Global Biogeochemical Cycles最新文献

The priming effects (PEs) of soil organic carbon (SOC) decomposition is a crucial process affecting the C balance of terrestrial ecosystems. However, there is uncertainty about how PEs will respond to climate warming. In this study, we sampled soils along a subtropical elevation gradient in China and conducted a 126-day lab-incubation experiment with and without the addition of ¹³C-labeled high-bioavailability glucose or low-bioavailability lignin. Based on the mean annual temperature (MAT) of each elevation (9.3–16.4°C), a temperature increase of 4°C was used to explore how PEs mediate the decomposition of SOC in response to warming. Our results showed that the magnitude of glucose-induced PEs (PE_glucose) was higher than lignin-induced PEs (PE_lignin), with both PEs linearly increasing with MAT. Across the MAT (i.e., elevation) gradient, short-term warming had a constant magnitude of negative effects on PE_glucose, whereas rising MAT exacerbated the negative effects of short-term warming on PE_lignin. Moreover, the temperature sensitivity of SOC decomposition decreased after adding glucose and lignin across the MAT gradient, suggesting that fresh C inputs may prime the microbial breakdown of labile SOC under warming. Taken together, warming alleviated SOC loss due to PEs through varying mechanisms depending on substrate bioavailability. Warming mediated the PE_glucose by increasing available nitrogen and weakening microbial nitrogen-mining but inhibited the PE_lignin by shifting from microbial nitrogen-mining to microbial co-metabolization. Our findings highlight the role of warming in regulating the PEs and suggest that incorporating the suppression effect of warming on PEs can contribute to the accurate prediction of soil C dynamics in a warming world.

Radium-226(²²⁶Ra) and barium (Ba) exhibit similar chemical behaviors and distributions in the marine environment, serving as valuable tracers of water masses, ocean mixing, and productivity. Despite their similar distributions, these elements originate from distinct sources and undergo disparate biogeochemical cycles, which might complicate the use of these tracers. In this study, we investigate these processes by analyzing a full-depth ocean section of ²²⁶Ra activities (T_1/2 = 1,600 years) and barium concentrations obtained from samples collected along the US GEOTRACES GP15 Pacific Meridional Transect during September–November 2018, spanning from Alaska to Tahiti. We find that surface waters possess low levels of ²²⁶Ra and Ba due to export of sinking particulates, surpassing inputs from the continental margins. In contrast, deep waters have higher ²²⁶Ra activities and Ba concentrations due to inputs from particle regeneration and sedimentary sources, with ²²⁶Ra inputs primarily resulting from the decay of ²³⁰Th in sediments. Further, dissolved ²²⁶Ra and Ba exhibit a strong correlation along the GP15 section. To elucidate the drivers of the correlation, we used a water mass analysis, enabling us to quantify the influence of water mass mixing relative to non-conservative processes. While a significant fraction of each element's distribution can be explained by conservative mixing, a considerable fraction cannot. The balance is driven using non-conservative processes, such as sedimentary, rivers, or hydrothermal inputs, uptake and export by particles, and particle remineralization. Our study demonstrates the utility of ²²⁶Ra and Ba as valuable biogeochemical tracers for understanding ocean processes, while shedding light on conservative and myriad non-conservative processes that shape their respective distributions.

Plant invasion and land reclamation have drastically transformed the landscape of coastal wetlands globally, but their resulting effects on soil organic nitrogen (SON) mineralization and nitrous oxide (N₂O) production remain unclear. In this study, we examined 21 coastal wetlands across southern China that have undergone habitat transformation from native mudflats (MFs) to Spartina alterniflora marshes (SAs), and subsequently to earthen aquaculture ponds (APs). We determined the SON net mineralization rate and the presence of pertinent enzyme-encoding genes, namely chiA, pepA, and pepN. The SON net mineralization rate increased by 46.7% following the conversion of MFs to SAs but decreased by 33.1% in response to the transformation of SAs to APs. Nevertheless, there was no significant difference in the estimated mineralization efficiency of soil microbes among the habitat types. The results of structural equation modeling showed that N-mineralization gene abundance played a major role in regulating SON mineralization. Although less than 20% of the SON was estimated to be labile/semi-labile, SON mineralization was important in sustaining soil N₂O production, with 5.8% of the mineralized N being fed into N₂O production. Overall, our findings showed that the presence of S. alterniflora increased both SON content and mineralization rate, which would in turn promote further proliferation of this exotic plant along the coast. The conversion of S. alterniflora marshes to APs partially mitigated the positive effects of exotic plant invasion on SON turnover.

