Biogeochemistry最新文献

Di-nitrogen (N₂) fixation rates, and the diversity of the organisms that fix N₂, remain largely unconstrained in the aquatic landscapes or aquascapes (e.g., lakes, wetlands, streams, rivers, estuaries) between land and sea. As a result, we lack a mechanistic understanding of the controls and contributions of N₂ fixation across disparate aquatic environments, and cannot accurately incorporate this process into local and global nitrogen (N) budgets. This special issue brings together papers highlighting current advances in understanding of N₂ fixation within and across all aquatic habitats, integrating novel methodology for studying N₂ fixation, quantification of N₂ fixation fluxes in understudied habitats, the role of N₂ fixation in biotic assemblages, and the rate and fate of fixed N in heterogeneous landscapes. Together, these papers address important gaps in understanding and highlight the frontiers of research on N₂ fixation in aquatic habitats.

Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N₂O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N₂O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N₂O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N₂O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N₂O:N₂ indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios.

Peatlands play a crucial role in the global carbon (C) cycle, making their restoration a key strategy for mitigating greenhouse gas (GHG) emissions and retaining C. This study analyses the most common restoration pathways employed in boreal and temperate peatlands, potentially applicable in tropical peat swamp forests. Our analysis focuses on the GHG emissions and C retention potential of the restoration measures. To assess the C stock change in restored (rewetted) peatlands and afforested peatlands with continuous drainage, we adopt a conceptual approach that considers short-term C capture (GHG exchange between the atmosphere and the peatland ecosystem) and long-term C sequestration in peat. The primary criterion of our conceptual model is the capacity of restoration measures to capture C and reduce GHG emissions. Our findings indicate that carbon dioxide (CO₂) is the most influential part of long-term climate impact of restored peatlands, whereas moderate methane (CH₄) emissions and low N₂O fluxes are relatively unimportant. However, lateral losses of dissolved and particulate C in water can account up to a half of the total C stock change. Among the restored peatland types, Sphagnum paludiculture showed the highest CO₂ capture, followed by shallow lakes and reed/grass paludiculture. Shallow lakeshore vegetation in restored peatlands can reduce CO₂ emissions and sequester C but still emit CH₄, particularly during the first 20 years after restoration. Our conceptual modelling approach reveals that over a 300-year period, under stable climate conditions, drained bog forests can lose up to 50% of initial C content. In managed (regularly harvested) and continuously drained peatland forests, C accumulation in biomass and litter input does not compensate C losses from peat. In contrast, rewetted unmanaged peatland forests are turning into a persistent C sink. The modelling results emphasized the importance of long-term C balance analysis which considers soil C accumulation, moving beyond the short-term C cycling between vegetation and the atmosphere.

The rewetting of long-term drained peatlands leads to the development of eutrophic shallow lakes, gradually inhabited by reed communities. These shallow lakes are characterized by significant nutrient and methane emissions. To comprehend the fate of organic compounds from decaying Phragmites australis litter in water and anaerobic soil layers, we conducted a 1.6-year decomposition experiment. The experiment employed bulk and lignin-derived phenol analysis, as well as Fourier-transform infrared spectroscopy. As anticipated, the highest level of decomposition was observed in the surface water body of the shallow lake, while the non-rooted degraded peat exhibited the lowest decay. The bulk mass loss of plant litter decreased with depth from 55 to 27% across the four decomposition environments. Analysis using infrared spectroscopy indicated that the decrease in mass loss was primarily driven by the breakdown of carbohydrates, which constitute a significant portion of plant litter. Interestingly, litter in the rooted degraded peat layer exhibited the highest degree of lignin decay. Furthermore, the study revealed a preferential loss of vanillin phenols and an accumulation of p-hydroxyl phenols. These findings suggest that the increased methane emissions in rewetted fens may be partially attributed to the demethoxylation of vanillin phenols and the subsequent formation of p-hydroxyl phenols. In conclusion, this study provides valuable insights into anaerobic lignin decomposition of plant litter and sheds light on potential mechanisms underlying elevated methane emissions in rewetted peatlands. Furthermore, the study’s findings hold significant implications for both carbon cycling and sequestration within these ecosystems, thereby stimulating further research into the microbial community and its extended effects.

