Global Biogeochemical Cycles最新文献

Northern peatlands have been a carbon sink since their initiation. This has been simulated by existing process-based models. However, most of these models are limited by lacking sufficient processes of the N cycle in peatlands. Here, we use a peatland biogeochemistry model incorporated with N-related processes of fixation, deposition, gas emission, loss through water flow, net mineralization, plant uptake and litterfall to project the role of the peatlands in future radiative forcing (RF). Simulations from 15-ka BP to 2100 are conducted driven by CMIP5 climate forcing data of IPSL-CM5A-LR and bcc-csm1-1, including warming scenarios of RCP 2.6, RCP 4.5 and RCP 8.5. During the Holocene, northern peatlands have an increasing cooling effect with RF up to −0.57 W m⁻². By 1990, these peatlands accumulate 408 Pg C and 7.8 Pg N. Under warming, increasing mineral N content enhances plant net primary productivity; the cooling effect persists. However, RF increases by 0.1–0.5 W m⁻² during the 21st century, mainly due to the stimulated CH₄ emissions. Northern peatlands could switch from a C sink to a source when the annual temperature exceeds −2.2 to −0.5°C. This study highlights that the improved N cycle causes higher CO₂-C sink capacity in northern peatlands. However, it also causes a significant increase in CH₄ emissions, which weakens the cooling effect of northern peatlands in future climate.

Arctic shelves receive a large load of nutrients from Arctic rivers, which play a major role in the biogeochemical cycles of the Arctic Ocean. In this study, we present measurements of dissolved silicon isotopes (δ³⁰Si(OH)₄) around the Laptev Sea and surface waters of the Eurasian shelves collected in October 2018 to document terrestrial silicon modifications on shelves and their contribution to the Arctic basin. Nitrogen was found to be depleted in surface waters and the limiting nutrient to primary production in the Laptev Sea, allowing excess silicon export to the central Arctic Ocean. Heavy δ³⁰Si(OH)₄ in the water column was linked to the strong biological removal of DSi on shelves, enabled by vigorous N recycling. From isotopically constrained processes, we estimate that >50% of the silicon from riverine inputs is removed within the Lena River delta and on the Laptev Sea shelf. Extrapolating this to major Siberian rivers, this leads to an export of 2.5 ± 0.8 kmol/s of riverine silicon through the Transpolar Drift. An updated isotopic budget of the Arctic Ocean reproduces the observed δ³⁰Si(OH)₄ signatures out of the Arctic Ocean and underlines the importance of biological processes in modulating silicon export. Given that opal burial fluxes on Artic shelves are controlled by denitrification and N-limitation, these processes are sensitive to ongoing climate change. As a consequence of higher riverine DSi inputs and shelf denitrification responding to productivity, it is inferred that silicon export from the Arctic Ocean could increase in the future, accompanied by lighter δ³⁰Si(OH)₄ signatures.

The role of manganese (Mn) in ecosystem carbon (C) biogeochemical cycling is gaining increasing attention. While soil Mn is mainly derived from bedrock, atmospheric deposition could be a major source of Mn to surface soils, with implications for soil C cycling. However, quantification of the atmospheric Mn cycle, which comprises emissions from natural (desert dust, sea salts, volcanoes, primary biogenic particles, and wildfires) and anthropogenic sources (e.g., industrialization and land-use change due to agriculture), transport, and deposition, remains uncertain. Here, we use compiled emission data sets for each identified source to model and quantify the atmospheric Mn cycle by combining an atmospheric model and in situ atmospheric concentration measurements. We estimated global emissions of atmospheric Mn in aerosols (<10 μm in aerodynamic diameter) to be 1,400 Gg Mn year⁻¹. Approximately 31% of the emissions come from anthropogenic sources. Deposition of the anthropogenic Mn shortened Mn “pseudo” turnover times in 1-m-thick surface soils (ranging from 1,000 to over 10,000,000 years) by 1–2 orders of magnitude in industrialized regions. Such anthropogenic Mn inputs boosted the Mn-to-N ratio of the atmospheric deposition in non-desert dominated regions (between 5 × 10⁻⁵ and 0.02) across industrialized areas, but that was still lower than soil Mn-to-N ratio by 1–3 orders of magnitude. Correlation analysis revealed a negative relationship between Mn deposition and topsoil C density across temperate and (sub)tropical forests, consisting with atmospheric Mn deposition enhancing carbon respiration as seen in in situ biogeochemical studies.

