Biogeochemistry最新文献

Carbon dioxide removal (CDR) via enhanced rock weathering (ERW) strongly depends on rock particle size. While ERW models typically link finer particle size to greater CDR, their tendency to aggregate with soil components such as organic matter (OM) may impede weathering. The inconsistent effects of ERW on soil OM storage in recent studies reinforce the need to clarify underlying mechanisms. We thus tested if finer basaltic rock promotes organo-mineral association while lowering CDR through incubation experiments (rock alone and rock-plant residue-sand mixture) under water regimes with or without weekly leaching. After six months, we analyzed total carbon, extractable metal(loid)s, organo-mineral aggregate formation (by density fractionation), and inorganic carbon contents (by XANES and leachates). Coarse basaltic rock (106–150 μm) showed faster abiotic and biologically induced weathering. Contrarily, fine basaltic rock (20–38 μm) led to greater organo-mineral aggregation and OM accrual, which was attributable to higher particle numbers, geometric surface area, and binding agents (inherent and increased reactive metal(loid)s). The amount of organic carbon stabilized in meso-density aggregates by basaltic rock was one order of magnitude higher than the estimated CDR, regardless of the water regimes. These results exhibit the first direct evidence that rock particle size could induce the trade-off between CO₂ removal and OM stabilization, which implies that the current ERW models may severely overestimate CDR potential due to basaltic rock interaction with OM and its weathering products. Further research into rock interactions with soil components is essential for improving model prediction and optimizing ERW applications.

Fluctuating groundwater levels in drained peatlands create a transition zone with seasonally changing oxygen availability. This zone drives dynamic iron (Fe) and sulphur (S) cycling under alternating anoxic and oxic conditions, influencing decomposition rates. This study investigated how Fe and S affect decomposition rates and resulting carbon dioxide (CO₂) emissions under fluctuating redox conditions in transition zone. In a controlled laboratory experiment, peat samples from two drained Dutch coastal peatlands were amended with ferric iron (Fe³⁺) and sulphate (SO₄²⁻) and incubated anoxically to mimic high groundwater tables. This was followed by an oxic phase simulating groundwater table drops. The cycle was repeated with lactate addition to replenish labile carbon. Carbon dioxide emission rates were monitored continuously throughout the anoxic–oxic cycles. Water soluble Fe and S concentrations, exoenzyme activities, and pH were measured before and after the experiment. Carbon dioxide emission rates increased under anoxic conditions with Fe³⁺ and SO₄²⁻ amendments potentially due to stimulation of microbial activity using these compounds as alternative electron acceptors. Short-term oxygenation suppressed emissions compared to controls without amendments. Water-soluble Fe remained stable across treatments, while water-soluble S concentrations changed significantly from initial levels. Exoenzyme activities were primarily influenced by pH, with minimal effects from amendments. The findings show that transition zone is an active redox zone where decomposition dynamics are determined by available electron acceptors in the system, influencing greenhouse gas (GHG) emissions from managed peatlands. This zone should be integrated into future models to improve the accuracy of reporting national GHG emissions.

Nitrogen (N) and phosphorus (P) concentrations in many northern prairie rivers have been increasing due to anthropogenic activities. While long-term trends in total N and P have been well documented, there remains limited knowledge regarding trends in dissolved fractions as well as the associated effects of shifting nutrient loadings on nutrient stoichiometry (i.e., N:P) of river water. We assessed long-term (25-year) trends in total and dissolved N and P concentrations and N:P at 11 monitoring stations situated on five rivers within the Red-Assiniboine River Basin in North America. We found that N and P concentrations and stoichiometry were changing through time at a majority of stations. Spatial patterns of trends were variable with no consistent directional changes in either nutrient concentrations or stoichiometry among stations, suggesting the importance of localized nutrient sources, such as wastewater treatment plants. Changes associated with catchment characteristics were the primary contributors to observed trends in nutrient concentrations and stoichiometry, whereas alterations in the streamflow regime played a comparatively minor role. Variations in the relative quantities of nutrients in the basin’s rivers may be influencing the potential for nutrient depletion, with some rivers undergoing stoichiometric shifts in the depleted nutrient. Consequently, nutrient management may need to occur at the sub-basin scale to mitigate point source nutrient pollution and protect riverine water quality throughout the basin.

