Biogeochemistry最新文献

Methane (CH₄) is the second-largest contributor to human-induced climate change, with significant uncertainties in its terrestrial sources and sinks. Tree stems play crucial roles in forest ecosystem CH₄ flux dynamics, yet much remains unknown regarding the environmental drivers of fluxes. We measured CH₄ flux from three tree species (Picea rubens, Tsuga canadensis, Acer rubrum) along an upland-to-wetland gradient at Howland Research Forest, a net annual sink of CH₄, in Maine USA. We measured fluxes every two weeks and at three heights from April to November 2024 to capture a range of environmental conditions. Tree species influenced CH4 flux more than any of the environmental variables considered. Among environmental variables, soil moisture was the most important driver of CH₄ flux, and our models suggested a significant interaction between soil moisture and soil temperature, such that the effect of higher soil moisture was greater at warmer soil temperatures. We determined a “breakpoint” in soil moisture along the upland-to-wetland gradient at ~ 60% volumetric water content, above which CH₄ flux rates increased dramatically. All stems measured were net CH₄ sources throughout the sampling period, with rare, isolate measurements of minimal uptake. The magnitude of flux varied by species: red maple stems were the largest emitters (1.946 ± 5.917 nmol m⁻² s⁻¹, mean ± SD), followed by red spruce (0.031 ± 0.065) and eastern hemlock (0.016 ± 0.027). This study highlights the contribution of these species to ecosystem CH₄ fluxes. Our results establish the sensitivity of stem flux rates to projected increases in regional precipitation and temperature, potentially shifting the site from a net CH₄ sink to a source.

Selenium (Se) biogeochemistry in boreal and permafrost-rich soils and sediments remains poorly constrained, despite its importance as both an essential micronutrient and potential contaminant. As climate change accelerates warming in northern ecosystems, the mobilization of vast carbon pools may significantly alter Se cycling and bioavailability, with cascading effects on aquatic food webs. In this context, we aim to investigate how temperature and organic matter (OM) lability influence Se redox dynamics in lake sediments, providing insights for predicting its behavior as these northern ecosystems continue to warm. We studied Se sequestration as a function of OM lability, temperature (4 and 23 °C) and Se speciation in minimally disturbed lacustrine sediments using flow-through reactors (FTRs). Initial sediments contained OM characterized as either labile (fresh) or recalcitrant (aged), and were fed with environmentally relevant, low Se concentrations and filtered lake water. We monitored Se concentration as well as speciation along with pH and the concentrations of dissolved OM, NO₃⁻, NO₂⁻, Fe(II), SO₄²⁻ and HS⁻ in the outflow of FTRs during 8 experimental phases. All sediments sequestered a large proportion of Se, with FTRs containing fresh OM removing 50% more Se than those containing aged OM. Along with a higher production of reduced species, such as ferrous Fe and sulfides, in the reactors with fresh OM, this result is consistent with reducing conditions promoting Se sequestration. Inflowing selenite was sequestered to a larger extent than inflowing selenate. Lastly, only selenate removal responded strongly to temperature. With an inflow concentration of 100 nM, selenate was sequestered at a rate of 92 pmol cm⁻³ d⁻¹ at 23 °C, which decreased to 80 pmol cm⁻³ d⁻¹ at 4 °C. In selenate removal experiments, outflow Se speciation consisted mostly of organic Se species at 23 °C and, in contrast, entirely of selenate at 4 °C. We hypothesize that selenate removal proceeded via microbial processes, consistent with temperature-dependent reactions catalyzed by enzymes. Overall, our findings suggest that the mobilization and warming of the boreal and permafrost carbon pools may increase the capacity of aquatic environments to sequester Se, lowering its bioavailability.