The Arctic is warming at a rate twice that of other global ecosystems and changing climate conditions in the Arctic are mobilizing long frozen permafrost stores of organic carbon. In ice-rich regions, permafrost thaw on sloping terrain can cause land subsidence, and the development of thaw-driven mass wasting. The Peel Plateau, Northwest Territories, Canada has extensive thaw-driven landslides called retrogressive thaw slumps that are exposing early Holocene age paleo-thaw layers and Pleistocene age glaciogenic material deposited by the Laurentide Ice Sheet. This study aimed to see if unique retrogressive thaw slump derived permafrost inputs could be readily observed in streams across six diverse thermokarst features via optical and ultrahigh-resolution mass spectrometry. Aquatic samples from water draining thermokarst slump features, and downstream of thermokarst inputs exhibited higher dissolved organic carbon concentrations and lower aromaticity as evidenced by optical parameters (e.g. declining SUVA₂₅₄, increasing S_275-295) and FT-ICR MS metrics (e.g. lower AI_mod and nominal oxidation state of carbon) versus upstream of thermokarst impacts. Increases in the relative abundances of assigned heteroatomic molecular formulae (e.g. CHON, CHOS, CHONS) were also greater within and downstream of thermokarst features. The unique molecular formulae present in permafrost thermokarst inputs were determined (n = 1844) and subsequently tracked downstream. These permafrost marker formulae were enriched in aliphatics and H/C, as well as heteroatoms and exhibited low aromaticity. A portion of the unique molecular fingerprint persisted downstream, highlighting the potential to not only assess thermokarst inputs but also to follow these inputs and their fate downstream throughout the aquatic network.

Nitrogen (N) is a critical element in many ecological and biogeochemical processes in forest ecosystems. Cycling of N is sensitive to changes in climate, atmospheric carbon dioxide (CO₂) concentrations, and air pollution. Streamwater nitrate draining a forested ecosystem can indicate how an ecosystem is responding to these changes. We observed a pulse in streamwater nitrate concentration and export at a long-term forest research site in eastern North America that resulted in a 10-fold increase in nitrate export compared to observations over the prior decade. The pulse in streamwater nitrate occurred in a reference catchment in the 2013 water year, but was not associated with a distinct disturbance event. We analyzed a suite of environmental variables to explore possible causes. The correlation between each environmental variable and streamwater nitrate concentration was consistently higher when we accounted for the antecedent conditions of the variable prior to a given streamwater observation. In most cases, the optimal antecedent period exceeded two years. We assessed the most important variables for predicting streamwater nitrate concentration by training a machine learning model to predict streamwater nitrate concentration in the years preceding and during the streamwater nitrate pulse. The results of the correlation and machine learning analyses suggest that the pulsed increase in streamwater nitrate resulted from both (1) decreased plant uptake due to lower terrestrial gross primary production, possibly due to increased soil frost or reduced solar radiation or both; and (2) increased net N mineralization and nitrification due to warm temperatures from 2010 to 2013. Additionally, variables associated with hydrological transport of nitrate, such as maximum stream discharge, emerged as important, suggesting that hydrology played a role in the pulse. Overall, our analyses indicate that the streamwater nitrate pulse was caused by a combination of factors that occurred in the years prior to the pulse, not a single disturbance event.

Rewetting drained agricultural peatlands aids in restoring their original ecosystem functions, including carbon storage and sustaining unique biodiversity. 30–60 cm of topsoil removal (TSR) before rewetting for Sphagnum establishment is a common practice to reduce nutrient concentrations and greenhouse gas emissions, and increase water conductivity. However, the topsoil is carbon-dense and preservation in situ would be favorable from a climate-mitigation perspective. The effect of reduced TSR on Sphagnum establishment and nutrient dynamics on degraded and rewetted raised bogs remains to be elucidated. We conducted a two-year field experiment under Sphagnum paludiculture management with three TSR depths: no-removal (TSR0), 5–10 cm (TSR5), and 30 cm (TSR30) removal. We tested the effects of TSR on Sphagnum establishment and performance, nutrient dynamics, and hotspot methane emissions. After two years, TSR5 produced similar Sphagnum biomass as TSR30, while vascular plant biomass was highest in TSR0. All capitula nitrogen (N > 12 mg/g) indicated N-saturation. Phosphorus (P) was not limiting (N/P < 30), but a potential potassium (K) limitation was observed in year one (N/K > 3). In TSR0, ammonium concentrations were > 150 µmol/l in year one, but decreased by 80% in year two. P-concentrations remained high (c. 100 µmol/l) at TSR0 and TSR5, and remained low at TSR30. TSR30 and TSR5 reduced hotspot methane emissions relative to TSR0. We conclude that all TSR practices have their own advantages and disadvantages with respect to Sphagnum growth, nutrient availability and vegetation development. While TSR5 may be the most suitable for paludiculture, its applicability for restoration purposes remains to be elucidated. Setting prioritized targets when selecting the optimal TSR with peatland rewetting is pivotal.

