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  • 1
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    Amazon forest response to repeated droughts (2016)
    T.R. Feldpausch, O.L. Phillips, R.J.W. Brienen, E. Gloor, J. Lloyd, G. Lopez-Gonzalez, A. Monteagudo-Mendoza, Y. Malhi, A. Alarcón, E. Álvarez Dávila, P. Alvarez-Loayza, A. Andrade, L.E.O.C. Aragao, L. Arroyo, G.A. Aymard C., T.R. Baker, C. Baraloto, J. Barroso, D. Bonal, W. Castro, V. Chama, J. Chave, T.F. Domingues, S. Fauset, N. Groot, E. Honorio C., S. Laurance, W.F. Laurance, S.L. Lewis, J.C. Licona, B.S. Marimon, B.H. Marimon-Junior, C. Mendoza Bautista, D.A. Neill, E.A. Oliveira, C. Oliveira dos Santos, N.C. Pallqui Camacho, G. Pardo-Molina, A. Prieto, C.A. Quesada, F. Ramírez, H. Ramírez-Angulo, M. Réjou-Méchain, A. Rudas, G. Saiz, R.P. Salomão, J.E. Silva-Espejo, M. Silveira, H. Steege, J. Stropp, J. Terborgh, R. Thomas-Caesar, G.M.F. Heijden, R. Vásquez Martinez, E. Vilanova, V.A. Vos
    Wiley
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    Publication Date: 2016-05-01
    Description: The Amazon Basin has experienced more variable climate over the last decade, with a severe and widespread drought in 2005 causing large basin-wide losses of biomass. A drought of similar climatological magnitude occurred again in 2010; however, there has been no basin-wide ground-based evaluation of effects on vegetation. We examine to what extent the 2010 drought affected forest dynamics using ground-based observations of mortality and growth utilizing data from an extensive forest plot network. We find that during the 2010 drought interval, forests did not gain biomass (net change: −0.43 Mg ha -1 , CI: −1.11, 0.19, n = 97), regardless of whether forests experienced precipitation deficit anomalies. This loss contrasted with a long-term biomass sink during the baseline pre-2010 drought period (1998 − pre-2010) of 1.33 Mg ha -1 yr -1 (CI: 0.90, 1.74, p  〈 0.01). The resulting net impact of the 2010 drought (i.e., reversal of the baseline net sink) was −1.95 Mg ha -1 yr -1 (CI:−2.77, −1.18; p  〈 0.001). This net biomass impact was driven by an increase in biomass mortality (1.45 Mg ha -1 yr -1 CI: 0.66, 2.25, p  〈 0.001), and a decline in biomass productivity (−0.50 Mg ha -1 yr -1 , CI:−0.78, −0.31; p  〈 0.001). Surprisingly, the magnitude of the losses through tree mortality was unrelated to estimated local precipitation anomalies, and was independent of estimated local pre-2010 drought history. Thus, there was no evidence that pre-2010 droughts compounded the effects of the 2010 drought. We detected a systematic basin-wide impact of drought on tree growth rates across Amazonia, with this suppression of productivity driven by moisture deficits in 2010, an impact which was not apparent during the 2005 event [ Phillips et al. , 2009]. Based on these ground data, both live biomass in trees and corresponding estimates of live biomass in roots, we estimate that intact forests in Amazonia were carbon neutral in 2010 (−0.07 PgC yr -1 CI:−0.42, 0.23), consistent with results from an independent analysis of airborne estimates of land-atmospheric fluxes during 2010 [ Gatti et al. , 2014]. Relative to the long-term mean, the 2010 drought resulted in a reduction in biomass carbon uptake of 1.1 PgC, compared to 1.6 PgC for the 2005 event [ Phillips et al . 2009].
