Global Biogeochemical Cycles
ISSN / EISSN : 0886-6236 / 1944-9224
Published by: American Geophysical Union (AGU) (10.1029)
Total articles ≅ 3,034
Latest articles in this journal
Global Biogeochemical Cycles, Volume 35; https://doi.org/10.1029/2020gb006933
Anthropogenic nitrogen deposition is widely considered to increase CO2 sequestration by land plants on a global scale. Here, we demonstrate that bedrock nitrogen weathering contributes significantly more to nitrogen-carbon interactions than anthropogenic nitrogen deposition. This working hypothesis is based on the introduction of empirical results into a global biogeochemical simulation model over the time period of the mid-1800s to the end of the 21st century. Our findings suggest that rock nitrogen inputs have contributed roughly 2 to 11 times more to plant CO2 capture than nitrogen deposition inputs since pre-industrial times. Climate change projections based on RCP 8.5 show that rock nitrogen inputs and biological nitrogen fixation contribute 2 to 5 times more to terrestrial carbon uptake than anthropogenic nitrogen deposition though year 2101. Future responses of rock N inputs on plant CO2 capture rates are more signficant at higher latitudes and in mountainous environments, where geological and climate factors promote higher rock weathering rates. The enhancement of plant CO2 uptake via rock nitrogen weathering partially resolves nitrogen-carbon discrepancies in Earth system models and offers an alternative explanation for lack of progressive nitrogen limitation in the terrestrial biosphere.
Global Biogeochemical Cycles, Volume 35; https://doi.org/10.1029/2021gb006964
The Pantanal region of Brazil is the largest seasonally flooded tropical grassland and, according to local chamber measurements, a substantial CH4 source. CH4 emissions from wetlands have recently become of heightened interest because global atmospheric 13CH4 data indicate they may contribute to the resumption of atmospheric CH4 growth since 2007. We have regularly measured vertical atmospheric profiles for two years in the centre of the Pantanal with the objectives to obtain an estimate of CH4 emissions using an atmospheric approach, and provide information about flux seasonality and its relation to controlling factors. Boundary layer - free troposphere differences observed in the Pantanal are large compared to other wetlands. Total emissions based on a planetary boundary layer budgeting technique are 2.0 to 2.8 TgCH4 yr-1 (maximum flux ∼0.4 gCH4m-2d-1) while those based on a Bayesian inversion using an atmospheric transport model are ∼3.3 TgCH4 yr-1. Compared to recent estimates for Amazonia (∼41± 3 TgCH4 yr-1, maximum flux ∼0.3 gCH4m-2d-1) these emissions are not that large. Our Pantanal data suggest a clear flux seasonality with CH4 being released in large amounts just after water levels begin to rise again after minimum levels have been reached. CH4 emissions decline substantially once the maximum water level has been reached. While predictions with prognostic wetland CH4 emission models agree well with the magnitude of the fluxes, they disagree with the phasing. Our approach shows promise for detecting and understanding longer term trends in CH4 emissions and the potential for future wetlands CH4 emissions climate feedbacks.
Global Biogeochemical Cycles; https://doi.org/10.1029/2020gb006908
Lakes are hotspots of dissolved organic matter (DOM) degradation and play an active role in carbon cycles. Alpine lakes are under the most immediate impacts of climate change and act as sentinels of alpine ecosystem's response to global warming, yet the primary environmental drivers of DOM variability in alpine lakes remain unidentified. Here, we sampled and analyzed DOM from 25 alpine lakes (>4000 m above mean sea level) across the Tibetan Plateau. We found that the water residence time (WRT) of lakes and mean annual temperature (MAT) accounted for 30–59% of the variance in the abundance of chromophoric DOM (CDOM) and fluorescent DOM (FDOM), which decreased with increasing WRT and MAT. Molecular-level analysis of selected DOM samples showed that a higher WRT corresponded to DOM with lower molecular intensities, lower proportions of aromatic, oxidized compounds, and higher proportions of reduced and N-containing compounds. These results show that in-lake DOM processing dominated the regional variability of CDOM and FDOM. Under the scenario of climate warming and lake enlargement due to increasing precipitation and glacier melting, more DOM could be processed and removed from the Tibetan lakes. Our findings are the first that identified WRT as a significant predictor of the amount of various DOM pools in Tibetan lakes and this observation may be broadly applicable to alpine lakes since they share similar DOM source-sink characteristics. The simple empirical models (DOM vs. WRT+MAT) can be integrated into more sophisticated numerical models in predicting changes in the carbon cycle of alpine lakes.
