deficit (VPD) ( = 0.45 vs. 0.65), indicating that fixed conductance formulations fail where nonlinear physiological regulation intensifies. SHAP diagnostics reveal that SIF and VPD are the dominant predictors of ET but their influence is conditional, with SIF‐ET coupling modulated by soil water content and VPD exhibiting threshold‐dependent behavior. PM‐ML analysis confirms that this limitation lies in the linear conductance formulation based on SIF, and global‐scale validation confirms these patterns across vegetated land. These results demonstrate that nonlinear coupling between photosynthetic activity, atmospheric demand, and soil water availability represents a first‐order control on ecosystem ET that current semi‐mechanistic formulations cannot adequately capture, and that interaction‐aware parameterizations of stomatal conductance are needed to reduce biases across ecosystems.
Abstract Understanding the spatial distribution of heavy metals in urban gardens is essential for mitigating contamination risks and supporting safe urban agriculture. This study examines factors influencing heavy metal concentrations in garden soil across San Antonio, Dallas, and Houston. A total of 215 soil samples, collected at the surface and down to 30 cm, were analyzed using Inductively Coupled Plasma–Mass Spectrometry for nine heavy metals, including arsenic ( As ), chromium ( Cr ), cadmium ( Cd ), and lead ( Pb ), which are elements of environmental and health concern due to their potential toxicity and, in some cases, carcinogenic properties. Over half (55%) of the samples exceeded the Environmental Protection Agency residential soil threshold for As (0.68 ppm). To identify key drivers, we applied both multivariate linear regression and a generalized mixed graphical network model. While regression highlights independent relationships between variables, the network model comprehensively captures nonlinearly interconnected relationships across variables. It revealed strong links between As and total soil carbon, Cd and clay content, Pb and distance to city center, and Cr and proximity to Superfund sites. A random forest model, informed by the top predictors from the network analysis, achieved its best performance for As (). A combined network model including all four metals emphasized distance to the city center and clay percentage as the most influential factors. The highest average concentrations occurred in gardens within five miles of the city center and in soils with <15% clay. These findings highlight the major drivers of metal concentrations in urban garden soils within the study region.
Abstract The Tibetan Plateau is home to Earth's largest alpine pastures, where livestock populations have more than doubled over the past half century, contributing widespread pasture degradation. In response, large‐scale restoration has been implemented (9% of pastures have been restored) despite a tripling in human population and food demand. The biogeochemical impacts of alpine pasture restoration, such as enhanced CO 2 uptake, are well documented, whereas its biophysical consequences remain poorly understood. Here, we show that restoration is associated with substantial litter accumulation, increased surface albedo, and consequent changes in the surface energy balance. Surveys of 11 paired restored pasture sites revealed 20%–45% increases in litter accumulation, consistent across alpine steppe, meadow, and wetland. Long‐term paired observations in representative alpine steppe and wetland ecosystems further showed that litter retained in the absence of herbivory increasingly covered the dwarf canopy and was associated with increases in surface albedo of 0.02–0.04 and a 7% reduction in net radiation. Sensitivity experiments suggest that increased albedo alters surface energy partitioning, reducing sensible heat flux by 0.15 W m −2 and increasing latent heat flux by 0.01 W m −2 over the Tibetan Plateau, with stronger responses within restoration areas. These lines of evidence highlight a previously underappreciated biophysical pathway, whereby restoration‐induced litter accumulation enhances surface reflectivity and alters land‐atmosphere energy exchange, contributing to a more comprehensive understanding of the climatic impacts of pasture restoration.
Abstract The intensification of disturbance regimes due to climate change is a growing concern in boreal forests, yet the long‐term consequences for forest recovery remain poorly understood, as boreal successional dynamics unfold over a century or longer and exceed the length of available observational records. To overcome this research gap we use the dynamic vegetation model LPJ‐GUESS to investigate future climate change impacts on boreal successional dynamics over the 21st and 22nd centuries. We first confirm that LPJ‐GUESS reproduces historic patterns of post‐disturbance recovery. We then show that under future warming, deciduous transients become more frequent, longer, and more pronounced due to a temperature‐mediated shift in competitive balance emerging in LPJ‐GUESS. Using a random forest framework, we find that the long‐term recovery pathway can be predicted with 90% accuracy from vegetation and environmental conditions in the first decade after disturbance alone. This supports the hypothesis that the changes in post‐disturbance recruitment currently observed in boreal forests are an indicator of more long‐lasting changes in vegetation composition in the future. While we find no evidence for permanent shifts toward deciduous tree cover in LPJ‐GUESS, the combination of prolonged deciduous transients and more frequent disturbances makes an increase in deciduous tree cover a likely future scenario.