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 2
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    Global oceanic emission of ammonia: constraints from seawater and atmospheric observations (2015)
    Wiley
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    Publication Date: 2015-07-19
    Description: Current global inventories of ammonia emissions identify the ocean as the largest natural source. This source depends on seawater pH, temperature, and the concentration of total seawater ammonia ( NH x ( sw )), which reflects a balance between remineralization of organic matter, uptake by plankton, and nitrification. Here, we compare [ NH x ( sw )] from two global ocean biogeochemical models (BEC and COBALT) against extensive ocean observations. Simulated [ NH x ( sw )] are generally biased high. Improved simulation can be achieved in COBALT by increasing the plankton affinity for NH x within observed ranges. The resulting global ocean emissions is 2.5 TgN a −1 , much lower than current literature values(7–23 TgN a −1 ), including the widely used GEIA inventory (8 TgN a −1 ). Such a weak ocean source implies that continental sources contribute more than half of atmospheric NH x over most of the ocean in the Northern hemisphere. Ammonia emitted from oceanic sources is insufficient to neutralize sulfate aerosol acidity, consistent with observations. There is evidence over the Equatorial Pacific for a missing source of atmospheric ammonia that could be due to photolysis of marine organic nitrogen at the ocean surface or in the atmosphere. Accommodating this possible missing source yields a global ocean emission of ammonia in the range 2–5 TgN a −1 , comparable in magnitude to other natural sources from open fires and soils.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 3
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    Partitioning N2O Emissions within the US Corn Belt using an Inverse Modeling Approach (2016)
    Wiley
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    Publication Date: 2016-08-09
    Description: Nitrous oxide (N 2 O) emissions within the US Corn Belt have been previously estimated to be 200-900% larger than predictions from emission inventories, implying that one or more source categories in bottom-up approaches are underestimated. Here we interpret hourly N 2 O concentrations measured during 2010 and 2011 at a tall tower using a time-inverted transport model and a scale factor Bayesian inverse method to simultaneously constrain direct and indirect agricultural emissions. The optimization revealed that both agricultural source categories were underestimated by the Intergovernmental Panel on Climate Change (IPCC) inventory approach. However, the magnitude of the discrepancies differed substantially, ranging from 42–58% and 200–525% for direct and indirect components, respectively. Optimized agricultural N 2 O budgets for the Corn Belt were 319 ± 184 (total), 188 ± 66 (direct), and 131 ± 118 Gg-N yr -1 (indirect) in 2010, versus 471 ± 326, 198 ± 80, and 273 ± 246 Gg-N yr -1 in 2011. We attribute the inter-annual differences to varying moisture conditions, with increased precipitation in 2011 amplifying emissions. We found that indirect emissions represented 41–58% of the total agricultural budget, a considerably larger portion than the 25–30% predicted in bottom-up inventories, further highlighting the need for improved constraints on this source category. These findings further support the hypothesis that indirect emissions are presently underestimated in bottom-up inventories. Based on our results, we suggest an indirect emission factor for runoff and leaching ranging from 0.014–0.035 for the Corn Belt, which represents an upward adjustment of 1.9–4.6 times relative to the IPCC and is in agreement with recent bottom-up field studies.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 4
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    Impacts of atmospheric nutrient deposition on marine productivity: Roles of nitrogen, phosphorus, and iron (2011)
    Wiley
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    Publication Date: 2011-06-23
    Description: Nutrients are supplied to the mixed layer of the open ocean by either atmospheric deposition or mixing from deeper waters, and these nutrients drive nitrogen and carbon fixation. To evaluate the importance of atmospheric deposition, we estimate marine nitrogen and carbon fixation from present-day simulations of atmospheric deposition of nitrogen, phosphorus, and iron. These are compared with observed rates of marine nitrogen and carbon fixation. We find that Fe deposition is more important than P deposition in supporting N fixation. Estimated rates of atmospherically supported carbon fixation are considerably lower than rates of marine carbon fixation derived from remote sensing, indicating the subsidiary role atmospheric deposition plays in total C uptake by the oceans. Nonetheless, in high-nutrient, low-chlorophyll areas, the contribution of atmospheric deposition of Fe to the surface ocean could account for about 50% of C fixation. In marine areas typically thought to be N limited, potential C fixation supported by atmospheric deposition of N is only ∼1%–2% of observed rates. Although these systems are N-limited, the amount of N supplied from below appears to be much larger than that deposited from above. Atmospheric deposition of Fe has the potential to augment atmospherically supported rates of C fixation in N-limited areas. In these areas, atmospheric Fe relieves the Fe limitation of diazotrophic organisms, thus contributing to the rate of N fixation. The most important uncertainties in understanding the relative importance of different atmospheric nutrients are poorly understood speciation and solubility of Fe as well as the N:Fe ratio of diazotrophic organisms.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 5
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    Fluxes and distribution of dissolved iron in the eastern (sub-) tropical North Atlantic Ocean (2012)
    Wiley
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    Publication Date: 2012-07-21
    Description: Aeolian dust transport from the Saharan/Sahel desert regions is considered the dominant external input of iron (Fe) to the surface waters of the eastern (sub-) tropical North Atlantic Ocean. To test this hypothesis, we investigated the sources of dissolved Fe (DFe) and quantified DFe fluxes to the surface ocean in this region. In winter 2008, surface water DFe concentrations varied between 1.5 nM) correlated positively with apparent oxygen utilization (AOU) and showed the importance of organic matter remineralization as an DFe source. As a consequence, vertical diffusive mixing formed an important Fe flux to the surface ocean in this region, even surpassing that of a major dust event.
