360 research outputs found
Effect of Foliage and Root Carbon Quantity, Quality, and Fluxes on Soil Organic Carbon Stabilization in Montane Aspen and Conifer Stands in Utah
Forest soils store as much carbon (C) as the vegetation that grows on them, and the carbon in soil is more stable than the C in biomass. Quaking aspen (Populus tremuloides Michx.) is the most widespread tree species in North America, and aspen forests in the Western US have been found to store more soil organic carbon (SOC) in the mineral soil than nearby conifers. Fire exclusion and grazing often promote the succession of aspen to conifer dominated forests due to their effect on aspen regeneration. So far the factors driving the differential SOC accumulation, and the effects of the vegetation shift on SOC pools, are not well understood.
In this dissertation I aimed to evaluate how various forest vegetation characteristics – tree type, detritus fluxes, detritus chemistry – affect SOC pools and stability from a global to a molecular level using two contrasting forest types – aspen and conifer. A meta-analysis showed that, while conifer forests worldwide had higher C pools in the forest floor, this difference did not translate into the mineral soil, suggesting that the mechanisms that control SOC storage differ between both soil compartments. Above- and belowground detritus input fluxes were similar between aspen and conifer forests, and did not explain the higher SOC pools under aspen. A sorption study revealed that the more labile aspen foliage dissolved organic carbon (DOC) was more effectively retained in soil than aspen root, and conifer substrate DOC. Furthermore, soils that contained aspen SOC retained new DOC better than soils with conifer SOC, irrespective of the source of the DOC. Finally, foliage and root specific compounds that were identified for aspen and subalpine fir provide a base for future studies aiming to identify the source of SOC under both overstory types.
Overall, the results of the dissertation suggest that substrate chemistry more than detritus fluxes drive the differences between SOC pools under aspen and conifer forests in Utah. This finding indicates that the link between C input amounts and SOC pools is not as direct as currently assumed in most SOC models. Furthermore, a tree species effect on SOC as distinct as aspen vs conifer is not common between all hardwood and conifer comparisons worldwide, thus suggesting that the effect of vegetation can be overridden by other factors
A tree species effect on soil that is consistent across the species\u27 range: the case of aspen and soil carbon in North America.
Trembling aspen covers a large geographic range in North America, and previous studies reported that a better understanding of its singular influence on soil properties and processes is of high relevance for global change questions. Here we investigate the potential impact of a shift in aspen abundance on soil carbon sequestration and soil carbon stability at the continental scale by conducting a systematic literature review using 23 published studies. Our review shows that aspen’s effect on soil carbon is relatively consistent throughout the species range. Aspen stores less C in the forest floor but similar amounts in the mineral soil relative to conifers. However, a robust set of indicators of soil C stability, for example, degree of organo-mineral associations, proportion of readily-available or labile C estimated during long-term soil incubations or using hot-water extraction, pattern of soil C distribution, and temperature sensitivity of soil heterotrophic respiration, reveals that the soil organic carbon (SOC) stock under aspen is more stable, rendering it more protected against environmental changes and soil disturbances. Therefore, our continental-scale analysis highlights that an increase in the abundance of trembling aspen in North American forests may increase the resistance and resilience of soil C stocks against global changes
Underscreened Kondo effect in S=1 magnetic quantum dots: Exchange, anisotropy and temperature effects
We present a theoretical analysis of the effects of uniaxial magnetic
anisotropy and contact-induced exchange field on the underscreened Kondo effect
in S=1 magnetic quantum dots coupled to ferromagnetic leads. First, by using
the second-order perturbation theory we show that the coupling to
spin-polarized electrode results in an effective exchange field
and an effective magnetic anisotropy . Second, we confirm these
findings by using the numerical renormalization group method, which is employed
to study the dependence of the quantum dot spectral functions, as well as
quantum dot spin, on various parameters of the system. We show that the
underscreened Kondo effect is generally suppressed due to the presence of
effective exchange field and can be restored by tuning the anisotropy constant,
when . The Kondo effect can also be restored by
sweeping an external magnetic field, and the restoration occurs twice in a
single sweep. From the distance between the restored Kondo resonances one can
extract the information about both the exchange field and the effective
anisotropy. Finally, we calculate the temperature dependence of linear
conductance for the parameters where the Kondo effect is restored and show that
the restored Kondo resonances display a universal scaling of Kondo
effect.Comment: 13 pages, 9 figures (version as accepted for publication in Physical
Review B
Spin anisotropy effects in dimer single molecule magnets
We present a model of equal spin dimer single molecule magnets. The
spins within each dimer interact via the Heisenberg and the most general set of
four quadratic anisotropic spin interactions with respective strengths and
, and with the magnetic induction . We solve the model
exactly for , and for antiferromagnetic Heisenberg couplings
(), present curves at low for these cases. Low-
curves for and electron paramagnetic susceptibility
for are also provided. For weak anisotropy
interactions, we employ a perturbative treatment, and show that the Hartree and
extended Hartree approximations lead to reliable analytic results at low
and large for these quantities and for the inelastic neutron scattering
cross-section . Our results are discussed with
regard to existing experiments on Fe dimer
single molecule magnets, and suggest that one of them contains a substantial
amount of single-ion anisotropy, without any sizeable global spin anisotropy.