Nickel (Ni) is a micronutrient that plays a role in nitrogen uptake and fixation in the modern ocean and may have affected rates of methanogenesis on geological timescales. Here, we present the results of a diagnostic model of global ocean Ni fluxes which addresses key questions about marine Ni cycling. Sparsely available observations of Ni concentration are first extrapolated into a global gridded climatology using tracers with better observational coverage such as macronutrients, and testing three different machine learning techniques. The physical transport of Ni is then estimated using the ocean circulation inverse model (OCIM2), revealing regions of net convergence or divergence. These diagnostics are not based on any assumption about Ni biogeochemical cycling, but their spatial patterns can be used to infer where biogeochemical processes such as biological Ni uptake and regeneration take place. Although Ni and silicate (Si) have similar concentration patterns in the ocean, we find that the spatial pattern of Ni uptake in the surface ocean is similar to phosphate (P) uptake but not to silicate (Si) uptake. This suggests that their similar distributions arise from different biogeochemical mechanisms, consistent with other evidence showing that Ni is not incorporated into diatom frustules. We find that Ni:P ratios at uptake do not decrease as Ni concentrations approach 2 nM, which challenges the hypothesis of a ∼2 nM pool of non-bioavailable Ni in the surface ocean. Finally, we find that the net regeneration of Ni occurs deeper in the ocean than for P, though not as deeply as for Si.

Phytoplankton stoichiometry modulates the interaction between carbon, nitrogen and phosphorus cycles. Environmentally driven variations in phytoplankton C:N:P can alter biogeochemical cycling compared to expectations under fixed ratios. In fact, the assumption of fixed C:N:P has been linked to Earth System Model (ESM) biases and potential misrepresentation of responses to future change. Here we integrate key elements of the Adaptive Trait Optimization Model (ATOM) for phytoplankton stoichiometry with the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) ocean biogeochemical model. Within a series of global ocean-ice-ecosystem retrospective simulations, ATOM-COBALT reproduced observations of phytoplankton N:P, and compared to static ratios, exhibited reduced phytoplankton P-limitation, enhanced N-fixation, and increased low-latitude export, improving consistency with observations and highlighting the biogeochemical implications of dynamic N:P. We applied ATOM-COBALT to explore the impacts of different physiological mechanisms hypothesized to underlie N:P variation, finding that two mechanisms together drove the observed patterns: proportionality of P-rich ribosomes in phytoplankton cells to growth rates and reductions in P-storage during scarcity. A third mechanism which linked temperature with phytoplankton biomass allocations to non-ribosomal proteins, led only to relatively modest impacts because this mechanism decreased the temperature dependence of phytoplankton growth rates, compensating for changes in N:P. We find that there are quantitative response differences that associate distinctive biogeochemical footprints with each mechanism, which are most apparent in highly productive low-latitude regions. These results suggest that variable phytoplankton N:P makes phytoplankton productivity and export resilient to environmental changes, and support further research on the physiological and environmental drivers of phytoplankton stoichiometry and biogeochemical role.

N₂ fixation is a central process of the marine nitrogen cycle, yet little is known about how this process varies from year-to-year. Here, we investigate this variability in the Western Tropical Atlantic (WTA), a region where N₂ fixation is prevalent, fueled, in part, by the nutrient input from the Amazon River. We use hindcast simulations from 1983 through 2019 with the Regional Oceanic Modeling System (ROMS) coupled to the Biogeochemical Elemental Cycling (BEC) model expanded to include Diatom-Diazotroph Assemblages (DDAs). Throughout the WTA, we find a substantial level of interannual variability of N₂ fixation, altering it by up to 33%, and locally by up to nearly 60%. Part of this interannual variability is driven by variations in the Amazon River discharge, which during high discharge events leads to reduced rates in the upper parts of the plume and strongly enhanced rates in the downstream part. This dipole pattern is a consequence of the riverine inputs of phosphorus and the competition with non-diazotrophs for this limiting resource. Another part of the N₂ fixation variability is driven by the Atlantic Meridional Mode (AMM), and the El Niño-Southern Oscillation (ENSO). These processes alter N₂ fixation primarily through the supply of the limiting nutrients phosphorus and iron by vertical mixing, while the role of top-down control through grazing is relatively limited in our model. The high, and so far not well recognized interannual N₂ fixation variability can lead to erroneous extrapolation of field measurements and inaccuracies in the marine nitrogen budget, if unaccounted for.

