| 1. |
- Sutton, M. A., et al.
(författare)
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Dynamics of ammonia exchange with cut grassland : Strategy and implementation of the GRAMINAE Integrated Experiment
- 2009
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Ingår i: Biogeosciences. - : Copernicus Publications (on behalf of the European Geosciences Union). - 1726-4170 .- 1726-4189. ; 6:3, s. 309-331
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Tidskriftsartikel (refereegranskat)abstract
- A major international experiment on ammonia (NH3) biosphere-atmosphere exchange was conducted over intensively managed grassland at Braunschweig, Germany. The experimental strategy was developed to allow an integrated analysis of different features of NH3 exchange including: a) quantification of nearby emissions and advection effects, b) estimation of net NH3 fluxes with the canopy by a range of micrometeorological measurements, c) analysis of the sources and sinks of NH3 within the plant canopy, including soils and bioassay measurements, d) comparison of the effects of grassland management options on NH3 fluxes and e) assessment of the interactions of NH3 fluxes with aerosol exchange processes. Additional technical objectives included the inter-comparison of different estimates of sensible and latent heat fluxes, as well as continuous-gradient and Relaxed Eddy Accumulation (REA) systems for NH3 fluxes. The prior analysis established the spatial and temporal design of the experiment, allowing significant synergy between these objectives. The measurements were made at 7 measurement locations, thereby quantifying horizontal and vertical profiles, and covered three phases: a) tall grass canopy prior to cutting (7 days), b) short grass after cutting (7 days) and c) re-growing sward following fertilization with ammonium nitrate (10 days). The sequential management treatments allowed comparison of sources-sinks, advection and aerosol interactions under a wide range of NH3 fluxes. This paper describes the experimental strategy and reports the grassland management history, soils, environmental conditions and air chemistry during the experiment, finally summarizing how the results are coordinated in the accompanying series of papers.
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| 2. |
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| 3. |
- Karlen, A, et al.
(författare)
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Nogo receptor 1 regulates formation of lasting memories
- 2009
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 1091-6490. ; 106:48, s. 20476-20481
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Tidskriftsartikel (refereegranskat)abstract
- Formation of lasting memories is believed to rely on structural alterations at the synaptic level. We had found that increased neuronal activity down-regulates Nogo receptor-1 (NgR1) in brain regions linked to memory formation and storage, and postulated this to be required for formation of lasting memories. We now show that mice with inducible overexpression of NgR1 in forebrain neurons have normal long-term potentiation and normal 24-h memory, but severely impaired month-long memory in both passive avoidance and swim maze tests. Blocking transgene expression normalizes these memory impairments. Nogo, Lingo-1, Troy, endogenous NgR1, and BDNF mRNA expression levels were not altered by transgene expression, suggesting that the impaired ability to form lasting memories is directly coupled to inability to down-regulate NgR1. Regulation of NgR1 may therefore serve as a key regulator of memory consolidation. Understanding the molecular underpinnings of synaptic rearrangements that carry lasting memories may facilitate development of treatments for memory dysfunction.
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| 4. |
- Mattsson, A. E., et al.
(författare)
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Nonequivalence of the generalized gradient approximations PBE and PW91
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 73:19, s. 195123-
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Tidskriftsartikel (refereegranskat)abstract
- Two of the most popular generalized gradient approximations used in the applications of the density functional theory, PW91 and PBE, are generally regarded as essentially equivalent. They produce similar numerical results for many simple properties, such as lattice constants, bulk moduli, and atomization energies. We examine more complex properties of systems with electronic surface regions, with the specific application of the monovacancy formation energies of Pt and Al. A surprisingly large and consistent discrepancy between PBE and PW91 results is obtained. This shows that despite similarities found between some simple material properties, PBE and PW91 are not equivalent. The differences obtained for the monovacancy formation energies are related to differences in surface intrinsic errors which are substantiated using the idealized, well-controlled, jellium surface model. In view of the differences obtained with the PW91 and PBE functionals we develop separate surface intrinsic error corrections for these and revisit some earlier results.
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| 5. |
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| 6. |
- Burkhardt, J., et al.