The biogeochemical cycle of copper (Cu) is mediated by its complexation with organic ligands. An emerging strategy for Cu uptake by marine and freshwater phytoplankton involves organic molecules of biological origin, such as cysteine (Cys), indicating an important role for Cys in controlling the redox state and uptake of Cu in surface waters. In this study, Cys-like compounds were detected in the surface layer of the Krka River estuary by cathodic stripping voltammetry (CSV), while high fractions of Cu(I) were simultaneously determined using an adapted solid phase extraction method. The affinity of Cys for associated redox reactions and for Cu(I) stabilisation depends on its complexation affinity towards Cu(I), which remains controversial in the literature, as values of reported stability constants for Cu(I)-Cys are inconsistent. Spectrophotometric and electrochemical approaches were used to determine the conditional stability constant of the Cu(I)-Cys complex (K″_CuL), which refers to the equilibrium constant conditional with respect to both Cu and the ligand. Spectrophotometric reverse titration against a known Cu(I) probe, bathocuproine disulfonate (BCS), under seawater conditions (0.55 mol/L NaCl) revealed a previously unrecognised effect of chloride (Cl⁻) on the stability of the [CuBCS₂]³⁻ complex due to the possible formation of ternary complexes involving Cu(I), BCS, and Cl⁻, which calls for caution when determining the stability of Cu(I) complexes this way. The results of the electrochemical reverse titration of the Cu(I)-Cys complex with BCS in Cl⁻-containing medium are consistent with the spectrophotometric results. The logarithm of the derived K″_CuL for the Cu(I)-Cys complex is 15.35 ± 0.11, corresponding to the conditional stability constants of the strong Cu-binding ligand class (L₁) in seawater, which further supports the importance of Cys for Cu uptake and redox cycling in seawater and estuarine water. By elucidating the stability of the Cu-Cys complex, this study provides insight into how biologically produced and naturally occurring thiol-like ligands influence Cu redox speciation in marine and estuarine waters, which can affect Cu transport and uptake and is therefore essential for accurately representing the Cu biogeochemical cycle.

Available phosphorus (P) concentrations are low in dryland soils due to high pH values linked to the presence of pedogenic carbonates. Thus, dissolution and mobilization of P compounds are important controls on P availability for plants, microbes, and biocrusts. One process that has been hypothesized as a way for dryland organisms to acquire P is the exudation of organic acids that can release bound P compounds. To explore this process, we assayed the critical thresholds of organic acid (citrate, malate, and oxalate) concentrations required to mobilize P in a range of dryland soils. Our results showed that: (1) Concentrations of oxalate or citrate, on the order of 1000 µmol/L are needed to effectively mobilize PO₄³⁻ in all landforms and microsites we examined. (2) The in situ organic acid concentrations in bulk soil core samples were < 100 µmol/L, both under plant canopies and in interspaces, suggesting they are below the needed threshold to mobilize P. However, hot spots such as the rhizosphere, though difficult to quantify, may still be locations where the concentration of organic acids approach the threshold. (3) Oxalate was the most effective in releasing PO₄³⁻, likely as a result of removing aqueous Ca via calcium oxalate precipitation. Overall, our results show that for P acquisition through organic acid production to be effective in dryland soils, a relatively high threshold of organic acid concentration that substantially exceeds bulk soil concentrations must be reached, suggesting that if it occurs, it is restricted to localized microsites within the soil matrix.

Pelagic dissolved (< 0.2 µm) nickel (dNi) concentrations are rarely depleted below 2 nmol L⁻¹ and normally remain tightly correlated with the macronutrients dissolved silicic acid and phosphate. Nickel is therefore widely disregarded as a possible limit on primary metabolism in modern-day aquatic environments. However, here we demonstrate that low dNi concentrations can arise in some polar regions due to low Ni concentrations in meltwater and thus there are environmental contexts in which Ni availability may plausibly constrain primary producers. Here we characterize the dNi concentrations of meltwater across a range of Arctic localities and present new pelagic measurements from Disko Bay (western Greenland) where previous measurements of dNi indicated the lowest concentrations ever measured in the Arctic or Atlantic. Across 10 (sub)Arctic glacier-fed streams/rivers we find a broad range of mean dNi concentrations from 0.40–132 nmol L⁻¹. Most of the surveyed glacier-fed streams had dNi concentrations within the range 3–13 nmol L⁻¹, which is comparable to major river systems worldwide. Yet three evidenced much lower concentrations with mean dNi < 2 nmol L⁻¹ which would act to dilute dNi concentrations almost anywhere in the ocean. Similarly, meltwater from iceberg fragments in southwest and west Greenland had very low dNi concentrations (mean 0.6 nmol L⁻¹ and 0.2 nmol L⁻¹, respectively). Meltwater therefore appears to be a possible driver of low dNi concentrations, especially when icebergs, rather than runoff enriched with dNi from bedrock weathering, are a dominant freshwater source. However, dNi flux budgets for Arctic fjords reveal that vertical entrainment of deep, Ni-rich waters driven by subglacial discharge plumes often also constitutes a measurable fraction of dNi supply to surface waters, limiting the impact of low-Ni meltwater except under very specific ice-melt stratified scenarios. Whilst in the modern-day ocean there appear to be only limited localized scenarios in which dilution by meltwater might deplete dNi concentrations to levels which could plausibly constrain primary producers, low dNi conditions might have been more prevalent during deglaciation events in Earth’s past when regions of the ocean were stratified by large meltwater fluxes.