Despite the growing interest in carbon cycling in tidal-wetland ecosystems we lack sufficient understanding of the degree to which saltmarsh ecosystems sequester carbon and the ecosystem dynamics of carbon dioxide (CO2) exchange, especially with tidal influence. The eddy covariance method was used to estimate a carbon budget at the ecosystem-scale for an Irish Atlantic saltmarsh and investigate the effects of tidal inundation on net ecosystem exchange (NEE). NEE was partitioned into gross primary productivity (GPP) and ecosystem respiration (R-eco). Biophysical drivers were determined on hourly, diurnal, and multiday timescales using wavelet analysis. Growing season R-eco and GPP during spring- and neap-tide periods were also investigated to assess tidal impacts on CO2 flux magnitude. Derrymore saltmarsh acted as a CO2 sink for all but 4 months of the year, sequestering 307 g C m(-2) yr(-1). Biophysical drivers of CO2 fluxes scaled temporally: hourly NEE and GPP were primarily regulated by latent energy (E), diurnal variations responded to both E and photosynthetically active radiation (PAR). On multiday scales, however, PAR emerged as the dominant driver. Similarly, while air temperature (T-a) governed R-eco at hourly and diurnal frequencies, multiday respiration was shaped by the combined influence of T-a, water temperature, and water level (H-w). Marsh full inundation (H-w > 0 m) by tidal waters decreased daytime GPP up to 73%, and night-time R-eco up to 45% during the growing season. This research shows that even in northern climates, saltmarshes can be productive and valuable carbon sinks, though their performance is strongly affected by tidal flooding.
Abstract Phosphorus (P) is an essential nutrient for plant growth and reproduction and may constrain primary production when its availability in soils is low. In natural systems, P exists in multiple forms, depending on soil properties and environmental conditions. Plants use different root trait strategies to target these different P pools, for instance by exuding a suite of molecules that can help release PO 4 3− for plants to uptake, or by either investing in more root surface area or in mycorrhizal fungi partners to optimize nutrient absorption. Recently, exudates in particular have emerged as a rich field of study, owing to the high variability in chemical compositions and the resulting cost and efficiency dynamics that can be expressed as strategic pathways. Here, we use the recent study by Keen et al. (2026, https://doi.org/10.1029/2025jg009380 ) to highlight the importance of a holistic understanding of plant P mining dynamics and compound production costs relative to plant‐scale P return, while also accounting for the complex abiotic and biotic interactions influencing plant‐soil P cycling, and making recommendations for next steps on this research area.
Abstract Microbial reductive dissolution of As‐bearing Fe(III) (oxyhydr)oxides is a primary mechanism of arsenic (As) mobilization in reducing aquifers; however, the relative contributions of mineral bioreactivity and microbial metabolic pathways remain insufficiently quantified. Here, we compared Fe(III)‐reducing bacteria ( Shewanella oneidensis MR‐1 and Geobacter sulfurreducens ) with a bacterium capable of both As(V) and Fe(III) reduction ( Clostridium sp. anHT01, isolated from a high As aquifer sediment from Inner Mongolia) to elucidate Fe and As release kinetics from As(V)‐bearing biogenic ferrihydrite‐goethite (Bio‐GF) and lepidocrocite (Bio‐Lp) under bicarbonate‐buffered reducing conditions. Biogenic mineralogy primarily governed the extent of Fe(III) reduction, with Bio‐GF exhibiting higher bioreactivity than Bio‐Lp, but did not dictate net As mobilization. In contrast, microbial metabolic pathways controlled the temporal relationship between Fe reduction and As release. Systems with active As(V) reduction exhibited delayed Fe(III) reduction yet rapid As(III) release, resulting in elevated As released /Fe released ratios and net As mobilization during the Fe‐reduction lag phase. Secondary Fe(II) minerals modulated As partitioning and retention but did not govern the onset of As mobilization. Together, these results demonstrate that As mobilization during microbial reduction of biogenic Fe(III) (oxyhydr)oxides is jointly governed by mineral bioreactivity and microbially mediated Fe‐As release relationships, providing a mechanistic basis for improving the prediction of As behavior in reducing aquifer systems.
Abstract Recent studies have shown changes in the production rates of nitrous oxide (N 2 O) in aerobic seawater in response to ocean acidification (OA). Understanding how N 2 O production responds to OA is crucial because N 2 O is a strong greenhouse gas and stratospheric ozone‐depleting substance emitted from the ocean. The pH dependence of N 2 O production rates on marine bacteria Nitrosococcus oceani strain NS58, one of the ammonia oxidizing bacteria that are relevant to nitrification occurring in eutrophic seawater, was investigated under several dissolved oxygen (DO) conditions. We also measured abundance ratios of N 2 O molecules substituted with rare stable isotopes (isotopocules) to distinguish the two major pathways of N 2 O production by nitrifiers: NH 2 OH oxidation and NO 2 − reduction. The ammonium oxidation rate () and N 2 O production rate () calculated respectively from the temporal change of the product concentrations were 4–34 × 10 −15 mol h −1 cell −1 and 1–15 × 10 −17 mol h −1 cell −1 . When compared in the stable phase ( t = 44–76 hr), decreased concomitantly with decreasing DO, also exhibiting a slight increase in acidified water. In contrast, was highest at 35% DO (air saturation), showing a 5%–60% increase by acidification (pH 7.7 vs. 8.0) depending on DO. Isotopocule ratios showed an increased contribution from NO 2 − reduction over NH 2 OH oxidation under 35% and 3% DO, but its pH dependence was negligible except under 3% DO. These results suggest that OA increases N 2 O emission in particular from eutrophic seawater and that both N 2 O production pathways can be stimulated to the same degree.
