Biogeochemistry最新文献

The Skagerrak is the main depot center for organic matter and anthropogenic pollutants from the entire North Sea. Changes in ocean circulation or suspended matter supply might impact the sediment redox conditions. Indeed, little is known about the response of Skagerrak sediment and associated pollutants to different oxygen levels. We investigated sediments from three stations within the Skagerrak and incubated them for up to twelve months under aerobic and anaerobic conditions. Furthermore, we present the first δ^98/95Mo data for Skagerrak sediment profiles and the incubations to be utilized as a redox tracer. The sediment profiles of metals reflected anthropogenic pollution (Cu, Ni, Pb) but differed regionally with redox conditions. We differentiated redox conditions mainly by sediment and porewater Fe, Mn, Mo and δ^98/95Mo. In aerobic incubations, no Mn or Fe reduction was detected, while under anaerobic conditions, initial Mn and Fe reduction decreased after approximately three months. Under anaerobic conditions, a strong isotopic fractionation of Mo in the dissolved phase was found, reaching up to 5.03 ± 0.10‰, probably caused by incomplete thiolation of molybdate under low hydrogen sulfide levels. During the incubations, Cd, Cu, Ni, Pb were mobilized. While Cu and Cd were mobilized under aerobic conditions, Ni and Pb mobilization depended mainly on remineralization and redox conditions. Our results show that changes in oxygen conditions in the Skagerrak can have significant effects on the (legacy) metals stored in the sediment over the past decades.

Graphical Abstract

Blue carbon represents the organic carbon retained in marine coastal ecosystems. Sabkhas (an Arabic for “mudflats”), formed in tidal environments under arid conditions, have been proposed to be capable of carbon sequestrating. Despite the growing understanding of the critical role of blue carbon ecosystems, there is a current dispute about whether sabkhas around the Persian Gulf can contribute to carbon retention as a blue carbon ecosystem. The arguments often lack data on a critical contributor, inorganic carbon in the form of carbonates, which can drive the net carbon exchange with the atmosphere. In this study we inventory organic and inorganic carbon retention capacity in two contrasting sabkhas of the Qatar Peninsula: carbonaceous Dohat Faishakh and siliciclastic Khor Al-Adaid. Despite the differences in organic carbon stock between the two sabkhas, the Dohat Faishakh sabkha has higher (37.17 ± 0.81 Mg C_org ha⁻¹) than it is in the Khor Al-Adaid sabkha (13.75 ± 0.38 Mg C_org ha⁻¹) for 0. 44 m sediment depth, the organic carbon retained in sabkhas is similar to those reported for mangroves and salt marshes. Notably, calculated CO₂ net sequestration indicated that both sabkhas evade CO₂ into the atmosphere. Thus, carbonate formation negated organic carbon accumulation in carbonaceous sabkha. Consequently, for proper evaluation of sabkhas as a blue carbon ecosystem, an inorganic carbon analysis, especially of carbonate formation, is inevitable. Considering only organic carbon stock may ay overestimate carbon sequestration capacity.

Streams serve as open windows for carbon emissions to the atmosphere due to the frequent supersaturation of carbon dioxide (CO₂) and methane (CH₄) that originates from large carbon input during runoff and associated in-stream processes. Due to the high spatial and temporal variability of the underlying environmental drivers (e.g., concentrations of dissolved CO₂ and CH₄, turbulence, and temperature), it has remained difficult to address the importance and upscale the emissions to annual whole-system and regional values. In this study, we measured concentrations and calculated emissions of CO₂ and CH₄ at diel and seasonal scales at 15 stations in a 1.4 km² stream network that drains a mixed lowland catchment consisting of agriculture (210 km²), forest (56 km²), and lakes, ponds, and wetlands (22 km²) in the upper River Odense, Denmark to evaluate environmental drivers behind the spatiotemporal variability. We used automatically venting floating chambers to calculate hourly diffusive fluxes of CO₂ and CH₄ and CH₄ ebullition. We found: 1) highly supersaturated CO₂ and CH₄ concentrations (median: 175 and 0.33 µmol L⁻¹, respectively) and high diffusive fluxes of CO₂ and CH₄ (median: 3,608 and 19 µmol m⁻² h⁻¹, respectively); 2) lower daytime than nighttime diffusive emissions of CO₂ in spring and summer, but no diel variability of CH₄; 3) higher concentrations and emissions of CH₄ at higher temperatures; and 4) higher emissions of CH₄ at stations located in sub-catchments with higher agricultural coverage. Ebullition of CH₄ peaked at two stations with soft organic sediment and low summer flow, and their ebullition alone constituted 30% of total annual CH₄ emissions from the stream network. Mean annual CO₂ emissions from the hydrological network (37.15 mol CO₂ m⁻² y⁻¹) exceeded CH₄ emissions 100-fold (0.43 mol CH₄ m⁻² y⁻¹), and their combined warming potential was 1.83 kg CO₂e m⁻² y⁻¹. Overall, agricultural sub-catchments had higher CH₄ emissions from streams, while lakes and ponds likely reduced downstream CH₄ and CO₂ emissions. Our findings demonstrate that CO₂ and CH₄ emissions data at high spatial and temporal resolution are essential to frame the heterogeneous stream conditions, understand gas emissions regulation, and upscale to annual values for hydrological networks and larger regions.