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  • 6
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    Atmospheric fluxes of organic N and P to the global ocean (2012)
    Wiley
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    Publication Date: 2012-09-14
    Description: The global tropospheric budget of gaseous and particulate non-methane organic matter (OM) is re-examined to provide a holistic view of the role that OM plays in transporting the essential nutrients nitrogen and phosphorus to the ocean. A global 3-dimensional chemistry-transport model was used to construct the first global picture of atmospheric transport and deposition of the organic nitrogen (ON) and organic phosphorus (OP) that are associated with OM, focusing on the soluble fractions of these nutrients. Model simulations agree with observations within an order of magnitude. Depending on location, the observed water soluble ON fraction ranges from ∼3% to 90% (median of ∼35%) of total soluble N in rainwater; soluble OP ranges from ∼20–83% (median of ∼35%) of total soluble phosphorus. The simulations suggest that the global ON cycle has a strong anthropogenic component with ∼45% of the overall atmospheric source (primary and secondary) associated with anthropogenic activities. In contrast, only 10% of atmospheric OP is emitted from human activities. The model-derived present-day soluble ON and OP deposition to the global ocean is estimated to be ∼16 Tg-N/yr and ∼0.35 Tg-P/yr respectively with an order of magnitude uncertainty. Of these amounts ∼40% and ∼6%, respectively, are associated with anthropogenic activities, and 33% and 90% are recycled oceanic materials. Therefore, anthropogenic emissions are having a greater impact on the ON cycle than the OP cycle; consequently increasing emissions may increase P-limitation in the oligotrophic regions of the world's ocean that rely on atmospheric deposition as an important nutrient source.
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    Topics: Biology , Chemistry and Pharmacology , Geography , Geosciences , Physics
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  • 7
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    Influence of chemical weathering and aging of iron oxides on the potential iron solubility of Saharan dust during simulated atmospheric processing (2011)
    Wiley
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    Publication Date: 2011-05-21
    Description: The flux of bioavailable Fe from mineral dust to the surface ocean is controlled not only by the processes in the atmosphere but also by the nature and source of the dust. In this study, we investigated how the nature of Fe minerals in the dust affects its potential Fe solubility (Fepsol) employing traditional and modern geochemical, mineralogical, and microscopic techniques. The chemical and mineralogical compositions, particularly Fe mineralogy, in soil samples as dust precursors collected from North African dust source regions were determined. The Fepsol was measured after 3 days of contact with sulfuric acid at pH 2 to simulate acid processes in the atmosphere. Fepsol of the soil dust samples were compared with calculated predictions of Fepsol based on the amount of individual Fe-bearing minerals present in the samples and Fe solubilities of corresponding standard minerals. The calculated Fepsol deviated significantly from the measured Fepsol of the soil dust samples. We attributed this to the variability in properties of Fe minerals (e.g., size of Fe oxides and heterogeneity of chemical compositions of clay minerals) in soil dusts in comparison to the standard minerals. There were, however, clear relationships between the degree of chemical weathering of North African soils and Fepsol. The Parker index and ratio of ascorbate plus dithionite Fe to total Fe ((FeA+FeD)/FeT) are positively and negatively correlated with Fepsol, respectively. In addition, the ratio of FeA/(FeA+FeD), which decreases with aging of the Fe oxides, was found to be positively correlated with Fepsol in the soil dusts. Overall, our results indicate that there is a significant regional variability in the chemical and Fe mineralogical compositions of dusts across North African sources, as a result of the differences in chemical weathering and aging of Fe oxides. Furthermore, the indices for these weathering processes can provide an estimate of the fraction of Fe which can be solubilized if acid processed in the atmosphere.
