Biogeochemistry最新文献

Ocean acidification due to anthropogenic carbon dioxide (CO₂) uptake threatens marine ecosystems, but coastal waters have complex and highly variable carbonate system dynamics. More coastal time series are therefore needed to better constrain coastal pH dynamics, but especially in the tropics such time series are rare. Southeast Asia’s Sunda Shelf Sea has exceptionally high marine biodiversity and rapidly increasing anthropogenic pressures, but we have little knowledge of coastal carbonate system dynamics in the region. We analyzed 7-years of monthly carbonate system data from the Singapore Strait in the central Sunda Shelf. Our results show consistently low seawater pH (total scale; pH_T; < 8.0) and aragonite saturation state (Ω_Ar, usually < 3.0), but with a clear seasonal pH_T variation by 0.11–0.19 units. The observed seasonality reflects the monsoon-driven advection of water masses that have been shaped by different biogeochemical processes. Specifically, during the southwest monsoon, remineralization of terrestrial dissolved organic carbon originating from regional peatlands lowers pH_T, while calcium carbonate (CaCO₃) formation and dissolution are more important during other seasons. Comparing our data to predicted values for purely conservative mixing of river water and seawater shows that these non-conservative biogeochemical processes are dominant drivers. Our time series shows a significant decreasing trend in pH_T of –0.043 units per decade, exceeding the theoretical trend detection time (TDT) of 5.0 ± 1.3 years. However, the seasonal pH_T variability itself shows interannual variability, and pH_T is also correlated with the El Niño Southern Oscillation. This longer-term climatic control may complicate trend quantification. Our study highlights how terrestrial dissolved organic carbon remineralization may enhance future ocean acidification in Sunda Shelf region, emphasizing the importance of continuing the time series to better quantify climatic drivers and long-term trends.

Biogenic methane, the largest contributor to atmospheric methane, is produced via different microbial methanogenic pathways, depending on the substrates and type of methanogens. Stable carbon and hydrogen isotope measurements (δ¹³C and δD) and the clumped isotopologues (Δ¹³CH₃D and Δ¹²CH₂D₂) of methane have emerged as important diagnostic tools, providing insights into methane sources and reaction pathways. Here, we investigate the pathway-specific bulk and clumped isotopic signatures of methane produced by microbial communities in sediments from a marine coastal system (lake Grevelingen, the Netherlands). Sediment batches were incubated with different substrates (acetate, carbon dioxide + hydrogen, methanol, and methanol + hydrogen) to promote the different methanogenic pathways. Our results show that the methanogenic pathways studied produce isotopically distinct methane. Analysis of the 16S rRNA gene from the sediment reveals a metabolically diverse methanogenic community capable of sustaining hydrogenotrophic, acetoclastic, and methylotrophic pathways, consistent with the isotopic variability observed in methane produced during incubations. The methylotrophic and acetoclastic pathways yield methane with significantly lower Δ¹²CH₂D₂ than the hydrogenotrophic pathway due to the combinatorial anti-clumping effect. The methane produced in situ in the sediments predominantly originates from the hydrogenotrophic pathway, with Δ¹³CH₃D and Δ¹²CH₂D₂ values closely matching incubations with carbon dioxide and hydrogen. Overall, the incubation results using lake sediments align well with previous pure culture studies, highlighting the potential of clumped isotope analysis to differentiate methane production pathways in natural environments.

Alien invasive species impact native plant communities, not only through direct negative effects on native species but also by altering nutrient cycling and availability. However, the mechanisms driving these changes—such as differences in litter decomposition rates and litter quality, or increased overall decomposition rates within invaded stands—remain unclear. This study examines how Solidago gigantea, an invasive species in Europe and Asia, affects decomposition. A climate chamber experiment tested whether S. gigantea litter decomposes faster than litter from co-occurring native species. The study examined whether differences in decomposition rates could be attributed to litter quality (using C/N ratio as a proxy) or to variations in soil microbiota (via inoculum from invaded vs. non-invaded plots). A field experiment measured overall decomposition rates in invaded and non-invaded plots using tea bags and wooden spatulas buried for 2–16 weeks. Soil moisture, carbon, and soil fauna activity were also assessed for their influence on decomposition. S. gigantea litter decomposed significantly faster than native graminoid species (decomposition rates of 0.91 and 0.33 g g⁻¹ day⁻¹ resp.), despite its higher C/N ratio (39.6 and 27.9 resp.). Invaded stands consistently had higher decomposition rates, which was attributed to abiotic changes, including reduced soil moisture and increased soil carbon, rather than to biotic changes in soil fauna or microbiota activity. These findings highlight S. gigantea’s substantial impact on nutrient cycling in invaded ecosystems. However, the extent—and potentially the direction—of these effects likely depends on the invaded plant community. Grass-dominated and wetter communities may experience the greatest increases in nutrient cycling, potentially enhancing primary productivity.

