Biogeochemistry最新文献

High flow and flood events are essential for sustaining river ecosystems, driving nutrient cycling, habitat diversity, and species dispersal.  However, widespread flow regulation via dams and reservoirs has disrupted natural hydrological processes, leading to river fragmentation and homogenization of flow regimes. While previous research has largely focused on the hydraulic and biological impacts of engineered flow events, less attention has been given to their influence on solute mobilization, transport, and biogeochemical transformations. This study addresses this gap by evaluating the geochemical and sediment dynamics of the first experimental spring high flow event (i.e., pulse event) on the Allegheny River (Pennsylvania, USA), conducted by the United States Army Corps of Engineers under the Sustainable Rivers Program. The pulse event, initiated on March 30, 2023, involved a sustained release of 451 cms from Kinzua Dam over 21 h. We hypothesized this experimental spring pulse would mobilize organic-rich sediment and nutrients stored behind Kinzua Dam, while also altering the geochemical signature of downstream waters through interactions with hyporheic zones, sediment scouring, and channel connectivity. To assess these impacts, we collected hourly water grab samples over a 48-h period spanning pre- and post-pulse conditions at multiple downstream locations. Samples were analyzed for dissolved metals, nutrients, total suspended solids, and nitrate isotopes. Results reveal distinct temporal shifts in water chemistry, with observed fluctuations in total suspended solids, dissolved metals, and nutrient concentrations highlighting hydrological connections between the main channel and riparian zones, reinforcing the importance of experimental pulse events in ecosystem restoration. Based on these findings, we propose a conceptual model linking controlled flood pulses to sediment and solute fluxes, which can be tested in other regulated river systems to evaluate the effectiveness of flow restoration strategies. These results provide key insights into the role of controlled high flow pulses in shaping sediment and solute dynamics, filling an important knowledge gap in understanding the biogeochemical implications of large-scale flow experiments.

Increasing concentrations of dissolved organic carbon (DOC) and changing dissolved organic matter (DOM) quality in surface waters, a phenomenon known as browning, have been observed at global scales with a range of implications for ecosystem structure and function, global carbon cycling and human health. Ecosystem recovery from chronic acidification resulting from rapid declines in acid deposition over recent decades has been the leading explanation for surface water browning. In this study, long-term dynamics of the quantity, quality, and seasonality of DOM in surface waters of an acid-resistant Adirondack lake and its forested watershed were investigated during a period of rapid regional changes in both acidic deposition and climate (1999–2018). Overall, we found that trends in DOM quality have occurred while the overall quantity and seasonality of DOC fluxes changed little during the same time frame. Lack of DOC trends was consistent with expectations for this acid-resistant ecosystem. Model reconstructions of DOM quality during this period indicated shifts towards a greater proportion of terrestrially-sourced DOM from the watershed, but with occasional ‘pulses’ of more microbially-processed DOM associated with periods of heavy rainfall and high discharge. Our findings suggest that ecologically meaningful changes in DOM quality may be occurring in acid-resistant ecosystems, aside from trends in DOC driven by ecosystem recovery from acid impairment.

We evaluated base cations, Al, Fe, Mn, REEs, DOC, anions, and P mobilization during three discharge events (E-1, E-2, E-3) at the headwater catchment W-9, Sleepers River, Vermont, USA. Peak discharge ranged from 3.696 (E-1) to 0.073 (E-3) mm h⁻¹. Eight samples from each event were speciated for total (unfiltered-acidified) and dissolved (0.45 µm filtered-acidified). During E-1, total Al, Fe, and Mn increased to maxima of 376 (1790X), 161 (194X), and 38 (45X) µmol L⁻¹, respectively. Concurrently, total La, Ce, Pr, and Nd increased to maxima of 87 (590X), 114 (671X), 15 (375X), and 53 (408X) nmol L⁻¹, respectively, greatly exceeding the dissolved fraction. Totals for Er > Yb > Tm > Lu reached comparable enrichments near or at maximum discharge. Discharge ranged from 0.41 to 0.94 mm h⁻¹ during E-2, a snowmelt event. Total Al was comparatively stable; total Fe and Mn increased 15X and 79X, remaining less than total Al. Total La, Ce, Pr, and Nd peaked with total Fe and Mn. E-3, a late summer rain, resembled E-1 but had much lower maxima for all REEs, P, Al, Fe, and Mn. Particulate, total, and dissolved REEs and P correlated with DOC and with total Al, Fe, and Mn over all discharges, with molar Al > Fe > Mn (most samples) for E-1 and E-3. Maximum total and dissolved P declined from 15.4 and 0.89 (E-1) to 0.98 and 0.27 µmol L⁻¹ (E-3), respectively. Particulate REEs correlated strongly (R² = 0.95–0.96) with Al, Fe, and Mn particulates eroded from the stream bed and continuously precipitated during and after high discharge of groundwater.