Global Biogeochemical Cycles; https://doi.org/10.1029/2020gb006876
Spatiotemporal patterns of crop nitrogen (N) budget have important implications for agricultural N management and environmental policy. Previous studies examined crop N budget in different countries but often overlooked cross-crop differences at sub-national scales. In this study, we synthesize multiple databases to examine the N budget of eight major crops in the United States at the county scale during 1970-2019. Our analyses show that national crop NUE increased from 0.55 kg N kg-1 N in the 1970s to 0.65 kg N kg-1 N in the 2010s. Four out of eight crops such as corn, rice, cotton, and sorghum demonstrated an increasing NUE trend during the study period, whereas the other crops overall presented a declining NUE trend. Nationwide, about 41% of the total N input was not used by these crops (N surplus) over the study period, of which temporal variation was mainly driven by corn due to its large planting area and high N input. The national N surplus first increased in the 1970s and remained relatively stable till the 2000s. Since the early 2010s, however, N surplus began to decline and approached the levels in the early 1970s—an encouraging development that may lead to decreased N pollution to the environment. The hotspots of national N surplus coincided with corn- and rice-producing counties. The sub-national variations and temporal dynamics in crop N budget revealed in this study highlight the urgent need to understand the farm-level crop N balance and the dominant factors controlling crop NUE for mitigating N pollution.
Global Biogeochemical Cycles; https://doi.org/10.1029/2021gb006985
To better quantify the ocean's biological carbon pump, we resolved the diversity of sinking particles that transport carbon into the ocean's interior, their contribution to carbon export, and their attenuation with depth. Sinking particles collected in sediment trap gel layers from four distinct ocean ecosystems were imaged, measured, and classified. The size and identity of particles was used to model their contribution to particulate organic carbon (POC) flux. Measured POC fluxes were reasonably predicted by particle images. Nine particle types were identified, and most of the compositional variability was driven by the relative contribution of aggregates, long cylindrical fecal pellets, and salp fecal pellets. While particle composition varied across locations and seasons, the entire range of compositions was measured at a single well-observed location in the subarctic North Pacific over one month, across 500 m of depth. The magnitude of POC flux was not consistently associated with a dominant particle class, but particle classes did influence flux attenuation. Long fecal pellets attenuated most rapidly with depth whereas certain other classes attenuated little or not at all with depth. Small particles (<100 μm) consistently contributed ∼5% to total POC flux in samples with higher magnitude fluxes. The relative importance of these small particle classes (spherical mini pellets, short oval fecal pellets, and dense detritus) increased in low flux environments (up to 46% of total POC flux). Imaging approaches that resolve large variations in particle composition across ocean basins, depth, and time will help to better parameterize biological carbon pump models.
Global Biogeochemical Cycles, Volume 35; https://doi.org/10.1029/2021gb007058
Continental shelves are important sources of iron (Fe) in the land-dominated Arctic Ocean. To understand the export of Fe from the Arctic to Baffin Bay (BB) and the North Atlantic, we studied the alteration of the Fe signature in waters transiting the Canadian Arctic Archipelago (CAA). During its transit through the CAA, inflowing Arctic Waters from the Canada Basin become enriched in Fe as result of strong sediment resuspension and enhanced sediment-water interactions (non-reductive dissolution). These high Fe waters are exported to BB, where approximately 10.7 kt of Fe are delivered yearly from Lancaster Sound. Furthermore, if the two remaining main CAA pathways (Jones Sound and Nares Strait) are included, this shelf environment would be a dominant source term of Fe (dFe + pFe: 26-90 kt y-1) to Baffin Bay. The conservative Fe flux estimate (26 kt y-1) is 1.7 to 38 times greater than atmospheric inputs, and may be crucial in supporting primary production and nitrogen fixation in BB and beyond.
Global Biogeochemical Cycles, Volume 35; https://doi.org/10.1029/2020gb006902
Systematic regional variations in the ratio of nutrient depth gradients of dissolved inorganic carbon (ΔDIC): nitrate (ΔNO3): phosphate (ΔPO4) in the upper layer (300m) of the Pacific Ocean are observed. Regional variations in the ΔDIC/ΔNO3/ΔPO4 are primarily the result of three processes, that is, the C/N/P of organic matter (OM) being exported and subsequently degraded, nitrogen fixation and air-sea CO2 gas exchange. The link between the observed dissolved ΔDIC/ΔNO3/ΔPO4 and the C/N/P of exported OM is established using surface layer dissolved DIC, NO3 and PO4 budgets. These budgets, in turn, provide a means to indirectly estimate the C/N/P of OM being exported from the surface layer of the ocean. The indirectly estimated C/N/P of exported OM reach maxima in the subtropical gyres at 177/22/1 that is significantly greater than the Redfield ratio and a minimum in the equatorial ocean at 109/16/1 with both results agreeing with available observed particle C/N/P and ocean biogeochemical models. The budget approach was applied to a bioactive trace element (TE) using the measured dissolved Cadmium (Cd) to PO4 gradients to estimate the Cd/P of exported OM in the Pacific Ocean. Combining the budget method with the availability of high-quality dissolved nutrient and trace element data collected during the GOSHIP and GEOTRACES programs could potentially provide estimates of the C/N/P/TE of exported OM on global ocean scales which would significantly improve our understanding of the link between the ocean’s biological pump and dissolved nutrient distributions in the upper ocean.