Methane oxidation has been observed in a wide range of aquatic environments worldwide, and measurements are rare in tropical floodplains. The Amazon floodplain is one of the largest tropical wetlands with seasonally flooded forests representing up to 80% of the area of aquatic habitats in the lowland Amazon. Hence, we measured methane oxidation rates (Mox) in two different flooded forests (várzea, in white waters; igapó, in black waters) and evaluated effects of dissolved oxygen and CH₄ concentrations, and water temperature on methane oxidation. We found high Mox in near-bottom waters associated with high CH₄ concentrations (1.0–2.4 µM) and hypoxia, with volumetric rates ranging from 9.8 to 73 mg C m⁻³ d⁻¹ in the igapó, and from 2.3 to 101.4 mg C m⁻³ d⁻¹ in the várzea. Depth integrated Mox rates ranged from 177 to 213 mg C m⁻² d⁻¹ for the igapó, and 159 mg C m⁻² d⁻¹ in the várzea, and were one to two orders of magnitude higher than CH₄ fluxes from water to the atmosphere, emphasizing the important role of Mox in attenuating CH₄ emissions from tropical flooded forests. The present study contributes to understanding of the complex processes involved in carbon dynamics on tropical floodplains.

Decomposition of organic compounds in peat soils requires atmospheric oxygen, which is limited when water fills soil pore spaces. We examined the thermodynamics of organic matter decomposition in Austrian peatlands and predicted greater thermodynamic constraints deeper in the soil profile where pore spaces are water filled. For mire types with stagnant water we hypothesize that thermodynamic closure of the pore space will occur deeper in the soil profile and there will be a greater extent of organic matter transformation. In this study peat cores from eight different peatlands were collected and analysed for their Gibbs free energy of formation ((Delta {text{G}}_{{text{f}}})), carbon oxidation state (C_ox), and degree of unsaturation (Ω). The experimental design included bogs and fens, as well as natural and degraded sites. The study showed that decomposition of organic matter was greater in fens and degraded sites than in bogs and undisturbed sites, respectively, and there was a consistent increase in Ω with depth that marked an evolution away from cellulose-dominated compositions and toward lignin-dominated compositions at depth. These results support our study hypothesis that greater water stagnation in sites results in less transformation and shows that peatlands can be distinguished between the stable and unstable, and by relative recalcitrance.

Anthropogenic inputs of reactive nitrogen (N) elevate nitrate–N (NO₃-N) levels in streams, potentially shifting their dissolved organic carbon (DOC) to N to phosphorus (P) ratios (DOC:N:P) toward N excess. Meanwhile, changes in riparian vegetation can alter light availability. Together, these factors may influence NO₃-N uptake by photoautotrophs and heterotrophs in surface (benthic) biofilms and by heterotrophs in subsurface (hyporheic) biofilms. Although these compartments may exhibit distinct rates and constraints on nutrient uptake and retention, the extent to which stoichiometric imbalances and light availability govern their macronutrient uptake remains largely unexplored. Here, we present results from a stream mesocosm experiment in which light availability and DOC:N:P were manipulated by adding labile DOC and inorganic P to create a physiologically more balanced stoichiometric composition of stream mesocosm water. We show (I) how the relative (macronutrient ratio) and absolute (particulate organic C, particulate N, and particulate P) macronutrient composition of benthic and hyporheic biofilms changes with different levels of light availability (20 and 90 µmol photons m⁻² s⁻¹) and different water DOC:N:P (350:940:1 and 73:40:1), (II) that benthic NO₃-N uptake rates increased with addition of labile DOC and P, whereas light had only a minor effect, and (III) that higher NO₃-N uptake rates due to labile DOC and P addition in benthic biofilms leads to higher N loss from biofilm biomass. This results in similar N retention times across treatments and highlights the importance of water column macronutrient stoichiometry as a predictor of in-stream N cycling.

