Global Biogeochemical Cycles最新文献

Large-scale geographical distributions in nitrogen and carbon stable isotope ratios (δ¹⁵N and δ¹³C) of particulate organic matter (POM) are essential to understand the variation in the baseline of pelagic food webs in the Pacific Ocean, where phytoplankton production and biological N₂ fixation are highly variable because of heterogeneity of nitrate and iron supply. Here, we determined their isoscapes during summer and discussed potential factors characterizing regional ecosystems from the viewpoint of nitrogen cycling. We collected a total of 2,289 and 2,278 isotope values for δ¹³C and δ¹⁵N, respectively, by synthesizing previously published data with our newly measured data, and analyzed their relationships with temperature, concentrations of nitrate and chlorophyll-a, and N₂ fixation activity, obtained from databases. POM δ¹³C and δ¹⁵N regionally varied in ranges of −30 to −18‰ and −4 to 14‰, respectively. POM δ¹³C was correlated positively with temperature throughout the ocean. In contrast, POM δ¹⁵N was negatively correlated with nitrate concentration at high latitudes and with N₂ fixation activity at low latitudes. High values (>8‰) of POM δ¹⁵N were identified mainly in the marginal area of equatorial upwelling; the highest values (10–14‰) were in the subtropical Southeastern Pacific. Using the isotopic values and nitrate concentration, we classified the ecosystems into 10 groups. Our data demonstrated the distribution patterns of ecosystems with different degrees of nitrate utilization, which are presumably associated with iron supply, and ecosystems sustained by different nitrogen sources: diazotrophic nitrogen and nitrate supplied below the nitracline and/or horizontally advected.

Savanna ecosystems in sub-Saharan Africa harbor substantial yet relatively unexplored reserves of soil organic carbon (SOC). Our study unravels for the first time the interplay between climate, reference soil groups, and anthropogenic disturbances in shaping SOC dynamics in these ecosystems. We analyzed SOC along climosequences in natural woodlands in Mozambique and Zambia, with mean annual temperature (MAT) of 20–24°C, and mean annual precipitation (MAP) of 365–1,227 mm. Anthropogenic disturbances were assessed through comprehensive field surveys and remote sensing of vegetation and indices change. MAT and evapotranspiration (PET) had no discernible effect on SOC. Bulk SOC, particulate organic matter, and mineral-associated organic matter stocks in the topsoil (0–10 cm) increased with MAP, though this relationship was not significant for the subsoil. MAP explained only 35% of topsoil SOC variability, limited by anthropogenic disturbances, which raised SOC stocks in the dry savanna but resulted in SOC losses at >600 mm MAP, which even extended into subsoil. For sites with little disturbance in the past decades, there were soil group-specific effects of MAP on SOC, explaining up to 85% of data variability. In disturbed sites, human presence altered the carbon (C) balance to an extent that, as rough estimate, could account for up to 2.6 Gt CO₂-C loss over 20 years in wetter sites, with another 2.4 Gt CO₂-C at risk as populations spread into these otherwise pristine environments. Accurate modeling of climate-change effects on the C cycle must therefore include the transformative impacts of current human activities, such as wood harvesting and grazing.

Monthly global sea-air CO₂ flux maps are created on a 1° by 1° grid from surface water fugacity of CO₂ (fCO_2w) observations using an extremely randomized trees (ET) machine learning technique (AOML-ET) over the period 1998–2020. Global patterns and magnitudes of fCO_2w from AOML-ET are consistent with other machine learning methods and with the updated climatology of Takahashi et al. (2009, https://doi.org/10.1016/j.dsr2.2008.12.009). However, the magnitude and trends of sea-air CO₂ fluxes are sensitive to the treatment of atmospheric forcing. In the default configuration of AOML-ET, the average global sea-air CO₂ flux is −1.70 PgC yr⁻¹ with a negative trend of −0.89 ± 0.19 PgC yr⁻¹ decade⁻¹. The large negative trend is driven by a small uptake at the beginning of the record. This leads to increasing sea-air fCO₂ gradients over time, particularly at high latitudes. However, changing the target variable in AOML-ET from fCO_2w to sea-air CO₂ fugacity difference, ∆fCO₂, results in a lower negative trend of −0.51 PgC yr⁻¹ decade⁻¹, though the average flux remains similar at −1.65 PgC yr⁻¹. This trend is close to the consensus trend of ocean uptake from machine learning and models in the Global Carbon Budget of −0.46 ± 0.11 PgC yr⁻¹ decade⁻¹ switching to a gas transfer parameterization with weaker wind speed dependence reduces uptake by 60% but does not affect the trend. Substituting a spatially resolved marine air CO₂ mole fraction product for the zonally invariant marine boundary layer CO₂ product yields greater influx by up to 20% in the industrialized continental outflow regions.

