Biogeochemistry最新文献

Nitrous oxide (N₂O) emissions play a significant role in global warming and stratospheric ozone depletion. Nitrification and denitrification represent the primary pathways of N₂O emissions in agroecosystems. However, modelling the responses of nitrification, denitrification, and subsequent N₂O emissions to soil conditions and nitrification inhibitors remains challenging, as the fractions of N₂O emissions derived from nitrification and denitrification used in model simulations cannot be directly measured. In this study, we estimated soil nitrification, denitrification, N₂O emissions, and their related parameters via data assimilation under various soil moisture levels [water-filled pore space (WFPS) at 50% and 70%], incubation temperature (15, 25 and 35 °C) and nitrification inhibitor application (DMPP, 3MPTZ and C₂H₂) in cereal and vegetable production systems in Australia. We found that the contribution of nitrification to N₂O emissions (i.e., the fraction of N₂O emitted from nitrification, ({f}_{{text{N}}_2{text{O}}_nit})) decreased with increasing temperature and moisture content, whereas denitrification dominated N₂O production (i.e., the fraction of N₂O emitted from denitrification, ({f}_{N2O_dni})) under 70% WFPS regardless of temperatures. Under fertilizer N application, the use of nitrification inhibitors decreased ({f}_{{text{N}}_2{text{O}}_nit}) but increased ({f}_{N2O_dni}). The efficacy of nitrification inhibitors in mitigating N₂O emissions varied with environmental conditions. In this study, we demonstrate the use of data assimilation to constrain key parameters for predicting nitrification, denitrification and associated N₂O emissions in response to soil environments and management practices. Integrating this technique into ecosystem process-based models has the potential to enhance model accuracy by reducing uncertainties and biases.

Litter traits are closely associated with soil organic carbon (SOC) persistence. However, quantified effects of litter quality and quantity on SOC formation and loss are still debated, as they depend on complex biotic and abiotic interactions. Specifically, it remains unclear how the elemental (e.g., carbon [C] and nitrogen [N]) stoichiometry impacts the SOC pool through its control over nutrient cycling and energy flow. Here, we quantified the variations in bulk SOC and its fractionations (particulate organic carbon [POC], mineral-associated organic carbon, dissolved organic carbon and microbial biomass carbon [MBC]) under different litter treatments varying in quality and quantity, using a 13-year detrital manipulation experiment in a temperate mixed forest. We found that double mixed litter input increased bulk SOC pool by 58.5%, with a 67.2% increase in POC at 0–10 cm depth. Litter removal reduced POC by 40.4% (0–10 cm) and 49.8% (10–20 cm). Notably, litter removal and double woody litter input reduced the carbon to nitrogen ratio (C:N) of bulk soil, particulate and mineral-associated fractions, but had no effect on the microbial biomass C:N. The MBC was positively correlated with POC and soil moisture at 0–10 cm depth. Our findings indicate that litter quantity dominates SOC dynamics by regulating POC. Double mixed litter exhibited non-additive effects on SOC formation, likely due to trade-offs between fresh C inputs and priming-induced C losses. While litter removal reduced the soil C:N, microbial biomass C:N was unchanged, suggesting the need for longer-term studies to understand these decoupled responses.

Excess nitrogen (N) in freshwater systems is harmful and can lead to eutrophication, loss of biodiversity and toxic cyanobacterial blooms. External loading of N is an important driver of eutrophication, however, internal processing can either exacerbate or relieve excess N through N₂ fixation or denitrification, respectively. Here, we aimed to determine how variation in N loading and hydrologic setting affect internal N processing in lakes in summer by quantifying N₂ saturation in 17 lakes across a land use gradient in Minnesota and Iowa. We hypothesized that lakes with the highest N loading rates would have the highest N₂ saturation values, indicative of net denitrification. We observed that lakes in agricultural regions had the highest N₂ saturation and all lakes showed the highest levels of supersaturation in June when runoff was maximal. Although seasonal changes affected the degree of N₂ saturation, all lakes were sources of N₂ to the atmosphere throughout the sample period suggesting that denitrification was more impactful to internal processing than was N₂-fixation. Peaks in N₂ supersaturation co-occurred with both low and high dissolved oxygen levels, the latter being somewhat paradoxical given that denitrification is an anaerobic process.

Ocean surface gravity waves facilitate gas exchanges primarily in two ways: (1) the formation of bubbles during wave breaking increases the surface area available for gas exchange, promoting CO(_2) transfer, and (2) wave-current interaction processes alter the sea surface partial pressure of CO(_2) and gas solubility, consequently affecting the CO(_2) flux. This study tests these influences using a global ocean-ice-biogeochemistry model under preindustrial conditions. The simulation results indicate that both wave–current interaction processes and the sea-state-dependent gas transfer scheme–which explicitly accounts for bubble-mediated gas transfer velocity–influence the air–sea CO(_2) flux, with substantial spatial and seasonal variations. In the equatorial region (10(^{circ })S–10(^{circ })N), both processes enhance the CO(_2) outgassing flux, with comparable magnitudes (more than 10% on average). However, in the region between approximately 10(^{circ }) and 35(^{circ }), the impact of ocean surface waves on the air-sea CO(_2) flux via the sea-state-dependent gas transfer velocity is greater than that of the wave-current interaction processes, with opposing directions of influence. During winter, the sea-state-dependent gas transfer velocity enhances the CO(_2) uptake flux, while in the summer season, it increases the CO(_2) outgassing flux. In regions poleward of 35(^{circ }), the impact of wave–current interaction processes on CO(_2) exchange dominates over that of the sea-state-dependent gas transfer velocity. It is worth noting that the impact of wave-current interaction processes on air-sea CO(_2) flux is primarily driven by changes in the ratio between the concentrations of dissolved inorganic carbon and total alkalinity, with variations in sea surface temperature exerting an opposite influence on pCO(_2), albeit with a smaller magnitude. Overall, wave-related processes should be considered in Earth System Models to better model the carbon cycle.

Climate warming is increasing thermal stratification depth, strength, and duration in lakes, leading to more frequent hypolimnetic oxygen depletion. Most research has focused on eutrophic temperate lakes, which differ significantly from boreal lakes that dominate Earth’s landscape. However, assessing the impact of hypoxia, confounded by browning, warming, and altered stratification, on biogeochemistry and ecological processes in boreal lakes is particularly challenging. Here, we test how oxygenating a hypoxic hypolimnion affects water chemistry, bacterial and primary production (BP and PP), and detritus degradation in a shallow humic boreal lake divided into two basins in an experimental four-year Before-After Control-Impact (BACI) design. After two control years, we oxygenated the hypolimnion of one basin during two stratified periods without disturbing the seasonal development of the thermocline. Hypolimnetic oxygen concentrations moderately impacted lake biogeochemistry. Reoxygenation altered nitrification pathways (increased NO₃⁻) of the hypolimnion, and slightly decreased epilimnion and lake BP (− 6.1% of annual average) and green tea degradation (− 6.0%), whereas Rooibos degradation slightly increased (7.3%). Other water chemistry parameters remained within natural variation. We compared our BACI approach, which separates natural variation, to the simpler Before vs After approach, which does not. We find that studies not accounting for seasonal and among-year variability may overestimate the effects of oxygenation on hypolimnion biogeochemistry, as much of the observed impact is due to natural climate variation. Climate warming and altered stratification patterns are therefore likely to impact boreal lake algal and bacterial production and degradation more than hypolimnion hypoxia during the stratified period.

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.