The application of surface agricultural practices (SAPs) to agricultural soils is gaining attention as a potential valuable method for sequestering carbon and improving soil fertility. However, the impacts of SAPs on the molecular properties of dissolved organic matter (DOM) in soil leachates are poorly understood. In this study, the molecular characteristics of DOM successively leached from agricultural soils applied with control, manure fertilization, lucerne planting, and straw return were unraveled by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results indicated that the greater proportion of low molecular weight labile DOM (lipids-like, proteins-like and carbohydrates-like) in initial soil leachates gradually changed to higher fractions of larger recalcitrant DOM (condensed aromatics-like and tannins-like) in later soil leachates. Compared to the control, the soil leachates treated with SAPs had greater percentage of labile DOM and lower percentage of recalcitrant DOM, along with higher abundance of CHNO and CHOS compounds. Furthermore, DOM in the manure, lucerne, and straw treatments showed smaller mass weights, higher H/C ratios and fewer double bonds, rings, and aromatic structures. DOM with different physicochemical properties play different roles in the processes of nitrogen cycling and arsenic migration. The implementation of SAPs may alleviate groundwater nitrogen pollution, but it may also enhance the potential risk of arsenic mobility in groundwater. This study deepens our understanding of the molecular characterization of DOM leached from agricultural soils applied with different SAPs, which holds significant implications for evaluating the environmental impacts of soil DOM leaching.

Wetlands provide important ecosystem services, including improving surface water quality through nutrient removal. Louisiana has experienced ~ 4800 km² of coastal wetland loss between 1932 and 2016 due to high relative sea level rise and reduced sediment from the Mississippi River due to levees. The 2023 LA Coastal Master Plan aims to restore Louisiana’s degraded coastline through restoration projects, including sediment diversions or river reconnection. The Mid-Barataria Sediment Diversion Project will reconnect the river sediment-laden water with the coastal wetlands of Barataria Basin to nourish degrading marshes. However, the diversion will also deliver substantial nitrate (NO₃⁻) to the basin, potentially negatively impacting water quality. We quantified NO₃⁻ reduction rates at these high (2 mg/L) and low (0.5 mg/L) water column concentrations for marsh and submerged estuarine sediments using intact cores and a laboratory incubation. An additional treatment where 2 cm of mineral river sediment was placed over the organic marsh soil as a future, post-diversion scenario to simulate sediment deposition on the marsh once the river is reconnected. We hypothesized that NO₃⁻ reduction rates would decrease once mineral sediment is deposited on the organic marsh soil. For an aerobic water column, nitrate reduction rates for the vegetated marsh, post-diversion marsh, submerged eroded marsh, and estuarine sediment zones were 71.1 ± 2.7, 27.8 ± 4.5, 19.7 ± 1.2, and 13.0 ± 0.75 mg N m⁻² d⁻¹, respectively. Thus, the post-diversion marsh NO₃⁻ reduction rate decreased by ~ 60% compared to the current vegetated marsh. However, we predict the newly deposited sediment will increase NO₃⁻ removal by 1.17 × in the eroded marsh and estuarine sediment zones, which are always flooded and will receive river sediment. The marsh is only flooded 31–48% of the time, lessening the impact of the reduction. These findings can improve predictive water quality models used to assess nutrient loading and fate more accurately across the basin under the river reconnection scenario and inform other deltaic regions as freshwater flows are restored to coastal systems globally.

Nitrification is a key biogeochemical process, with higher rates indicative of higher soil nitrogen availability and potential nitrogen losses from soils to waterways and the atmosphere. Heterotrophic microbes and plants compete with nitrifiers for mineralized nitrogen, thereby influencing the fraction of ammonium converted by nitrifiers to nitrate. Higher soil carbon availability fuels heterotrophic microbial ammonium demand, which can weaken the positive relationship between net nitrogen mineralization and nitrification by limiting ammonium supply to nitrifiers. Whether soil carbon availability remains a central control on the coupling of these processes under altered plant nitrogen demand remains relatively unexplored even as disturbances that reduce plant biomass increase globally. Using partially disturbed forests that vary in aboveground biomass and soil carbon availability, we test the generalizability of microbially available carbon as a control on the coupling of net nitrogen mineralization and nitrification. We analyze differences between harvested and unharvested forest stands, changes over time since harvest, and the effects of retained overstory trees. Higher levels of disturbance consistently strengthened the positive relationship between net nitrogen mineralization and nitrification. Yet reduced plant biomass, rather than microbially available carbon, primarily mediated the coupling of these processes. Our findings suggest that plant-mediated nitrogen demand can be a stronger control on the decoupling of nitrogen mineralization and nitrification than heterotrophic soil microbes following partial canopy disturbances. These results have important implications for understanding coupled carbon and nitrogen cycling processes in forests globally, highlighting a need to consider how shifting disturbance regimes could influence controls on nitrification.