Global Biogeochemical Cycles, Volume 35; https://doi.org/10.1029/2020gb006883
Atmospheric methane (CH4) and its isotopic composition trends over the last decades are explained by various flux scenarios, from tropical wetland emission increases through to reductions in global hydroxyl (OH). In this work we develop a modelling framework to assess the potential usefulness of clumped isotope measurements to distinguish between the main drivers of change in the CH4 burden. We model interhemispheric differences of 0.12 ‰ and 0.38 ‰ and seasonal cycles of 0.02-0.04 ‰ and 0.21-0.32 ‰ for Δ13CH3D and Δ12CH2D2, respectfully, which is insignificant relative to the uncertainty of measurements that could eventually be made. We show, however, that measurements of Δ12CH2D2 specifically could provide constraints for understanding trends in the global total source and sink magnitudes, which has not been possible with the current sets of observables. Changes in OH concentration of 10 % developed across three decades results in a difference of up to 2 ‰ in Δ12CH2D2, which would be observable given current measurement uncertainty limits. For this type of global scale analysis we show that measurements of Δ13CH3D would be unlikely to provide additional useful information. We suggest an emphasis should now be on developing the methods to make measurements from ambient air samples, followed by measurements of Δ13CH3D and Δ12CH2D2 from sampling at clean Northern and Southern Hemisphere sites, combined with more accurate and precise laboratory measurements of the clumped kinetic isotope effects relevant for the atmospheric sinks.
Global Biogeochemical Cycles, Volume 35; https://doi.org/10.1029/2020gb006904
Increasing mercury isotope ratios from pre-industrial (1510 – 1850) to present-day (1990 – 2014) in lake sediment cores have been suggested to be a global phenomenon. To assess factors leading to spatiotemporal changes, we compiled mercury concentration (THg) and mercury isotope ratios in twenty-two lake sediment cores located at various regions of the world. We find that the positive δ202Hg shifts together with THg increases from pre-industrial to present-day are a widespread phenomenon. This is caused by increased contribution of mercury from local to regional anthropogenic mercury emission sources, which have higher δ202Hg (-1.07±0.69‰, 1 s.d.) than pre-industrial sediments (-1.55±0.96‰, 1 s.d.). The positive Δ199Hg shifts were observed in fifteen lake sediment cores, which have low pre-industrial Δ199Hg (-0.20±0.32‰) compared to the sediment cores with near-zero to positive pre-industrial Δ199Hg (0.08±0.07‰). The magnitudes of δ202Hg (r2 = 0.09) and Δ199Hg (r2 = 0.20, both p > 0.05) changes from pre-industrial to present-day did not correlate with the magnitude of THg changes. Instead, the magnitudes of δ202Hg and Δ199Hg changes decreased with increasing pre-industrial δ202Hg and Δ199Hg values, suggesting that the baseline mercury isotope ratios play a more important role in determining the magnitude of mercury isotope changes compared to the degree of THg input. We suggest that the spatiotemporal assessments of δ202Hg in lake sediment cores can be used as an important proxy for monitoring changes in anthropogenic mercury sources for the Minamata Convention on Mercury.
Global Biogeochemical Cycles, Volume 35; https://doi.org/10.1029/2021gb006956
The ACT-America Earth Venture mission conducted five airborne campaigns across four seasons from 2016-2019, to study the transport and fluxes of Greenhouse gases across the eastern United States. Unprecedented spatial sampling of atmospheric tracers (CO2, CO, COS) related to biospheric processes offers opportunities to improve our qualitative and quantitative understanding of seasonal and spatial patterns of biospheric carbon uptake. Here, we examine co-variation of boundary layer enhancements of CO2, CO, and COS across three diverse regions: the crop-dominated Midwest, evergreen-dominated South, and deciduous broadleaf-dominated Northeast. To understand the biogeochemical processes controlling these tracers, we compare the observed co-variation to simulated co-variation resulting from model- and satellite- constrained surface carbon fluxes. We found indication of a common terrestrial biogenic sink of CO2 and COS and secondary production of CO from biogenic sources in summer throughout the eastern US, driven by stomatal conductance. Upper Midwest crops drive CO2 and COS depletion from early to late summer. Northeastern temperate forests drive CO2 and COS depletion in late summer. The unprecedented ACT-America flask samples uncovered evidence that southern humid temperate forests photosynthesize and absorb CO2 and COS, and emit CO precursors, deep into the growing season. Satellite- constrained carbon fluxes capture much of the observed seasonal and spatial variability, but underestimate the magnitude of net CO2 and COS depletion in the South, indicating a stronger than expected net sink of CO2 in late summer. Additional sampling of the South will more accurately constrain underlying biological processes and climate sensitivities governing southern carbon dynamics.