(författare)
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Modelling the dynamic chemical interactions of atmospheric ammonia with leaf surface wetness in a managed grassland canopy
- 2009
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Ingår i: Biogeosciences. - Göttingen : Copernicus Publications. - 1726-4170 .- 1726-4189. ; 6:1, s. 67-84
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Tidskriftsartikel (refereegranskat)abstract
- Ammonia exchange fluxes between grassland and the atmosphere were modelled on the basis of stomatal compensation points and leaf surface chemistry, and compared with measured fluxes during the GRAMINAE intensive measurement campaign in spring 2000 near Braunschweig, Germany. Leaf wetness and dew chemistry in grassland were measured together with ammonia fluxes and apoplastic NH4+ and H+ concentration, and the data were used to apply, validate and further develop an existing model of leaf surface chemistry and ammonia exchange. Foliar leaf wetness which is known to affect ammonia fluxes may be persistent after the end of rainfall, or sustained by recondensation of water vapour originating from the ground or leaf transpiration, so measured leaf wetness values were included in the model. pH and ammonium concentrations of dew samples collected from grass were compared to modelled values.The measurement period was divided into three phases: a relatively wet phase followed by a dry phase in the first week before the grass was cut, and a second drier week after the cut. While the first two phases were mainly characterised by ammonia deposition and occasional short emission events, regular events of strong ammonia emissions were observed during the post-cut period. A single-layer resistance model including dynamic cuticular and stomatal exchange could describe the fluxes well before the cut, but after the cut the stomatal compensation points needed to numerically match measured fluxes were much higher than the ones measured by bioassays, suggesting another source of ammonia fluxes. Considerably better agreement both in the direction and the size range of fluxes were obtained when a second layer was introduced into the model, to account for the large additional ammonia source inherent in the leaf litter at the bottom of the grass canopy. Therefore, this was found to be a useful extension of the mechanistic dynamic chemistry model by keeping the advantage of requiring relatively little site-specific information.
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| 7. |
- Herrmann, B., et al.
(författare)
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Vertical structure and diurnal variability of ammonia exchange potential within an intensively managed grass canopy
- 2009
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Ingår i: Biogeosciences. - Kaltenburg-Lindau : Copernicus on behalf of the European Geosciences Union. - 1726-4170 .- 1726-4189. ; 6:1, s. 15-23
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Tidskriftsartikel (refereegranskat)abstract
- Stomatal ammonia compensation points (χs) of grass species on a mixed fertilized grassland were determined by measurements of apoplastic [NH4+] and [H>+] in the field. Calculated χs values were compared with in-canopy atmospheric NH3 concentration (χa) measurements.Leaf apoplastic [NH4+] increased by a factor of two from the lowest level in the canopy to the top level. Bulk leaf [NH4+] and especially [NO3−] slightly increased at the bottom of the canopy and these concentrations were very high in senescent plant litter. Calculated χs values were below atmospheric χs at all canopy levels measured, indicating that the grassland was characterized by NH3 deposition before cutting. This was confirmed by the χa profile, showing the lowest χa close to the ground (15 cm above soil surface) and an increase in χa with canopy height. Neither χs nor χa could be measured close to the soil surface, however, the [NH4+] in the litter material indicated a high potential for NH3 emission.A diurnal course in apoplastic [NH4+] was seen in the regrowing grass growing after cutting, with highest concentration around noon. Both apoplastic and tissue [NH4+] increased in young grass compared to tall grass. Following cutting, in-canopy gradients of atmospheric χa showed NH3 emission but since calculated χs values of the cut grass were still lower than atmospheric NH3 concentrations, the emissions could not entirely be explained by stomatal NH3 loss. High tissue [NH4+] in the senescent plant material indicated that this fraction constituted an NH3 source. After fertilization, [NH4+] increased both in apoplast and leaf tissue with the most pronounced increase in the former compared to the latter. The diurnal pattern in apoplastic [NH4+] was even more pronounced after fertilization and calculated χs values were generally higher, but remained below atmospheric [NH3].
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| 8. |
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| 9. |
- Kiselev, A, et al.
(författare)
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Solar light decomposition of DFP on the surface of anatase and rutile TiO2 prepared by hydrothermal treatment of microemulsions
- 2005
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Ingår i: Surface Science. - : Elsevier BV. - 0039-6028 .- 1879-2758. ; 584:1, s. 98-105
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Tidskriftsartikel (refereegranskat)abstract
- The photocatalytic decomposition of diisopropylfluorophosphate (DFP) over nanostructured anatase and rutile TiO2 powder was investigated by FTIR and XPS. Upon irradiation with artificial solar light DFP decomposed on both polymorphs as evidenced by FTIR. For both crystalline structures acetone and subsequently coordinated formate and carbonate were observed on the surface during the photocatalytic reaction is the isopropyl groups dissociated from DFP. XPS revealed that small amounts of phosphates and inorganic fluoride (Ti-F) gradually built up on both TiO2 surfaces, while organic F was present only on the rutile phase. From repeated cycles of intermittent DFP adsorption and irradiation measurements, the decomposition rates and formation of residuals on the surface were deduced. It was found that the overall oxidation yield is higher on anatase than rutile. The oxidation rate decreases with increasing irradiation time. an effect that is more pronounced on rutile. We find that both the difference between the polymorphs and the initial decrease of the oxidation yield can largely be explained by variations in surface area rather than poisoning by POx or F species. In particular, we observe a dramatic decrease of the specific area or rutile as a function of photocatalytic oxidation cycle. (c) 2005 Elsevier B.V. All rights reserved.
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