The Canadian Prairie Pothole Region is a notable hotspot for cyanobacteria-dominated lakes. This study found minor variations in cyanobacterial genera across these lakes yet observed significant differences in standing biomass, as the lakes ranged from oligotrophic to hypereutrophic classifications. A correlational analysis of nutrients, specifically total phosphorus (TP) and total nitrogen (TN) revealed that the limiting nutrients varied considerably across the region. Of the lakes studied, cyanobacterial biomass was P-limited in 21 lakes, N-limited in 3 lakes, and co-limited by both P and N in 23 lakes. Surprisingly, in 32 lakes, the biomass was limited by neither P nor N. In these lakes, iron (Fe) emerged as the most likely limiting nutrient, given a relatively narrow range of free ferric Fe (pFe) between 18 and 26. Cyanobacteria can create biomass under Fe stress by producing Fe-scavenging siderophores that target pFe. However, in neither P- nor N-limited lakes, there was a lack of correlation between siderophore concentrations and cyanobacterial biomass (r = 0.05), indicating that the siderophores were unable to scavenge Fe and thereby utilize the available P and N to produce further cyanobacterial biomass. Our findings suggest that these Fe-starved eutrophic lakes exhibited a paradox of slow-growing yet high cyanobacterial biomass, challenging the notion that only oligotrophic lakes embody slow-growing metabolisms. Overall, our study highlights the importance of considering nutrient limitations on cyanobacterial growth and incorporating macro- (P and N) and micro- (Fe) nutrient limitation considerations into existing nutrient management strategies to mitigate cyanobacterial dominance effectively.

We have investigated the bacterial decomposition of dissolved organic matter (DOM) extracted from the organic layer of a boreal forest soil and filtered at a pore size of 0.2 µm. This DOM source has previously been extensively characterized and contains approximately equal amounts by carbon of a colloidal fraction, mainly composed of carbohydrates, and a fraction of molecularly dissolved DOM. Here, extracts were inoculated with soil bacteria and the decomposition of DOM was followed over a period of 2 months, during which it was analyzed with scattering methods and ¹H NMR, and by measuring the concentration of total organic carbon. A comparison was also made with dialyzed extract. Results showed that while the bacteria fully decomposed the molecular fraction within approximately two weeks, the colloidal fraction was stable with no visible decomposition within the 2 months. The results indicate the importance of distinguishing small molecules from colloidal aggregates in decomposition studies, and demonstrate the usefulness of combining scattering methods with ¹H NMR for this purpose.

In seasonally snow-covered ecosystems such as northern hardwood forests of the northeastern U.S., spring snowmelt is a critical transition period for plant and microbial communities, as well as for the biogeochemical cycling of nitrogen (N). However, it remains unknown how shifting snowmelt dynamics influence soil and plant processing and uptake of N in these forests, which are experiencing reductions in N availability relative to demand, a process known as oligotrophication. We characterized the role of changing spring snowmelt timing on root production and N pools and fluxes by manipulating snowmelt timing along a climate elevation gradient at the Hubbard Brook Experimental Forest in New Hampshire. We manually halved or doubled snow water equivalent (SWE) in experimental plots in March of 2022 and 2023 to accelerate or delay by an average of one week, respectively, the onset of spring snowmelt. Earlier snowmelt led to reduced snowpack depth and duration, as well as deeper, more sustained soil frost during the snowmelt period in 2022, but soil freezing did not occur in 2023. Soil nitrate and net nitrification rates were significantly lower with shallower snowpack and earlier snowmelt compared to plots with deeper snow and later snowmelt. Shallower snowpack and early snowmelt were also associated with decreased foliar N concentrations and δ¹⁵N values, indications that earlier snowmelt contributes to lower N availability relative to plant N uptake and demand. Our study provides evidence that early snowmelt resulting from shallower snowpack contributes to N oligotrophication, primarily through impacts on soil nitrate supply and uptake of N by trees.

Spring can be a critical time of year for stem and soil methane (CH₄), nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions as soil freeze–thaw events can be hot moments of gas release. Greenhouse gas fluxes from soil, Downy birch (Betula pubescens) and Norway spruce (Picea abies) stems were quantified using chamber systems and gas analysers in spring 2023 in a northern drained peatland forest. Dissolved gas concentrations in birch sap and soil water, environmental parameters, soil chemistry, and functional gene abundances in the soil were determined. During spring, initially low soil and stem CH₄, N₂O, and CO₂ emissions increased towards late April. Temperature emerged as the primary driver of soil and stem fluxes, alongside photosynthetically active radiation influencing stem fluxes. Soil hydrologic conditions had minimal short-term impact. No clear evidence linked stem CH₄ emissions to birch sap gas concentrations, while relationships existed for CO₂. Functional gene abundances of the N and CH₄-cycles changed between measurement days. Potential for methanogenesis and complete denitrification was higher under elevated soil water content, shifting to methanotrophy and incomplete denitrification as the study progressed. However, our results highlight the need for further analysis of relationships between microbial cycles and GHG fluxes under different environmental conditions, including identifying soil microbial processes in soil layers where tree roots absorb water.

Flocculation of riverine dissolved organic matter (DOM) in estuaries is crucial for transforming and removing terrestrial carbon inputs across the land-to-ocean aquatic continuum. We measured variations in chromophoric DOM (CDOM) absorption and fluorescence of riverine DOM through mixing experiments conducted across various seasons and environments, identifying patterns in salt-induced flocculation. Our observations show a systematic reduction in CDOM absorption in the 250–450 nm range at salinity 2, with a sharper decrease at higher wavelengths. Flocculation led to decreased relative fluorescence intensity below emission wavelength of 360 nm and an increased intensity at higher emission wavelengths across the excitation spectrum measured (250–450 nm). We introduce a new metric, red shift ratio, a fluorescence-based metric calculated as the ratio of emission intensity at 300–350 nm to that at 360–500 nm, at excitation wavelengths between 250 and 300 nm, for detecting flocculation-induced changes in CDOM across estuarine systems. The observed sensitivity of CDOM to flocculation in low salinities challenges its use as a conservative tracer in coastal gradients, suggesting that recalibrations are required for remote sensing algorithms and carbon flux estimations across land-sea continuum, particularly in systems with similar characteristics.