Biogeochemistry最新文献

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.

Black carbon has been shown to suppress microbial methane production by promoting anaerobic oxidation of organic carbon, diverting electrons from methanogenesis. This finding represents a new process through which black carbon, such as wildfire char and biochar, can impact the climate. However, the mechanism and capacity of black carbon to support metabolism remained unclear. We hypothesized black carbon could support microbial growth exclusively through its electron storage capacity (ESC). The electron contents of a wood biochar was quantified through redox titration with titanium(III) citrate before and after Geobacter metallireducens growth, with acetate as an electron donor and air-oxidized biochar as an electron acceptor. Cell number increased 42-fold, from 2.8(± 0.6) × 10⁸ to 1.17(± 0.14) × 10¹⁰, in 8 days based on fluorescent cell counting and the result was confirmed by qPCR. The qPCR results also showed that most cells existed in suspension, whereas cell attachment to biochar was minimal. Graphite, which conducts but does not store electrons, did not support growth. Through electron balance and use of singly ¹³C-labeled acetate (¹³CH₃COO^–), we showed (1) G. metallireducens could use 0.86 mmol/g, or ~ 19%, of the biochar's ESC for growth, (2) 84% and 16% of the acetate was consumed for energy and biosynthesis, respectively, during biochar respiration and (3) ca. 80 billion electrons were deposited into biochar for each cell produced. This is the first study to establish electron balance for microbial respiration of black carbon and to quantitatively determine the mechanism and capacity of biochar-supported growth.

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