The air-sea transfer of carbon dioxide can be viewed as a dynamic system through which atmospheric and oceanic processes push surface waters away from thermodynamic equilibrium, while diffusive gas transfer pulls them back toward local equilibrium. These push/pull processes drive significant sub-seasonal, seasonal, and interannual variability in air-sea carbon fluxes, the quantification of which is critical both for diagnosing the ocean response to fossil fuel emissions and for attempts to mitigate anthropogenic climate disruption through intentional modification of surface ocean biogeochemistry. In this study, we present a new approach for attributing air-sea carbon fluxes to specific mechanisms. The new framework is first applied to a two-box ocean nutrient and carbon cycle model as an illustrative example. Next, outputs from a regional eddy-resolving model of the Southern Ocean are analyzed. The roles of multiple physical and biogeochemical processes are identified. The decomposition of the seasonal air-sea carbon flux shows the dominant role of biological carbon pumps that are partially compensated by the transport convergence. Finally, the framework is used to diagnose the response to mesoscale iron and alkalinity release, explicitly quantifying transport feedback and eventual impacts on net air-sea carbon flux. Ocean carbon transport has divergent influences between iron and alkalinity release, due to opposing near-surface gradients of dissolved inorganic carbon. We suggest that our attribution framework may be a useful analytical technique for monitoring natural ocean carbon fluxes and quantifying the impacts of human intervention on the ocean carbon cycle.

Riparian zones are known to control the hydrology and biogeochemistry of forest headwater catchments. Some evidence suggests that these riparian-stream connections are shaped by a relatively small volume of soil, or dominant source layer (DSL), through which most water and solutes are routed laterally. However, the hydrological and biogeochemical significance of the DSL has not been broadly evaluated. We compiled data from four forest headwaters, each from different European sites (boreal, temperate, subhumid Mediterranean, semiarid Mediterranean) to test whether DSL dimensions and biogeochemical characteristics vary predictably across ecoregions based on differences in hydroclimate, topography, and soil features. Boreal DSLs were shallow and thin, whereas small-scale topographic heterogeneity shaped DSL dimensions at the temperate site. In the Mediterranean sites, DSLs were deeper and thicker, but upper riparian layers that seldomly connected to the streams had a large influence on the overall lateral flux. Contrasting hydroclimates and soils led to high dissolved organic carbon concentrations in riparian solutions in both boreal and Mediterranean sites. By contrast, nitrate concentrations were driven by differences in soil saturation, being orders of magnitude higher in dry Mediterranean than in wet temperate and boreal riparian soils. Notably, stream chemistry did not consistently reflect riparian DSL chemistry across flow conditions and ecoregions. We hypothesize that ecoregion-specific water sources bypassing the riparian zone, as well as ecoregion-specific in-stream biogeochemical processes could explain these discrepancies. Overall, conceptualizing the varied roles of the DSL across diverse systems can aid in both scientific assessments and management of land-water connectivity in river networks.

We examined the nitrogen (N) biogeochemistry of adjacent cyclonic and anticyclonic eddies near Hawai'i in the North Pacific Subtropical Gyre (NPSG) and explored mechanisms that sustain productivity in the cyclone after the initial intensification stage. The top of the nutricline was uplifted into the euphotic zone in the cyclone and depressed in the anticyclone. Subsurface nutrient concentrations and apparent oxygen utilization at the cyclone's inner periphery were higher than expected from isopycnal displacement, suggesting that shallow remineralization of organic material generated excess nutrients in the subsurface. The excess nutrients may provide a supply of subsurface nutrients to sustain productivity in maturing eddies. The shallow remineralization also raises questions regarding the extent to which cyclonic eddies promote deep carbon sequestration in subtropical gyres such as the NPSG. An upward increase in nitrate ¹⁵N/¹⁴N isotope ratios below the euphotic zone, indicative of partial nitrate assimilation, coincided with negative preformed nutrients—potentially signaling heterotrophic bacterial consumption of carbon-rich (nitrogen-poor) organic material. The ¹⁵N/¹⁴N of material collected in shallow sediment traps was significantly higher in the cyclone than in the anticyclone and showed correspondence to the ¹⁵N/¹⁴N ratio of the nitrate supply, which is acutely sensitive to sea level anomaly in the region. A number of approaches were applied to estimate the contribution of N₂ fixation to export production. Results among approaches were inconsistent, which we attribute to non-steady state conditions during our observation period.

