Global Biogeochemical Cycles最新文献

Dissolved organic matter (DOM) is a key component of the lake biogeochemistry. Hydrology link variables influencing lake DOM at local and watershed scales, but its role at macroscales remains less understood. We studied the DOM concentration and composition from 548 lakes across the five major Canadian continental basins using absorption spectroscopy and parallel factor analysis, and ultra-high resolution mass spectroscopy, and linked this to deuterium excess (d-excess), derived from stable water isotopes as a proxy for evaporation, water residence time, and regional hydrology. DOM concentration and composition varied greatly within and across basins, with strong correlations between molecular and optical properties. At a continental scale, d-excess and TP concentration were the main drivers of DOM concentration and composition. TP positively influenced DOM concentration, and specific DOM components (e.g., Aliphatics), suggesting nutrient-driven effects on lake metabolism that varied regionally. DOM concentration declined with d-excess, but the relationships between individual DOM molecular composition classes and d-excess differed among components and basins, resulting in regional differences in DOM composition along hydrologic gradients. The inferred source composition DOM based on these patterns had subtle regional differences, with Aliphatics related to the average regional altitude and Aromatics related to the average regional soil organic content. We show that DOM processing along the hydrologic continuum is the key factor establishing differences in DOM composition in lakes at a continental scale. Overall, TP influenced DOM through effects on primary production and metabolism, whereas d-excess integrated the selective degradation and accumulation of DOM along the aquatic network.

The Tibetan Plateau (TP) has become warmer and wetter, which has led to permafrost degradation, increased soil erosion and higher CO₂ emissions from water bodies. There are ongoing debates on whether the TP will transition from a carbon sink to a source, but relevant analysis accounting for both terrestrial and aquatic carbon processes at the catchment scale remains lacking. Here, we develop a process-based distributed water-carbon coupling model applicable for the permafrost region GBEHM-Carbon, which integrates the vertical water-heat-carbon fluxes through the soil-vegetation-atmosphere continuum, the lateral water-carbon fluxes transported from hillslopes to river channels, as well as the water-sediment-carbon dynamics in river networks along the river routing process. The model is then applied in the Yellow River Source Region (YRSR) on the eastern TP. The results show that the ecosystem carbon budget of the YRSR is 3.3 Tg C/yr on average, with an increasing trend during 1960–2019. Lateral carbon fluxes played a substantial role, accounting for 30.1% of the net ecosystem production (NEP) on average, with their relative contribution rising to 37.8% during the 2010s. Regions undergoing permafrost degradation are identified as potential hotspots for carbon sink-to-source transitions, primarily driven by reduced vegetation productivity and enhanced heterotrophic soil respiration. In addition, riverine carbon fluxes show distinct spatial patterns associated with stream order and are strongly modulated by hydrological conditions. This study provides critical insights into the long-term variations of water and carbon fluxes in the TP headwater catchments, and offers valuable guidance for water resource and ecological management in alpine river systems.

The twilight zone plays a pivotal role in the oceanic carbon cycle because large amounts of organic carbon (OC) exported from the euphotic zone are degraded within this layer. However, the magnitude and variability of this remineralization flux remain incompletely understood, particularly across varying regional scales. Using a new particulate excess barium (PBa_xs)-oxygen utilization rate transfer function, this study examines spatial patterns of twilight zone OC remineralization fluxes (F_{OC_remineral}) in the western North Pacific (wNP) and the South China Sea (SCS). Results reveal a pronounced latitudinal gradient in wNP open waters, with PBa_xs concentrations and F_{OC_remineral} in the upper 150–600 m increasing from the oligotrophic western North Pacific Subtropical Gyre (NPSG) to the nutrient-rich North Pacific Transition Zone (NPTZ). Despite its subtropical location, the SCS basin shows values comparable to those of the NPTZ. Satellite-derived net primary production (NPP) and export production (EP) distributions generally align with F_{OC_remineral} patterns, indicating a strong association between upper and mesopelagic ocean carbon cycling. Using NPP, EP, and F_{OC_remineral} data, we estimated the twilight zone OC remineralization ratio (r-ratio) relative to the euphotic zone biological pump carbon export efficiency (e-ratio). The NPTZ shows enhanced carbon storage potential, with higher e-ratio and lower r-ratio values, while the western NPSG exhibits an opposite trend, indicating reduced carbon sequestration capacity. This spatial variability underscores the need for a comprehensive understanding of twilight zone OC remineralization and its connection to upper-ocean carbon cycling, in order to accurately assess the biological pump's role in the long-term pelagic carbon sink.