Croplands that experience seasonal soil freezing and thawing have been shown to be significant sources of N₂O emissions. Yet, there is a paucity of year-round N₂O emission data for one of the most significant crop production regions that seasonally freeze, the Prairies. Here, we present micrometeorological N₂O fluxes measured over 4 years in Saskatchewan, Canada, to evaluate the magnitude of freeze-thaw N₂O emissions and investigate its driving factors. Significant thaw related emissions occurred in 2 of the 4 years and were associated with relatively higher fall nitrate levels and a more gradual soil thawing period. Overall, fall soil nitrate levels were a strong explanatory variable for the differences in non-growing season (NGS) N₂O emission (r² = 0.485). Measured cumulative N₂O emissions for the NGS were 123–938 g N ha⁻¹ and were much smaller than those obtained at other cold climate sites but amounted to 52% of annual totals on average. The November to April period contributed 30% of the annual total emissions in years without major thaw events, but 70% in years with significant thaws. NGS N₂O emissions were not explained by cumulative freezing degree days unlike most other cold climate sites. We propose that NGS N₂O emissions are more strongly influenced by thaw dynamics during freezing-thawing conditions in dry regions, whereas freezing intensity is the dominant factor for wetter regions. Our results indicate that even for a semi-arid region freeze-thaw is an important source of N₂O emissions and must be considered for more accurate reporting and development of mitigation strategies.

Soil microbial biomass (SMB) is a fundamental contributor to soil ecosystem services. Mycorrhizal fungi, a significant group of soil microbes, play essential roles in regulating carbon allocation and nutrient cycles. Acknowledging the profound importance of SMB and mycorrhizal symbiosis, our objective was to explore how mycorrhizal types modulate the global patterns of SMB across varied land use types (LUTs). Using data from 329 independent studies, we categorized vegetation species with defined mycorrhizal types into arbuscular mycorrhizal (AM) type (with 958 observations) or mixed AM and ectomycorrhizal (AM + ECM) type (with 481 observations). This categorization served as the foundation for our investigation into the impacts of various LUTs and environmental conditions (mean annual temperature, and mean annual precipitation, MAP) on global SMB patterns associated with specific mycorrhizal associations. The overall mean value of SMB was remarkably higher under AM + ECM type (92.23 ± 4.73 nmol/g) compared with that under AM type (49.45 ± 1.87 nmol/g) at a global scale. The primary factor contributing to this difference was the natural system. Additionally, the AM + ECM type (0.19 ± 0.01) exhibited a higher F:B ratio (Fungi-to-bacteria ratio) than the AM type (0.16 ± 0.001), attributed to the cumulative effects of different LUTs. Furthermore, SMB was markedly positively affected by aridity index under AM type and negatively influenced by temperature under AM + ECM type. Besides, MAP had a pronounced positive impact on SMB under AM type, while exhibiting a negative impact under AM + ECM type. Our study presented evidence affirming the essential role of mycorrhizal associations in shaping global patterns of SMB in response to environmental factors across varied LUTs.

With almost 700 Pg of carbon, marine dissolved organic carbon (DOC) stores more carbon than all living biomass on Earth combined. However, the controls behind the persistence and the spatial patterns of DOC concentrations on the basin scale remain largely unknown, precluding quantitative assessments of the fate of this large carbon pool in a changing climate. Net removal rates of DOC along the overturning circulation suggest lifetimes of millennia. These net removal rates are in stark contrast to the turnover times of days to weeks of heterotrophic microorganisms, which are the main consumers of organic carbon in the ocean. Here, we present a dynamic “MICrobial DOC” model (MICDOC) with an explicit representation of picoheterotrophs to test whether ecological mechanisms may lead to observed decadal to millennial net removal rates. MICDOC is in line with >40,000 DOC observations. Contrary to other global models, the reactivity of DOC fractions is not prescribed, but emerges from a dynamic feedback between microbes and DOC governed by carbon and macronutrient availability. A colimitation of macronutrients and organic carbon on microbial DOC uptake explains >70% of the global variation of DOC concentrations, and governs characteristic features of its distribution. Here, decadal to millennial net removal rates emerge from microbial processes acting on time scales of days to weeks, suggesting that the temporal variability of the marine DOC inventory may be larger than previously thought. With MICDOC, we provide a foundation for assessing global effects on DOC related to changes in heterotrophic microbial communities in a future ocean.