We urge further experiments of the above types on single crystals of Fe and
on some [Mn] dimers, in order to elucidate the precise values
of the various microscopic interactions.Comment: 30 pages, 25 figures, submitted to Phys. Rev.
Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single molecule magnets
We study single-ion and exchange anisotropy effects in equal-spin
tetramer single molecule magnets exhibiting , , ,
, , or ionic point group symmetry. We first write the
group-invariant quadratic single-ion and symmetric anisotropic exchange
Hamiltonians in the appropriate local coordinates. We then rewrite these local
Hamiltonians in the molecular or laboratory representation, along with the
Dzyaloshinskii-Moriay (DM) and isotropic Heisenberg, biquadratic, and
three-center quartic Hamiltonians. Using our exact, compact forms for the
single-ion spin matrix elements, we evaluate the eigenstate energies
analytically to first order in the microscopic anisotropy interactions,
corresponding to the strong exchange limit, and provide tables of simple
formulas for the energies of the lowest four eigenstate manifolds of
ferromagnetic (FM) and anitiferromagnetic (AFM) tetramers with arbitrary .
For AFM tetramers, we illustrate the first-order level-crossing inductions for
, and obtain a preliminary estimate of the microscopic
parameters in a Ni from a fit to magnetization data.
Accurate analytic expressions for the thermodynamics, electron paramagnetic
resonance absorption and inelastic neutron scattering cross-section are given,
allowing for a determination of three of the microscopic anisotropy
interactions from the second excited state manifold of FM tetramers. We also
predict that tetramers with symmetries and should exhibit both
DM interactions and multiferroic states, and illustrate our predictions for
.Comment: 30 pages, 14 figures, submitted to Phys. Rev.
Molecular switching in iron complexes bridged via tin-cyanides observed by Mössbauer and ESR spectroscopy
The precursor [FeIII(L)Cl] (LH2 N,N'-bis(2'-hydroxy- 3'-X-benzyliden)-1,6-diamino-3-N-hexane) is a high-spin (S 5/2) complex (with X -CH3, -O-CH3). This precursor is combined with the bridging unit [SnIV(CN)4] to yield star-shaped pentanuclear clusters, [(L-X-FeIII)4Sn(CN) 4]Cl4 57Fe-Mössbauer, 119mSn- Mössbauer, and ESR spectroscopy are used to study our samples. For X -CH3 the 57Fe-Mössbauer data show a multiple spin transition between iron(III) in the high-spin and low-spin state. Changing the functional group from X -CH3 to X -O-CH3 turns the switchability off
A Tetranuclear Dysprosium Schiff Base Complex Showing Slow Relaxation of Magnetization
A tetranuclear dysprosium Schiff base complex was isolated by reacting dysprosium chloride with 2-hydroxy-3-methoxybenzaldehyde and 2-(aminomethyl)pyridine in-situ under basic conditions. The isolated Dy(III) complex was characterized by elemental analyses, single crystal X-ray diffraction and molecular spectroscopy. The complex crystallizes in the triclinic space group P-1 with unit cell parameters of a = 10.2003 (4), b = 13.8602 (5), c = 14.9542 (6), α = 94.523 (3), β = 109.362 (4), and γ = 99.861 (3). The magnetic properties of 1 have been investigated by DC and AC susceptibility measurements. The DC measurements reveal weak exchange coupling of antiferromagnetic nature. In the AC measurement, the complex shows a slow relaxation of magnetization in the absence of an external magnetic field
Field-Induced Single Molecule Magnetic Behavior of Mononuclear Cobalt(II) Schiff Base Complex Derived from 5-Bromo Vanillin
A mononuclear Co(II) complex of a Schiff base ligand derived from 5-Bromo-vanillin and 4-aminoantipyrine, that has a compressed tetragonal bipyramidal geometry and exhibiting field-induced slow magnetic relaxation, has been synthesized and characterized by single crystal X-ray diffraction, elemental analysis and molecular spectroscopy. In the crystal packing, a hydrogen-bonded dimer structural topology has been observed with two distinct metal centers having slightly different bond parameters. The complex has been further investigated for its magnetic nature on a SQUID magnetometer. The DC magnetic data confirm that the complex behaves as a typical S = 3/2 spin system with a sizable axial zero-field splitting parameter D/hc = 38 cm⁻¹. The AC susceptibility data reveal that the relaxation time for the single-mode relaxation process is τ = 0.16(1) ms at T = 2.0 K and BDC = 0.12 T
Triangulo -{Er$_{3}} complex showing field supported slow magnetic relaxation
The triangulo-{Er} complex [ErCl(o-van)(OH)(HO)]Cl·nHO (n = 9.4; H(o-van) = o-vanillin) (1) was generated by an in situ method. The isolated Er(III) complex 1 was characterized by elemental analysis and molecular spectroscopy. The results of single crystal X-ray diffraction studies have shown that 1 is built up of trinuclear [ErCl(o-van)(OH)(HO)] complex cations, chloride anions and water solvate molecules. Within the complex cation the three Er(III) central atoms are placed at the apexes of a triangle which are bridged by three (o-van) ligands with additional chelating functions and two μ-OH ligands. Additionally five aqua and one chlorido ligands complete the octa-coordination of the three Er(III) atoms. AC susceptibility measurements reveal that the compound exhibits slow magnetic relaxation with two relaxation modes
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