Small artificial waterbodies are larger emitters of carbon dioxide (CO₂) and methane (CH₄) than natural waterbodies. The Intergovernmental Panel on Climate Change (IPCC) recommends these waterbodies are accounted for in national emission inventories, yet data is extremely limited for irrigated landscapes. To derive a baseline of their greenhouse gas footprint, we investigated 38 irrigation farm dams in horticulture and broadacre cropping in semi-arid NSW, Australia. Dissolved CO₂, CH₄, and nitrous oxide (N₂O) were measured in spring and summer, 2021–2022. While all dams were sources of CH₄ to the atmosphere, 52% of irrigation farm dams were sinks for CO₂ and 70% were sinks for N₂O. Relationships in the linear mixed effect models indicate that CO₂ concentrations were primarily driven by dissolved oxygen (DO), ammonium, and sediment carbon content, while N₂O concentration was best explained by an interaction between DO and ammonium. Methane concentrations did not display any relationship with typical biological variables and instead were related to soil salinity, trophic status, and size. Carbon dioxide-equivalent emissions were highest in small (< 0.001 km²) dams (305 g CO₂-eq m⁻² season⁻¹) and in those used for recycling irrigation water (249 g CO₂-eq m⁻² season⁻¹), with CH₄ contributing 70% of average CO₂-eq emissions. However, irrigation dams had considerably lower CH₄ emissions (mean 40 kg ha⁻¹ yr⁻¹) than the IPCC emission factor (EF) of 183 kg CH₄ ha⁻¹ yr⁻¹ for constructed ponds and lower N₂O EF of 0.06% than the indirect EF for agricultural surface waters (0.26%). This synoptic survey reveals existing models may be severely overestimating (4–5 times) farm dam CH₄ and N₂O emissions in semi-arid irrigation areas. Further research is needed to define these artificial waterbodies in emissions accounting.

The greenhouse gas (GHG) emissions of spring cereal monoculture under long-term conventional tillage (CT) and no-till (NT) treatment established in 2018 were measured in a peatland in Southwestern Finland during the period 2018–2021. Nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) fluxes were measured with chambers approximately every two weeks throughout the period under study. Net ecosystem exchange was measured during the growing seasons, and hourly ecosystem respiration (ER) and gross photosynthesis (GP) were modelled with empirical models. Across the whole period, annual emissions were 6.8 ± 1.2 and 5.7 ± 1.2 Mg CO₂–C ha ⁻¹ yr⁻¹ (net ecosystem carbon balance), 8.8 ± 2.0 and 7.1 ± 2.0 kg N₂O–N ha⁻¹ yr⁻¹, and − 0.43 ± 0.31 and − 0.40 ± 0.31 kg CH₄-C ha⁻¹ yr⁻¹ for CT and NT, respectively. The global warming potential was lower in NT (p = 0.045), and it ranged from 26 to 34 Mg CO₂ eq. ha⁻¹ yr⁻¹ in CT and from 19 to 31 Mg CO₂ eq. ha⁻¹ yr⁻¹ in NT. The management effect on the rates of single GHGs was not consistent over the years. Higher GP was found in CT in 2019 and in NT in 2020. Differences in ER between treatments occurred mostly outside the growing season, especially after ploughing, but the annual rates did not differ statistically. NT reduced the N₂O emissions by 31% compared to CT in 2020 (p = 0.044) while there were no differences between the treatments in other years. The results indicate that NT may have potential to reduce slightly CO₂ and N₂O emissions from cultivated peat soil, but the results originate from the first three years after a management change from CT to NT, and there is still a lack of long-term results on NT on cultivated peat soils.