Biogeochemistry最新文献

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.

Bransfield Strait has been identified as a climate hotspot for understanding regional environmental changes with global impact. This study focuses on enhancing the understanding of carbon cycle dynamics and its interactions with hydrographic variables in Bransfield Strait, located on the northern Antarctic Peninsula. The stable carbon isotopes of dissolved inorganic carbon (δ¹³C_DIC) were investigated in the study region during comprehensive sampling in 2023 along the major ocean basins. Bransfield Strait is influenced by two main source water masses: the Circumpolar Deep Water (CDW), which intrudes into the region from the Antarctic Circumpolar Current meander, and Dense Shelf Water (DSW), which is advected by coastal currents from the Weddell Sea continental shelf. The study reveals CDW’s dominant role in 2023, accounting for ~60% of the water mass mixture in the region and limiting the highest contribution of DSW to the deep layer of the central basin. The spatial variation of δ¹³C_DIC signatures showed that biogeochemical processes predominantly shape the δ¹³C_DIC distribution along the water column. Photosynthesis enriched the surface waters with the heavier carbon isotope, with signatures ranging from 2 to 1.5‰, while organic matter remineralization depleted it below the mixed layer (ranging from 0 to − 2‰). Horizontally, δ¹³C_DIC distribution was influenced by the higher contribution of each source water mass. Thermodynamic fractionation contributed to the enrichment of δ¹³C_DIC (~ 1 to 1.5‰) in the CDW layer in Bransfield Strait. Conversely, the predominance of younger and colder DSW exhibited a depletion of δ¹³C_DIC (− 1 to − 2‰). Therefore, δ¹³C_DIC is identified as an additional tracer to provide new insights into the biogeochemical and hydrodynamic processes of Bransfield Strait.

Polyphosphate is formed by polyphosphate-accumulating organisms occurring in various terrestrial, freshwater, and marine ecosystems as well as industrial environments. Although polyphosphate-accumulating organisms and polyphosphate have been well studied in enhanced biological phosphorus (P) removal from wastewater treatment plants, their role in the internal P cycle of natural lakes remains unclear. Several studies have shown that polyphosphate storage is widespread in lake sediments. In this study, ³¹P nuclear magnetic resonance spectroscopy was used to analyse the seasonal dynamics of polyphosphate and its drivers at the sediment surface of three stratified German lakes with strong seasonality of hypolimnetic oxygen concentrations. Similar seasonal patterns of polyphosphate were observed in all three lakes. Polyphosphate content increased by a factor of three to five at the beginning of summer stratification, with the maximum content observed in May when oxygen was already very low. During this period, strong redox gradients prevailed within the topmost sediment layer, and highly soluble reactive P concentrations were present in the pore water due to the reductive release of P bound to iron(III)oxides and oxide-hydroxides. Polyphosphate acted as a temporary P storage and was released after a delay, which may mitigate sedimentary P release into the water body during the (early) summer stratification. The observed seasonal dynamics of polyphosphate at the sediment surface offer a novel insight into the link between the P and iron cycles in lakes.

Ponds can store large amounts of organic matter (OM) in their sediments, often accumulated over long periods of time. Sediment OM is largely protected from aerobic mineralization under water saturated conditions but are vulnerable when exposed to oxygen during periods of drought. As climate change progresses, drought periods are likely to occur more frequently and may affect OM mineralization, and thus the release of greenhouse gases (GHGs) such as carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) from pond ecosystems. Therefore, we aimed to test how GHG emissions and concentrations in the sediment respond to drought by gradually decreasing water levels to below the sediment surface. To this end, undisturbed sediment cores from two small ponds with distinct watershed and water chemistry characteristics were incubated in mesocosms for 118 days at 20 °C. Water levels were sequentially tested at 3 cm above the sediment surface (Phase I) and at the level of the sediment surface (Phase II). In Phase III, water levels were continuously lowered either by evaporation or by active drainage including evaporation. Mean CH₄ fluxes of both ponds were high (21 and 87 mmol m⁻² d⁻¹), contributing 90 and 96% to the GHG budget over the three phases. The highest CH₄ fluxes occurred in Phase II, while active drainage strongly reduced CH₄ fluxes in Phase III. A multivariate analysis suggests that dissolved organic carbon and sulphate were important drivers of CH₄ fluxes in Phase III. CO₂ and N₂O fluxes also responded to declining water levels, but their contribution to the GHG budget was rather small. Both gases were primarily produced in the upper sediment layer as indicated by highest concentrations at 5 cm sediment depth. Compaction of sediment cores by water level lowering increased bulk density and maintained high water contents. This side effect, retarding the drying of the sediment surface, was possibly relevant for the GHG net emission of the sediments in Phase II and III. Overall, GHG fluxes from the sediments exhibited high sensitivity to falling water levels. This study suggests that drying pond sediments have great potential to emit large amounts of GHGs to the atmosphere in the event of drought, representing hot spots of GHGs in the landscape.