Biogeochemistry最新文献

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.

Carbon sequestration in arid areas is a crucial component of the carbon cycle in terrestrial ecosystems. As an alluvial plain in the upper reaches of the yellow river, saline soils in Bayannur have a huge capacity for carbon sequestration. Weathering of coarse-grained silicate minerals (feldspar) from the Langshan Mountain generate CO₃²⁻ or HCO₃⁻, which combine with Ca²⁺ transported by the Yellow River, forming inorganic carbon sequestration. Additionally, humic substances produced by agricultural activities, alongside microbial residues, contribute to organic carbon sequestration. This research examines the processes and influencing factors of organic and inorganic carbon sequestration in arid regions by analyzing the soluble salts, minerals, elements, and dissolved organic matter (DOM) in the topsoil of Bayannur. The results showed that the topsoil (0–10 cm) was highly alkaline, with pH levels ranging from 8.07 to 9.94. The dominant soluble salts in the soil were Na⁺ and SO₄²⁻. Soil minerals content in descending order was quartz (Qtz), clay minerals (Clay), plagioclase (Pl), calcite (Cal), K-feldspar (Kfs), and dolomite (Dol). The soil organic carbon (SOC) content ranged from 0.16 to 0.89%, while the soil inorganic carbon (SIC) content ranged from 0.93 to 1.86%. The SOC content in the topsoil of Bayannur (cultivated saline soils) surpasses that in natural saline soils (uncultivated), likely due to increased carbon input from crops and agricultural fertilization. Similarly, the SIC content is also higher than that in natural saline soils. This is attributed to the irrigation process, which increases the concentration of Ca²⁺ in the soil and accelerates the weathering of the topsoil.

We used the catchment mass balance approach to investigate mercury (Hg) cycling at the 14 forested GEOMON catchments of the Czech Geological Survey. The temperate forest catchments had variable exposure to historic high sulfur (S) and Hg emissions, and span a range of size and elevation. We monitored monthly Hg inputs (bulk precipitation, throughfall, litterfall) and outputs (stream runoff) during 2020–2022. The catchments spanned a large gradient of historic Hg deposition, but current Hg patterns more closely aligned with catchment factors like local climate, as influenced by elevation, dissolved organic carbon (DOC) concentrations, and geology. The dominant pathway of Hg input was litterfall (averaging 44.5 ± 15.7 µg m⁻² yr⁻¹; > 91% of total input). Two surprising findings were that GEOMON had low Hg concentrations and fluxes in general but had the highest litterfall Hg fluxes in Europe, and these increased even further in forested areas that had bark beetle infestations. Gaseous elemental mercury (GEM), measured using passive samplers, was consistently low (1.25 to 1.66 ng m⁻³) across the 14 catchments. Stream Hg output varied across catchments and averaged 1.5 ± 1.7 µg m⁻² yr⁻¹. The average Hg retention rate at the 14 GEOMON catchments, calculated as the fraction of average Hg inputs (throughfall + litterfall) that remained in the catchment and did not run off in streamwater, was 97%. The high catchment Hg retention combined with its strong association with DOC suggests that with climate change intensification of carbon cycling, these catchments will be a Hg source for decades to come.

Soils are the largest terrestrial carbon sink on Earth, yet substantial uncertainty in the size and stability of this pool remains. Much of this uncertainty stems from the characterization of bulk density, which is the mass of a soil sample divided by its volume, a key property in the calculation of soil organic carbon (SOC) stocks. We used data from nearly 2900 plots in the United States (U.S.) Nationwide Forest Inventory to quantify SOC stocks in forests with three common methods of calculating soil bulk density. Mean SOC stocks calculated with these methods varied by up to 13 Mg ha⁻¹, a difference equivalent to more than 70 percent of the 2022 economy-wide carbon dioxide emissions in the U.S. when scaled across all forest area. These differences were primarily driven by inconsistent treatment of coarse materials (i.e. rocks and roots) in soil bulk density calculations, which led to an overestimation of SOC content by 32 percent of the mean SOC stock across all U.S. forests. The largest discrepancies were found in soils with high coarse fragment content, which are more common in ecologically sensitive ecosystems like alpine zones and drylands, and in commercially important softwood forest types. Quantifying the size and stability of SOC in the land sector is essential to understanding how this carbon pool may serve as a nature-based solution to climate change. Consistent and transparent methods are necessary when estimating and reporting SOC content and when comparing SOC dynamics across ecological gradients, with disturbance, and over time.

Methane is emitted from lakes by diffusion and ebullition. Methane diffusion is constrained by diffusion from sediments to water and water to the atmosphere, as well as oxidation. Methane ebullition from shallow water sediments bypasses these constraints but requires high methane production to form bubbles. We tested if ebullition dominates at high emissions with a Danish dataset and a global dataset comprising 973 measurements. Upper limits of methane diffusion were more constrained than ebullition. During periods of low total emissions, diffusive methane emissions predominated, whereas ebullition prevailed during periods of high emissions. The relative contribution of ebullition changed predictably, being 50% at 1.5–1.6 mmol m⁻² d⁻¹ and 75% at 5.1–6.4 mmol m⁻² d⁻¹ total methane emission. The probability of ebullitive flux was highly affected by the magnitude of the diffusive flux, and water temperature. Thus, when data was divided into the water temperature intervals ≤10, 10–20, and >20 °C, ebullition occurred in 69, 69 and 95% of the observations, respectively, and emission increased from 0.29, 0.71 to 3.6 mmol m⁻² d⁻¹ between the three temperature intervals. Summed across all measurements, ebullition accounted for the majority (75–83%) of total methane emissions. Thus, to attain reliable whole-lake emission and global estimates, many ebullition measurements are required to cover their extensive spatial and temporal variability.