Reconstructing long-term records of air pollution and comparing present-day air quality with that of the pre-Industrial Revolution era are crucial for predicting the trajectory of recovery from sulfur-based air pollution. The annual rings of trees contain sulfur compounds that have accumulated over time, and the sulfur stable isotope ratio (δ³⁴S) reflects the past δ³⁴S values in the atmosphere. Although δ³⁴S in tree rings has been analyzed in the past, there are no δ³⁴S data in tree rings from before the Industrial Revolution (1760), when humans began to emit large amounts of sulfur compounds to the atmosphere. Therefore, we analyzed long-term variations in δ³⁴S in the annual rings of two giant Japanese cedars (Cryptomeria japonica) from the Okute-Shinmei Shrine and Ise Jingu in central Japan, spanning over 350 years. The fluctuation trend of δ³⁴S values was statistically divided into periods before and after industrialization. Before industrialization, δ³⁴S values were stable and high, averaging + 6.0 ± 0.6‰ in Okute and + 10.5 ± 0.9‰ in Ise. The values closely match previously reported adjusted δ³⁴S values from Greenland ice cores. After industrialization, values declined sharply owing to fossil fuel use. This decline stabilized around the 1990s, and values increased slightly. Despite these reductions in sulfur emissions, δ³⁴S values remain significantly lower than pre-industrial levels, implying that air quality has not yet returned to pre-industrial conditions.

Understanding mercury (Hg) accumulation and distribution patterns in alpine timberline forests is essential for understanding the biogeochemical cycling of Hg in these ecosystems. To this end, we systematically analyzed Hg concentrations and isotopic compositions in the soil–plant system to elucidate the spatial distribution and source contributions of Hg in the timberline ecotone. The transition from coniferous forests to shrubbery resulted in a distinct decrease in Hg pool size—67% in vegetation and 19% in soil, respectively. The Hg isotopic mixing model further demonstrated that vegetation-induced atmospheric Hg⁰ deposition, with an average contribution of 74 ± 10%, was the dominant source of soil Hg in the timberline ecotone. Hg concentration exhibited an increasing trend from Oi to Oe soil horizons, followed by a decline in the mineral layers. The rising Hg concentrations in organic soils resulted from accelerated organic carbon loss during decomposition, while the decreasing gradient in mineral soils was primarily driven by the combined effects of geological Hg sources and long-term Hg depletion during soil formation. Vegetation transition across the timberline ecotone significantly reduced Hg pool dynamics in the soil–plant system. The upward migration of fir forests driven by climate warming may enhance atmospheric Hg deposition, underscoring the need for a comprehensive assessment of Hg cycling in warming alpine ecosystems.

The abundant lakes in the boreal region are an active zone for cycling of organic carbon (OC), functioning as simultaneous sources and sinks of atmospheric carbon dioxide. While previous studies have documented increased OC loading and accelerated carbon accumulation to inland waters, the drivers of this increase remain ambiguous. In this study we investigate carbon accumulation rates in 208 boreal lakes in the context of land use. We show that modern sediment carbon accumulation rates in boreal lakes have increased to median of 24.5 g m⁻² y⁻¹, representing a roughly fivefold increase over Holocene record, mainly due to increased terrestrial organic carbon inputs. Land use has become the most significant predictor of modern lake sediment carbon accumulation with lake morphometrics primarily affecting the efficiency of carbon burial. Recalcitrant terrestrial carbon from forested and peatland areas is buried more effectively, whereas increased soil erosion and enhanced mineralization of labile OC from agricultural runoff reduces the relative burial of terrestrial OC.

Rivers are significant sources of dissolved inorganic nitrogen, contributing to coastal eutrophication and hypoxia. While the impact of large rivers is well documented, less is known about small rivers that directly discharge into the sea after draining urban areas. Their aggregate biogeochemical significance should not be overlooked, given their potentially high loads of nitrogen. There are various kinds of human activities within a river catchment that may contribute different types and amounts of inorganic nitrogen. Identifying the relative contribution of each of these sources is important to facilitate the planning of environmental management measures that can effectively reduce riverine nutrient loading. Focusing on the subtropical Lam Tsuen River in Hong Kong, this study made use of dual nitrogen and oxygen isotopes to identify sources of nutrients within the catchment. Results indicate that nutrients were mainly released into the system in lowland areas in the form of sewage originating from anthropogenic activities. Downstream sites had a significantly higher proportion of nitrate originating from sewage than upstream sites. Nitrification generally accounted for more than 40% of the nitrite and nitrate found in the catchment. Assimilation in the river was found to be negligible, suggesting that most nutrients were transported into the seasonally hypoxic Tolo Harbour. Compared to streams worldwide that do not drain urbanised areas, the Lam Tsuen River generally discharges 10 times more inorganic nitrogen per catchment area. This nitrogen delivered to the coast ultimately consumes roughly 2% of Tolo Harbour’s oxygen. Together with four other similar-sized or even larger rivers, up to 10% of the harbour’s dissolved oxygen will be consumed. Overall, our findings highlight the need for a better accounting of the impact of these small rivers on marine nitrogen budgets.