The Editors of the Global Biogeochemical Cycles express their appreciation to those who served as peer reviewers for the journal in 2023.

Release of iron (Fe) from continental shelves is a major source of this limiting nutrient for phytoplankton in the open ocean, including productive Eastern Boundary Upwelling Systems. The mechanisms governing the transport and fate of Fe along continental margins remain poorly understood, reflecting interaction of physical and biogeochemical processes that are crudely represented by global ocean biogeochemical models. Here, we use a submesoscale-permitting physical-biogeochemical model to investigate processes governing the delivery of shelf-derived Fe to the open ocean along the northern U.S. West Coast. We find that a significant fraction (∼20%) of the Fe released by sediments on the shelf is transported offshore, fertilizing the broader Northeast Pacific Ocean. This transport is governed by two main pathways that reflect interaction between the wind-driven ocean circulation and Fe release by low-oxygen sediments: the first in the surface boundary layer during upwelling events; the second in the bottom boundary layer, associated with pervasive interactions of the poleward California Undercurrent with bottom topography. In the water column interior, transient and standing eddies strengthen offshore transport, counteracting the onshore pull of the mean upwelling circulation. Several hot-spots of intense Fe delivery to the open ocean are maintained by standing meanders in the mean current and enhanced by transient eddies and seasonal oxygen depletion. Our results highlight the importance of fine-scale dynamics for the transport of Fe and shelf-derived elements from continental margins to the open ocean, and the need to improve representation of these processes in biogeochemical models used for climate studies.

Wetlands are integral to the global carbon cycle, serving as both a source and a sink for organic carbon. Their potential for carbon storage will likely change in the coming decades in response to higher temperatures and variable precipitation patterns. We characterized the dissolved organic carbon (DOC) and dissolved organic matter (DOM) composition from 12 different wetland sites across the USA spanning gradients in climate, landcover, sampling depth, and hydroperiod for comparison to DOM in other inland waters. Using absorption spectroscopy, parallel factor analysis modeling, and ultra-high resolution mass spectroscopy, we identified differences in DOM sourcing and processing by geographic site. Wetland DOM composition was driven primarily by differences in landcover where forested sites contained greater aromatic and oxygenated DOM content compared to grassland/herbaceous sites which were more aliphatic and enriched in N and S molecular formulae. Furthermore, surface and porewater DOM was also influenced by properties such as soil type, organic matter content, and precipitation. Surface water DOM was relatively enriched in oxygenated higher molecular weight formulae representing HUP_{High O/C} compounds than porewaters, whose DOM composition suggests abiotic sulfurization from dissolved inorganic sulfide. Finally, we identified a group of persistent molecular formulae (3,489) present across all sites and sampling depths (i.e., the signature of wetland DOM) that are likely important for riverine-to-coastal DOM transport. As anthropogenic disturbances continue to impact temperate wetlands, this study highlights drivers of DOM composition fundamental for understanding how wetland organic carbon will change, and thus its role in biogeochemical cycling.

Lithogenic materials such as terrigenous lithogenic particles (TLP) can efficiently promote the formation and sinking of mineral-associated marine organic matter, acting as important ballast and potentially playing an important role in the global carbon cycle. To assess the influence of TLP on fluxes of particulate organic carbon (POC) and other biogeochemical cycles, we construct TLP forcing fields based on global riverine suspended sediment data and then apply them to the Community Earth System Model, version 2 (CESM2) modified with the TLP ballasting effect term. Simulations forced by different concentrations of TLP transported in the surface ocean or along the bottom of continental shelves and slopes are conducted. When the TLP transports seaward along the bottom, simulated POC fluxes at 100 and 2,000 m decrease about 11% and 19%, respectively, for the global ocean, and about 9% and 12%, respectively, for the oceanic regions of continental margins. The initial abiotic ballast processes triggered by TLP input increase POC fluxes, causing additional removal and burial of dissolved iron in continental margins. This further enhances the accumulation of macronutrients in the upwelling regions and their advection transport to neighboring subtropical gyres, thus altering regional productivity when simulations reach quasi-equilibrium. When consider the impacts of TLP in simulations, the simulated POC flux exhibits an increase in subtropical gyres but a decrease in tropical Pacific and mid-high latitude regions. The present work highlights the importance of TLP in global biogeochemical cycles, suggesting that the amount of carbon sequestration might be overestimated without TLP in models.