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  • 8
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    Estimation of atmospheric nutrient inputs to the Atlantic Ocean from 50°N to 50°S based on large-scale field sampling: Iron and other dust-associated elements (2013)
    Wiley
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    Publication Date: 2013-07-14
    Description: [1]  Atmospheric inputs of mineral dust supply iron and other trace metals to the remote ocean and can influence the marine carbon cycle due to iron's role as a potentially limiting micronutrient. Dust generation, transport and deposition are highly heterogeneous and there are very few remote marine locations where dust concentrations and chemistry (e.g. iron solubility) are routinely monitored. Here we use aerosol and rainwater samples collected during 10 large-scale research cruises to estimate the atmospheric input of iron, aluminium and manganese to four broad regions of the Atlantic Ocean over two 3-month periods for the years 2001 – 2005. We estimate total inputs of these metals to our study regions to be 4.2, 17 and 0.27 Gmol in April – June and 4.9, 14 and 0.19 Gmol in September – November, respectively. Inputs were highest in regions of high rainfall (the inter-tropical convergence zone and South Atlantic storm track) and rainfall contributed higher proportions of total input to wetter regions. By combining input estimates for total and soluble metals for these time periods, we calculated overall percentage solubilities for each metal that account for the contributions from both wet and dry deposition and the relative contributions from different aerosol types. Calculated solubilities were in the range 2.4 – 9.1% for iron, 6.1 – 15% for aluminium and 54 – 73% for manganese. We discuss sources of uncertainty in our estimates and compare our results to some recent estimates of atmospheric iron input to the Atlantic.
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  • 9
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    Reconciling the differences between top-down and bottom-up estimates of nitrous oxide emissions for the US Corn Belt (2013)
    Wiley
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    Publication Date: 2013-07-16
    Description: [1]  Nitrous oxide (N 2 O) is a greenhouse gas with a large global warming potential and is a major cause of stratospheric ozone depletion. Croplands are the dominant source of N 2 O, but mitigation strategies have been limited by the large uncertainties in both direct and indirect emission factors (EFs) implemented in “bottom-up” emission inventories. The Intergovernmental Panel on Climate Change (IPCC) recommends EFs ranging from 0.75 to 2% of the anthropogenic nitrogen (N) input for the various N 2 O pathways in croplands. Consideration of the global N budget yields a much higher EF ranging between 3.8 and 5.1% of the anthropogenic N input. Here, we use two years of hourly high-precision N 2 O concentration measurements on a very tall tower to evaluate the IPCC bottom-up and global “top-down” EFs for a large representative subsection of the United States Corn Belt, a vast region spanning the US Midwest that is dominated by intensive N inputs to support corn cultivation. Scaling up these results indicates that agricultural sources in the Corn Belt released 420 ± 50 Gg N (mean ± 1 standard deviation; 1 Gg = 10 9 g) in 2010, in close agreement with the top-down estimate of 350 ± 50 Gg N and 80% larger than the bottom-up estimate based on the IPCC EFs (230 ± 180 Gg N). The large difference between the tall-tower measurement and the bottom-up estimate implies the existence of N 2 O emission hot spots ormissing sources within the landscape that are not fully accounted for in the IPCC and other bottom-up emission inventories. Reconciling these differences is an important step toward developing a practical mitigation strategy for N 2 O.
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  • 10
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    Impact of atmospheric deposition on the contrasting iron biogeochemistry of the North and South Atlantic Ocean (2013)
    Wiley
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    Publication Date: 2013-06-15
    Description: [1]  Dissolved iron (dFe) distributions, atmospheric and vertical subduction fluxes of dFe were determined in the upper water column for two meridional transects of the Atlantic Ocean. The data demonstrate the disparity between the iron biogeochemistry of the North and South Atlantic Ocean and show well defined gradients of size fractionated iron species in surface waters between geographic provinces. The highest dFe and lowest mixed layer residence times (0.4-2.7 y) were found in the northern tropical and subtropical regions. In contrast, the South Atlantic Gyre had lower dFe concentrations (〈0.4 nM) and much longer residence times (〉 5 y), presumably due to lower atmospheric inputs and more efficient biological recycling of iron in this region. Vertical input fluxes of dFe to surface waters ranged from 20 - 170 nmol m -2 d -1 in the North Atlantic and tropical provinces, whereas average fluxes of 6 - 13 nmol m -2 d -1 were estimated for the South Atlantic. Our estimates showed that the variable dFe distribution over the surface Atlantic (〈0.1 – 2.0 nM) predominantly reflected atmospheric Fe deposition fluxes (〉50% of total vertical Fe flux to surface waters) rather than upwelling or vertical mixing. This demonstrates the strength of the connection between land-derived atmospheric Fe fluxes and the biological cycling of carbon and nitrogen in the Atlantic Ocean.
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