Microbial nitrogen (N) demand substantially influences soil N transformations during vegetation succession, its dynamics across contrasting successional pathways remain poorly understood yet. We used vector model, GeoChip 5.0, and ¹⁵N-tracer to investigate microbial N metabolism and gross N transformation across primary succession (20–130 years post-glacier retreat) and secondary succession (grassland to primary forest) in the eastern Tibetan Plateau. During primary succession, microbial N limitation was progressively alleviated, coincident with increasing plant biomass and richness, as well as the accumulation of soil labile carbon (C), total N, and phosphorus (P). Increases in soil C, N, and P pools enhanced both gross and net N mineralization rates directly and indirectly by elevating ureC abundance and 4-β-N-acetylglucosaminidase activity. Microbial N limitation emerged in the late coniferous stage of secondary succession. This pattern was associated with declines in plant richness, reductions in soil labile C and pH from early stage (grassland/shrubland) or mid-successional broadleaf stages to late coniferous stages, and concomitant decreases in leucine aminopeptidase activity. Such changes led to a lower gross N mineralization rate and reduced soil N availability in late secondary forests. Overall, our results indicate that microbial N limitation is gradually relieved during soil development following glacier retreat as plant communities and soil C and nutrients become more favorable, whereas late-stage secondary coniferous forests experience microbial N limitation driven by more homogeneous vegetation and reduced soil N mineralization rates. These findings imply that succession-specific management strategies are needed to conserve soil N cycling and ecosystem resilience in fragile subalpine ecosystems.

Particulate excess barium (pBa_xs), collected in situ, which is thought to exist as barite (BaSO₄), is potentially a powerful proxy of water column respiration. Here we use this proxy along the US GEOTRACES GA03 North Atlantic and GP16 East Pacific transects, comparing respiration rates derived from pBa_xs distributions, using previously proposed algorithms, and respiration rates calculated using ²³⁰Th-normalized particulate organic carbon (POC) fluxes in the water column. Both transects traversed upwelling regimes, oxygen deficient zones (ODZs), near-shore, and open-ocean gyre stations, providing a more robust evaluation of the methodology than previous work. Respiration rates were estimated over two different depth intervals in the mesopelagic zone (100–500 m and 100–1,000 m). Generally, respiration rates were different (p < 0.05) between biogeochemical provinces (i.e., gyre vs. Oxygen deficient zone stations) irrespective of method. Rates along GA03 were more difficult to establish using ²³⁰Th-normalized fluxes due to the absence of observed POC flux maxima in the top ∼0–100 m. Still, rate estimates using depth weighted average pBa_xs concentrations and Th-normalized POC fluxes in the 100–500 m interval agreed well within certain biogeochemical provinces: for example, at GP16 ODZ stations, average Th-normalized POC respiration rate estimates were 3.3 ± 1.6 whereas pBa_xs-based estimates were 2.9 ± 0.5 m mol C m⁻² day⁻¹. Excess particulate Ba appears to be a reasonable proxy for water column POC respiration. We suggest that average excess pBa_xs concentrations may be used as a method to calculate respiration rates in the 100–500 m depth interval if other methods are not available.

Zinc (Zn) is biogeochemically important due to its crucial role in biological processes. In the global ocean, there is an apparent coupling between the concentrations of zinc and silicon (Si), and the ratio between their concentrations is nearly constant in the global ocean. However, this coupling is observed to be disrupted locally for example, in the subarctic North Pacific (NP) Ocean. The aim of the current study was to investigate the roles of uptake parameters, continental-shelf supply, and regeneration of Zn on the observed Zn-Si decoupling in the subarctic NP, employing two distinct circulation fields. Model experiments using two different circulation fields led to the two different conclusions about the cause of the Zn-Si decoupling: continental-shelf supply or regeneration. A comparison between the two circulation fields revealed that older water mass in the NP and greater POC export there led to more regenerated Zn and a higher probability of decoupling without the continental shelf supply. For more quantitative evaluation on relative important of regeneration and continental-shelf supply, both refining biogeochemical models and a circulation field that realistically reproduces regenerated nutrient distribution are required.

Seagrasses are key carbon sinks in the biosphere and, when intentionally conserved or restored, constitute a promising natural solution for climate change mitigation. Unfortunately, they are also experiencing major anthropogenic and climatic pressures that can lead to seagrass degradation or even result in difficult-to-reverse abrupt shifts (i.e., tipping point responses) to complete loss. Although the possibility of tipping point responses in seagrass ecological dynamics has been acknowledged, the potential cascading effect of tipping points on biogeochemical dynamics, shifting seagrass ecosystems from carbon sinks to carbon sources, remains largely unexplored. In this context, we developed a mechanistic stoichiometric model that couples ecological and biogeochemical functioning to assess the effects of three major stressors—mechanical damage, eutrophication, and warming—on the carbon storage capacity of seagrass ecosystems. After parameterizing our model for the Mediterranean seagrass Posidonia oceanica (L.) Delile, we explored these stress cases to identify the processes and feedbacks that can cause ecological tipping points leading to changes in biogeochemical dynamics. The model shows that when ecological tipping points occur, they cascade into biogeochemistry and precipitate abrupt losses of carbon storage. Importantly, even without a tipping point, carbon storage still declined abruptly rather than gradually along stressor gradients. Yet, the dynamics of carbon losses depended on the type of stressor, indicating the need to further test the relative contribution of biotic and abiotic drivers in shifting seagrasses from carbon sinks to carbon sources.