Dissolved black carbon (DBC) is a key component of the global carbon cycle, yet its seasonal dynamics and river-to-sea transport remain poorly understood, particularly in Southeast Asia where anthropogenic pressures are intense. This study investigates the spatial and seasonal variability of DBC along with dissolved organic matter (DOM) in the main branch of the Red River (North Vietnam) based on three sampling campaigns conducted in March (dry season), June (early wet season), and September 2023 (late wet season). DBC concentrations increased from 29 μg C L⁻¹ in March to 66 μg C L⁻¹ in September, following rainfall-driven inputs. This seasonal pattern was accompanied by changes in DOM quality, as inferred from optical indices: higher SUVA₂₅₄ (specific UV absorbance at 254 nm), a_CDOM(350) (absorption coefficient of chromophoric DOM at 350 nm), and HIX (humification index) in September indicated more terrestrial and humified material, while higher BIX (biological index) in March suggested a higher contribution of fresher, autochthonous DOM. Spatial trends showed a downstream decrease in DBC in June, likely due to abiotic degradation (particularly photodegradation) and dilution. This contrasted with the increasing concentrations from Hanoi to the estuary in March and September, which may be linked to local inputs during dry-season groundwater dynamics and rainfall. DOM optical indices support a contribution of low-DBC groundwater near Hanoi in March. Estimated DBC fluxes at the estuary reached up to 20.7 Gg yr⁻¹, representing 0.11% of the global riverine DBC flux to the ocean during the wet season. These results emphasize the role of tropical rivers as dynamic conveyors of combustion-derived carbon, where seasonality and local processes, such as rainfall, photodegradation, and groundwater inflow, strongly shape DBC transport from land to sea.

As one of the major global biomes, grasslands contribute substantially to the storage of soil organic carbon (SOC) and macroelements such as nitrogen (N) and sulfur (S). However, while SOC and N storage in grassland soils have been extensively studied in the past, similar assessments for S are missing. We conducted a meta-analysis to determine which soil, climate or management parameters were most relevant in controlling S storage compared to SOC and N storage. We collected data on SOC, total N, and total S concentrations in grassland topsoils from previously published studies, along with data on mean annual temperature, mean annual precipitation, soil pH, and soil texture. We additionally classified conditions at each site according to Reference Soil Groups, Koeppen climate classes and management systems. Our data set includes a total of 248 observations from 30 studies in 15 different countries published between 1958 and 2024. We generally observed similar patterns in SOC, N, and S storage, with Reference Soil Group and Koeppen climate class as the most relevant parameters determining total element concentration or element ratios in grassland topsoils, while the type of grassland management did not consistently affect element concentrations and ratios. However, we observed that S concentration was increased especially in soils that were influenced by geogenic inputs of S from the parent material or with groundwater influx, with corresponding changes in C:S and N:S ratios. This resulted in larger variability in S storage in grassland soils compared to SOC and N storage.

Symptoms of eutrophication are increasingly evident in remote clearwater lakes. To identify the sources and dynamics of phosphorus release, we measured total phosphorus (TP) in sediments and soluble reactive phosphorus (SRP) fluxes across the sediment–water interface at 54 locations in a deep temperate lake. Once renowned for its clear waters, Lake Stechlin has experienced a fourfold increase in water column TP over the past decade. SRP fluxes from sediments generally increased with water depth across all three lake basins, although there were significant variations in SRP concentrations, up to threefold, among sampling locations at the same depth. Notably, the lake´s total mean SRP flux in June (1.12 mg m⁻² day⁻¹) was higher than that determined in October (0.74 mg m⁻² day⁻¹). This result can be attributed to the substantial contribution (about 31% of total SRP release) of shallower sediments (0–20 m), which are not affected by seasonal anoxia. Our findings highlight a notable spatial variability of SRP fluxes and underscore the importance of considering often overlooked shallow sediments when assessing P dynamics in lakes.