Deep Chlorophyll Maxima (DCMs) are ubiquitous in low-latitude oceans, and of recognized biogeochemical and ecological importance. DCMs have been observed in the Southern Ocean, initially from ships and recently from profiling robotic floats, but with less understanding of their onset, duration, underlying drivers, or whether they are associated with enhanced biomass features. We report the characteristics of a DCM and a Deep Biomass Maximum (DBM) in the Inter-Polar-Frontal-Zone (IPFZ) south of Australia derived from CTD profiles, shipboard-incubated samples, a towbody, and a BGC-ARGO float. The DCM and DBM were ∼20 m thick and co-located with the nutricline, in the vicinity of a subsurface ammonium maximum characteristic of the IPFZ, but ∼100 m shallower than the ferricline. Towbody transects demonstrated that the co-located DCM/DBM was broadly present across the IPFZ. Large healthy diatoms, with low iron requirements, resided within the DCM/DBM, and fixed up to 20 mmol C m⁻² d⁻¹. The BGC-ARGO float revealed that DCM/DBM persisted for >3 months. We propose a dual environmental mechanism to drive DCM/DBM formation and persistence within the IPFZ: sustained supply of both recycled iron within the subsurface ammonium maxima, and upward silicate transport from depth. DCM/DBM cell-specific growth rates were considerably slower than those in the overlying mixed layer, implying that phytoplankton losses such as herbivory are also reduced, possibly because of heavily silicified diatom frustules. The light-limited seasonal termination of the observed DCM/DBM did not result in a “diatom dump”, rather ongoing diatom downward export occurred throughout its multi-month persistence.

While modeling efforts have furthered our understanding of marine iron biogeochemistry and its influence on carbon sequestration, observations of dissolved iron (dFe) and its relationship to physical, chemical and biological processes in the ocean are needed to both validate and inform model parameterization. Where iron comes from, how it is transported and recycled, and where iron removal takes place are critical mechanisms that need to be understood to assess the relationship between iron availability and primary production. To this end, hydrographic and trace metal observations across the GO-SHIP section SR3, south of Tasmania, Australia, have been analyzed in tandem with the novel application of an optimum multiparameter analysis. From the trace-metal distribution south of Australia, key differences in the drivers of dFe between oceanographic zones of the Southern Ocean were identified. In the subtropical zone, sources of dFe were attributed to waters advected off the continental shelf, and to recirculated modified mode and intermediate water-masses of the Tasman Outflow. In the subantarctic zone, the seasonal replenishment of dFe in Antarctic surface and mode waters appears to be sustained by iron recycling in the underlying mode and intermediate waters. In the southern zone, the dFe distribution is likely driven by dissolution and scavenging by high concentrations of particles along the Antarctic continental shelf and slope entrained in high salinity shelf water. This approach to trace metal analysis may prove useful in future transects for identifying key mechanisms driving marine dissolved trace metal distributions.

The drivers governing the air-sea CO₂ exchange and its variability in the coastal ocean are poorly understood. Using a global ocean biogeochemical model, this study quantifies the influences of thermal changes, oceanic transport, freshwater fluxes, and biological activity on the spatial and seasonal variability of CO₂ sources/sinks in the global coastal ocean. We identify five typical coastal behaviors (dominated by biological drawdown, vertical transport, land imprint, intracoastal alongshore currents, and weak CO₂ sources and sinks coastal regions) and propose a new processed-based delineation of the coastal ocean based on the quantification of these controlling processes. We find that the spatiotemporal variability of CO₂ sources/sinks is dominated by strong exchanges with the open ocean and intracoastal processes, while continental influences are restricted to hotspot regions. In addition, where thermal changes appear to drive the seasonal CO₂ variability, it often results from compensating effects between individual non-thermal terms, especially biological drawdown and vertical transport.

Hydrogen sulfide is produced by heterotrophic bacteria in anoxic waters and via carbonyl sulfide hydrolysis and phytoplankton emissions under oxic conditions. Apparent losses of dissolved cadmium (dCd) and zinc (dZn) in oxygen minimum zones (OMZs) of the Atlantic and Pacific Oceans have been attributed to metal-sulfide precipitation formed via dissimilatory sulfate reduction. It has also been argued that such a removal process could be a globally important sink for dCd and dZn. However, our studies from the North Pacific OMZ show that dissolved and particulate sulfide concentrations are insufficient to support the removal of dCd via precipitation. In contrast, apparent dCd and dZn deficits in the eastern tropical South Pacific OMZ do reside in the oxycline with particulate sulfide maxima, but they also coincide with the secondary fluorescence maxima, suggesting that removal via sulfide precipitation may be due to a combination of dissimilatory and assimilatory sulfate reduction. Notably, dCd loss via precipitation with sulfide from assimilatory reduction was found in upper oxic waters of the North Pacific. While dissimilatory sulfate reduction may explain local dCd and dZn losses in some OMZs, our evaluation of North Pacific OMZs demonstrates that dCd and dZn losses are unlikely to be a globally relevant sink. Nevertheless, metal sulfide losses due to assimilatory sulfate reduction in surface waters should be considered in future biogeochemical models of oceanic Cd (and perhaps Zn) cycling.