The signaling compound nitric oxide (NO) might play an important, yet unquantified role in mediating soil biogeochemical Carbon and Nitrogen cycles. This study quantified the effects of different soil-typical exogenous NO concentrations on the microbial community, on fertilizer N turnover, and on C and N trace gas fluxes of agricultural soil. For this, we repeatedly established soil NO concentrations of either 0, 200, 400, and ppbv˗NO in soil mesocosms for in total of 12 days, followed by high-resolution automated measurements of CO₂, NO, CH₄, and N₂O fluxes, molecular analysis of microbial community composition and ¹⁵N-isotope-tracing based assessment of fertilizer N turnover. We found no effects of different NO levels on microbial communities and CO₂, CH₄, and NO fluxes. However, at 200 ppbv concentration, exogenous NO promoted microbial assimilation of fertilizer N. In contrast, at 400 ppbv˗NO concentration, microbial biomass N was reduced, and microbial uptake of fertilizer N was inhibited, accompanied by a 33% reduction of N₂O emissions. This suggested a promoting effect of 200 ppbv˗NO on the physiology of cells involved in heterotrophic microbial N turnover, probably reinforcing the role of cell-endogenous NO. In contrast, the higher exogenous NO concentrations of 400 ppbv seemed to inhibit heterotrophic microbial inorganic N assimilation, with however no increase in N₂O emissions due to detoxification mechanisms. In conclusion, our pioneering study provides first insights into impacts of exogenous NO on soil C and N biogeochemistry in natural soil systems and reveals a NO concentration-dependent regulation of microbial N retention.

Biologically available nitrogen from human activities have altered nutrient dynamics across landscapes and aquatic ecosystems. Small spatial changes in land use and river management, may contribute to altered nutrient dynamics and influence denitrification and assimilatory uptake in river systems. Human actions can influence the stoichiometry of rivers. Construction of Libby Dam and the creation of the transboundary Koocanusa Reservoir has resulted in sequestration of approximately 60%–80% of the phosphorus entering the reservoir. Recent and ongoing expansion of surficial mining operations in one tributary upstream of Koocanusa Reservoir, the Elk River, has increased nitrate loading tenfold or more to Koocanusa Reservoir and to the Kootenai River. The combination of excessive nitrate loading and decreased phosphorus availability has skewed the N:P ratio to greater than 200:1 in both the river and reservoir. To address how this altered stoichiometry influences nitrogen spiraling in a large river, we estimated nitrate uptake over 16 years in five reaches of the Kootenai River. Reaches spanned 224 river km and types were based on natural and anthropogenically-influenced geomorphology. Although we documented a decline in nitrate moving longitudinally downstream indicating that nitrate is being used by the biota, the magnitude and timing of areal nitrate uptake varies among the reaches. Areal nitrate uptake did not differ between the early years with lower nitrate concentrations and the later years with higher nitrate concentrations suggesting that the Kootenai River is nitrogen saturated. Phosphorus addition, used as a management tool to offset P sequestration in the reservoir, increased areal nitrate uptake and extended the period of higher areal nitrate uptake. Without increases to the ecosystem functions of nitrogen transformation and removal, the ecosystem becomes saturated and the entire load is being transported downstream.