This study synthesizes the budgets of three greenhouse gases (GHG, namely CO₂, CH₄, N₂O) for Russia over two decades (2000–2009 and 2010–2019) using bottom-up and top-down approaches, as part of the Regional Carbon Cycle Assessment and Processes, Phase 2 (RECCAP2). Published estimates of natural sources and sinks of these GHGs in Russia vary widely. Here, bottom-up estimates are based on eddy covariance measurements, the Integrated Land Information System of Russia (ILIS-LEA), field data, Dynamic Global Vegetation Models (DGVMs), and regional models. The bottom-up approach estimated Net Ecosystem Exchange (NEE) at −0.64 ± 0.17 and −0.57 ± 0.14 Pg C yr⁻¹, for decades 2000–2009 and 2010–2019, respectively. Top-down atmospheric inversions provide similar NEE carbon flux estimates with comparable uncertainties at −0.56 ± 0.26 and −0.73 ± 0.27 Pg C yr⁻¹ for the two decades. Differences between these approaches arise from distinct flux components and structural assumptions. ILIS-LEA indicates a slightly declining carbon sink in 2010–2019, driven by increased disturbances. In contrast, DGVMs suggest a stable carbon sink over both decades but they do not fully simulate the effects of disturbances and recovery. Top-down inversions reveal an increasing CO₂ sink, suggesting with additional observed constraints on biomass carbon increment that soil and non-forest biomes absorb more carbon than predicted by DGVMs and ILIS-LEA models. A Bayesian averaging approach estimates natural ecosystems acting as a GHG sink with a land-to-atmosphere flux of −1.55 ± 0.91 and −1.47 ± 0.82 Pg CO₂-eq. yr⁻¹. Accounting for both natural and anthropogenic emissions across the Russian territory shifts the net GHG balance to a source around 1.2 Pg CO₂-eq. yr⁻¹.

Atmospheric deposition is an important pathway for delivering micronutrient and pollutant trace elements (TEs) to the surface ocean. In the central Arctic, much of this supply takes place onto sea ice during winter, before eventual delivery to the ocean during summertime melt. However, the seasonality of aerosol TE loading, solubility, and deposition flux are poorly studied over the Arctic Ocean, due to the difficulties of wintertime sampling. As part of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, aerosols collected during winter and spring (December–May) were analyzed for soluble, labile, and total TE concentrations. Despite low dust loading, mineral aerosol accounted for most of the variation in total Fe, Al, Ti, V, Mn, and Th concentrations. In contrast, soluble TE concentrations were more closely linked to non-sea-salt sulfate, and Fe solubility was significantly higher during Arctic winter (median = 6.5%) than spring (1.9%), suggesting an influence from Arctic haze. Beryllium-7 data were used to calculate an average bulk deposition velocity of 613 ± 153 m d⁻¹ over most of the study period, which was applied to calculate seasonal deposition fluxes of total, labile, and soluble TEs to the central Arctic. Total TE fluxes (173 ± 145 nmol m⁻² d⁻¹ for Fe) agreed within a factor of two or three with earlier summertime estimates, with generally higher wintertime concentrations offset by a lower deposition velocity. Cumulative seasonal deposition of total, labile, and soluble Fe to the central Arctic Ocean was calculated at 25 ± 21, 5 ± 3, and 2 ± 2 μmol m⁻², respectively.

Ocean acidification (OA) threatens coral calcification by reducing the carbonate ion concentration that corals need to build their skeletons. However, assessments of the impacts of long-term OA are scarce, limiting our understanding of the response and acclimatization of corals to high pCO₂ levels. Here we present a 42-year (1968–2010) seasonal δ¹¹B and B/Ca records from Porites corals at Dongsha Atoll, located in the northern South China Sea. Our results reveal a rapid decline in seawater pH over this period, at a rate of −0.0021 ± 0.0008 pH units per year. Of special interest is that the interannual variability in seawater pH appears to be primarily co-regulated by hydrological changes in the Pearl River and fluctuations in the strength of Kuroshio intrusion. These factors are linked to large-scale climate systems and interannual-to-decadal variability, including the Pacific Decadal Oscillation, El Nino-Southern Oscillation, and East Asian Winter Monsoon. Meanwhile, reconstructed carbonate chemistry from the coral calcifying fluid suggests that Porites corals at Dongsha Atoll are able to physiologically modulate their internal pH. This up-regulation of internal pH not only buffers seasonal fluctuations in the aragonite saturation state and sustains stable calcification rates year-round, but also aids in long-term resistance to the detrimental effects of OA.