Greenhouse gas (GHG) emissions from reservoirs are quantitatively relevant for atmospheric climatic forcing. The magnitude of these fluxes depends on the mechanisms promoting the production of carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄), and on the physical forces determining their emission. GHG emissions exhibit large temporal variability, with diel changes accounting for a substantial part of this variability. However, most GHG flux estimations rely on upscaling discrete measurements taken at daytime and typically overlook nighttime emissions. This study explored the diel patterns of CO₂, N₂O, and both diffusive and ebullitive CH₄ fluxes in two eutrophic reservoirs with different morphometries, using hourly GHG flux measurements over a summer day in two different years. Daytime emissions of CO₂, N₂O, and diffusive CH₄ were on average 159, 267, and 194% higher than nighttime emissions, respectively. Despite the different production pathways, the diffusive fluxes showed strong daily synchrony, suggesting an external common driver for the three of them. Daily emissions of CO₂, N₂O, and diffusive CH₄ were positive and significantly related to wind speed and solar time. In contrast, ebullitive CH₄ fluxes showed no consistent daily pattern, and were influenced by reservoir management (i.e., water level drawdown) in the shallowest system. Ebullitive CH₄ fluxes represented an average of 51% of the total CH₄ emitted. Our study suggests that diel variability in GHG emissions may be as relevant as spatial or inter-system differences and should be integrated into future GHG budgets to improve their accuracy.

Coastal wetlands can serve as natural laboratories for assessing the future impacts of sea-level rise and the intricacies of the effect of sulfate (SO₄²⁻) on emissions of greenhouse gases, such as methane (CH₄) and carbon dioxide (CO₂). In the case of previously drained and freshened coastal wetlands, we can observe how freshwater terrestrial microbial communities react and adapt to intrusion of SO₄²⁻ rich saline waters. We conducted a 3-month anoxic incubation experiment with soil extracted from a coastal peatland on the German Baltic coast which was rewetted with brackish water in late 2019 to examine how microbial communities at the site had adapted to the new conditions after two years. Soil slurries were incubated at a temperature of 15 °C at two different salinities (reflecting surface water and average peat soil water salinity) and sampled at 8 timepoints. At each timepoint 5 replicates of each treatment were destructively harvested and sampled for concentrations of CH₄, dissolved inorganic carbon (DIC), total aqueous organic carbon, sulfate (SO₄²⁻), ammonium (NH₄⁺), and other major ions, pH values, δ¹³DIC and δ¹³CH₄ values, microbial community composition via 16S rRNA amplicon sequencing and functional gene analysis via shotgun metagenomic sequencing. Carbon, nitrogen, and sulfur elemental analysis (CNS) and X-ray fluorescence (XRF) analysis of soil cores from nearby monitoring locations were included to give background on the biogeochemical conditions of the soil. Contrary to expectations, the legacy of SO₄²⁻ exposure from a previous connection with the Baltic Sea, as evidenced by high sulfate concentrations, was the strongest influence on the biogeochemistry of each treatment, rather than the new salinity and SO₄²⁻ introduced during rewetting. The different salinities tested had little impact on the methane emissions as the microbial community was already well adapted to saline and SO₄²⁻-rich conditions and displayed a considerable amount of functional gene equivalency. We conclude from our results that we need to pay more attention to the legacy effects in coastal peatlands and how they affect methane cycling and microbial community composition for years to decades.

Permafrost soils constitute a large part of the terrestrial carbon pool that is vulnerable to future climate warming. Continued warming of the low Arctic is also leading to the encroachment of large shrubs and trees into tundra ecosystems with effects on microbial community composition, organic matter cycling and physical soil parameters. To date it is still largely unknown how such vegetation shifts affect soil organic matter cycling in permafrost soils on short and long timescales. Here, we investigated differences in soil organic matter properties under graminoid tussock (Eriophorum vaginatum), birch shrub (Betula glandulosa), spruce tree (Picea mariana) and alder shrub (Alnus viridis) vegetation by density fractionation and subsequent measurements of organic carbon, total nitrogen, δ¹³C, and lignin phenol biomarker contents. Particulate organic matter constituted 1.3–11.3% of soil weight and stored between 29 and 89% of the total soil lignin, 12–60% of organic carbon and 6–40% of total nitrogen. The contribution of particulate organic matter generally decreased with soil depth. Soils under Alnus viridis showed significantly higher amounts of particulate organic matter and stored more lignin, organic carbon and total nitrogen in particulate form in all soil depths. Sites dominated by Eriophorum vaginatum exhibited higher lignin content and lower degradation state in the subsoil, which was associated with water saturation and low active layer depth. We conclude that the effect of vegetation changes on soil organic matter cycling is dependent on plant species with the encroachment of Alnus viridis shrubs potentially increasing the deposition of particulate organic matter into permafrost soils.