Pyrogenic carbon (PyC) from biomass burning is a large, but poorly quantified, slow-cycling component of the soil organic carbon pool. Modeling of soil carbon dynamics can be improved by including the processes governing the input and cycling of PyC in the soil. The carbon isotope composition of PyC (δ¹³C_PyC) provides a tracer for the partitioning of PyC into the soil from biomass. We report the stocks and δ¹³C values for PyC and organic carbon (OC) for 41 regions dominated by savannas and seasonally wet to arid regions of Australia and Africa. Stocks of PyC in the 0–5 cm interval ranged from 0 to 1.17 MgC ha⁻¹ (mean 0.43 ± 0.25 MgC ha⁻¹) and in the 0–30 cm interval ranged from 0.25 to 3.89 MgC ha⁻¹ (mean 1.65 ± 0.77 MgC ha⁻¹). PyC stocks averaged 8% (but were up to 25%) of total organic carbon (TOC) stocks. Stocks tended to highest in relatively wet, but seasonally dry, regions such as tropical savannas. PyC abundance could be predicted (r = 0.8 to 0.95) from environmental variables only. δ¹³C_PyC values varied widely between regions, but with no systematic differences within regions related to current vegetation or sample depth, likely due to the long residence time of PyC in the soil. δ¹³C_PyC values were strongly correlated with δ¹³C_OC values but were systematically 1–2‰ higher even in C₃ only regions.

Export rates of organic matter (OM) were determined based on PO₄³⁻, NO₃⁻ and O₂ budgets during GEOTRACES cruise GP15 in the Pacific Ocean that crossed subpolar, subtropical and equatorial regimes. Lowest OM export rates at 3–5 mmol C/m²/yr were found in the subtropical regions and highest rates at 9–12 mmol C/m²/yr were found in the equatorial and subpolar regions. Satellite based OM export rates showed similar regional trends but with a significantly larger range. The budget and satellite-based OM export rates were 3–15× higher than estimates of particle loss rates based on ²³⁴Th and sediment trap collections, with the differences primarily due to non-particle forms of OM export and different integration times of methods. The efficiency of export varied from 0.1 to 0.3, with the lowest efficiencies in the subtropics and highest efficiencies in the subpolar and equatorial regions.

The eastern tropical North Pacific oxygen deficient zone (ETNP-ODZ) exhibits a distinct physical and biological environment compared to other oxygenated water columns, leading to a unique scenario of particulate organic matter (POM) production and vertical transport. To elucidate these biological pump processes, we present the first comparison of δ¹⁵N values of nitrate, phenylalanine (Phe), and glutamic acid (Glu) within two distinct size fractions of particles collected along a productivity gradient in the ETNP-ODZ. Low δ¹⁵N_Phe and δ¹⁵N_Glu values in both particle pools at sites with prominent secondary chlorophyll maximum (SCM), compared to the ambient δ¹⁵N-NO₃⁻, suggest the presence of recycled N-utilizing primary producers distinct from those at the primary chlorophyll maximum and their contribution to export. We observed reduced ¹⁵N enrichment of Phe in small particles and a narrower δ¹⁵N_Phe disparity between the two particle size fractions compared to the results from oxic waters, likely due to slower heterotrophic microbial degradation of small particles. Unique δ¹⁵N_Phe and δ¹⁵N_Glu signatures of particles were found at the lower oxycline, potentially attributable to chemoautotrophic production and zooplankton mediation. These findings underscore the need for further investigations targeting particles generated at the SCM, their subsequent alteration by zooplankton, and the new production by chemoautotrophs. This will allow for a better evaluation of the efficiency of the biological pump in the globally expanding ODZs under contemporary climate change.

Terrestrial dissolved inorganic carbon (DIC) and total alkalinity (TAlk) loads have contrasting effects on the pH and carbonate chemistry of the coastal ocean. While TAlk can buffer against ocean acidification, elevated exports of free CO₂ can further exacerbate ocean acidification. In this study, we quantify terrestrial DIC and TAlk loads from rivers and mangrove floodplains across six bioregions and varying flow conditions to assess their impact on the buffering capacity of the Great Barrier Reef (GBR) lagoon in Australia. For a mid-flow year, median terrestrial DIC and TAlk loads ranged from 0.72 to 0.89 Tg C yr⁻¹ and 0.26 to 1.03 Tg C yr⁻¹, respectively. We find that mangrove-dominated terrestrial inputs only have a small influence on the whole GBR but contribute 12.5% (range: 1.9%–45.7%) of the DIC and 18.7% (range: 2.8%–68.2%) of the TAlk inner shelf inventory. Depending on the approach used to estimate TAlk loads, mangroves have a potential short-term buffering effect on near-shore coastal waters due to higher TAlk loads. However, long-term mangrove TAlk production via pyrite formation complicates this interpretation, highlighting the need for ongoing monitoring to understand the complex interplay between terrestrial inputs and their effect on the GBR carbonate chemistry.