Nitrification, or the microbial transformation of ammonium (NH₄⁺–N) to nitrate, is influenced by NH₄⁺–N and dissolved oxygen availability, water temperature, and carbon-to-nitrogen ratios. Open-canopy agricultural streams receive excess inorganic nitrogen (N) from the surrounding landscape and the mineralization of organic-rich sediments, and the form and timing of these N inputs varies throughout the year. Compared to forested streams, the seasonality of nitrification rates in agricultural streams are not well documented. We conducted nitrification assays on stream sediments to estimate seasonal rates in three agricultural streams from summer 2020 to spring 2021. We documented seasonal variation in nitrification rates and identified changes in environmental controls [e.g., stream temperature, NH₄⁺–N and dissolved organic carbon (DOC) availability, chlorophyll-a]. Nitrification rates were highest in spring (54.4 ± 12.7 mg N m⁻² d⁻¹; p = 0.02), coinciding with elevated NH₄⁺–N and higher stream temperatures relative to winter (p < 0.001). Rates were lowest in autumn (19.9 ± 3.5 mg N m⁻² d⁻¹) when organic carbon concentrations peaked (17.2 ± 10.3 mg C L⁻¹; p = 0.01). Algal senescence in autumn may allow heterotrophs to outcompete nitrifiers for NH₄⁺–N. However, partial least square regression analyses indicated that sediment organic matter (as %OM) is an important positive predictor of nitrification, suggesting carbon can be an indirect positive control on nitrification. In the context of previous studies, agricultural streams had elevated NH₄⁺–N concentrations, but nitrification rates were comparable to those in less impacted systems. Although complex interactions exist among rates and drivers, rates from this study help expand documentation of nitrification in agricultural streams, and provide insight into temporal variation and dominant controls.

Nitrogen (N) fixation in association with mosses could be a key source of new N in tropical montane cloud forests since these forests maintain high humidity levels and stable temperatures, both of which are important to N fixation. Here, nutrient availability could be a prominent control of N fixation processes. However, the mechanisms and extent of these controls, particularly in forests at different successional stages, remains unknown to date. To address this knowledge gap, we investigated the impact of N, phosphorus (P) and molybdenum (Mo) additions on moss-associated N fixation in tropical montane cloud forests of two successional stages, an old-growth forest and an early-successional natural regrowth forest. We hypothesized that if N is available, N fixation rates would be rapidly reduced, while P and Mo would promote nitrogenase activity. Our results show that Mo additions did not affect N fixation rates, whereas N and P additions, in different doses and combinations, immediately reduced N fixation in both forests. Nonetheless, rates recovered within 1 year of nutrient additions. Nitrogen fixation rates associated with ground-covering mosses were similar in both forests. Interestingly, one year after the nutrient additions, N fixation rates across all the treatments were higher in the natural regrowth forests than the mature forests, suggesting more nutrient limitation in these regrowing forests, likely as a result of higher demand for growth. Our study highlights how moss-associated N fixation responds to changes in nutrient availability across distinct successional stages, deepening our understanding of processes that contributes to tropical montane cloud forests.