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.

Climate warming is causing more extreme weather conditions, with both larger and more intense precipitation events as well as extended periods of drought in many regions of the world. The consequence is an alteration of the hydrological regime of streams and rivers, with an increase in the probability of extreme hydrological conditions. Mediterranean-climate regions usually experience extreme hydrological events on a seasonal basis and thus, freshwater Mediterranean ecosystems can be used as natural laboratories for better understanding how climate warming will impact ecosystem structure and functioning elsewhere. In this paper, we revisited and contextualized historical and new datasets collected at Fuirosos, a well-studied Mediterranean intermittent stream naturally experiencing extreme hydrological events, to illustrate how the seasonal alternation of floods and droughts influence hydrology, microbial assemblages, water chemistry, and the potential for biogeochemical processing. Moreover, we revised some of the most influential conceptual and quantitative frameworks in river ecology to assess to what extent they incorporate the occurrence of extreme hydrological events. Based on this exercise, we identified knowledge gaps and challenges to guide future research on freshwater ecosystems under intensification of the hydrological cycle. Ultimately, we aimed to share the lessons learned from ecosystems naturally experiencing extreme hydrological events, which can help to better understand warming-induced impacts on hydrological transport and cycling of matter in fluvial ecosystems.

The processes involved in the carbon cycle are essential for marine trophic networks and global climate regulation. Interactions within the microbial loop play key roles in carbon transformation and transport across the food web. The Argentine Patagonian Shelf in the Southwestern Atlantic Ocean is a hotspot for carbon sequestration. However, our understanding of microbial impacts on carbon cycling in this area remains limited. This study examines the microbial community structure and its role in the carbon transformation during a progression of the spring bloom along the Patagonian shelf-break and adjacent ocean. This progression was studied in a latitudinal track where we observed a gradient of Dissolved Organic Matter (DOM) complexity. In the northern area, the bloom termination was characterised by low Chlorophyll-a concentrations, with smaller organisms (Synechococcus) dominating the autotrophic plankton biomass, and high viral concentrations. DOM showed high humification and aromaticity, indicating an intensified microbial activity by heterotrophic bacteria that followed the production of phytoplankton-derived DOM. In the southern area, high Chlorophyll-a was mainly attributed to large protist plankton, accompanied by abundant heterotrophic bacteria and bioavailable DOM from recent phytoplankton blooms. These results showed that during bloom termination, bacterial production of refractory compounds significantly immobilises carbon, suggesting a potential pathway for carbon sequestration. Additionally, data suggest high carbon retention on the shelf side of the front by microbial transformation and efficient trophic transfer within the microbial community, while the side influenced by the Malvinas Current, presents high carbon export by advection and a higher degree of unutilised carbon from bacterial origin. These findings highlight rapid shifts in carbon dynamics driven by microbial successions during different bloom phases.

The increased mining of metals required to meet future demands also generates vast amounts of waste rock that depending on the ore, can contain substantial amounts of metal sulfides. Unconstrained storage of these mining biproducts results in the release of acidic metal laden effluent (termed ‘acid rock drainage’) that causes serious damage to recipient ecosystems. This study investigated the development of 16S rRNA gene based microbial communities and physiochemical characteristics over two sampling occasions in three age classes of rock, from newly mined to > 10 years in a boreal metal sulfide waste repository. Analysis of the waste rocks showed a pH decrease from the youngest to oldest aged waste rock suggesting the development of acid rock leachate. The microbial communities differed between the young, mid, and old samples with increasing Shannon’s H diversity with rock age. This was reflected by the young age microbial community beta diversity shifting towards the mid aged samples suggesting the development of a community adapted to the low temperature and acidic conditions. This community shift was characterized by the development of iron and sulfur oxidizing acidophilic populations that likely catalyzed the dissolution of the metal sulfides. In conclusion, the study showed three potential microbial community transitions from anaerobic species adapted to underground conditions, through an aerobic acidophilic community, to a more diverse acidophilic community. This study can assist in understanding acid rock drainage generation and inform on strategies to mitigate metal and acid release.

Boreal peatlands store abundant carbon (C) belowground because of saturated conditions and cold temperatures, which inhibit the enzymatic release of dissolved organic carbon (DOC) from organic matter. However, metals may also bind DOC, as well as nitrogen (N) and phosphorus (P), and their impact may vary among peatlands with differing hydrology. To assess variation of metal-C-nutrient interactions within and among peatlands and with depth, we sampled cores from seven peatlands in the Marcell Experimental Forest, Minnesota, including bogs, poor fens, and a rich fen. We extracted peat with sodium sulfate to release elements bound with exchangeable metals such as calcium (Ca) or aluminum (Al), and with sodium dithionite to release elements bound with the redox-active metals iron (Fe) and manganese (Mn). We compared extracted elements to long-term peat porewater measurements. Mean DOC extracted by sulfate or dithionite in the bogs and poor fens was 5 or 8 times greater, respectively, than porewater DOC, and in the rich fen it was 8 or 38 times greater. Similarly, N and P extracted by sulfate and dithionite were 10–24 times higher than porewater in the bogs and poor fens and 7–55 times higher in the rich fen. The ratio and absolute values of redox-sensitive and ion-exchangeable elements varied by element among peatland types and with peat depth and values were not always greater in fens than bogs. We conclude that both redox-active (Fe) and non-redox-active (Ca and Al) metals bind important pools of peatland C and nutrients regardless of peatland hydrologic type and despite the very low total mineral content of these boreal peats.