526 research outputs found
Modelling long-term impacts of fertilization and liming on soil acidification at Rothamsted experimental station
Liming is widely used to reduce the impacts of soil acidification and optimize soil pH for agricultural production. Whether models can simulate the effect of liming on soil pH, and base saturation (BS), and thereby guide lime application, is still largely unknown. Long-term experimental data from a grassland (Park Grass, 1965–2012)and arable land (Sawyers Field, 1962–1972) at Rothamsted Research, UK, were thus used to assess the ability of the VSD+ model to simulate the effects of long-term fertilization and liming on soil acidification. The VSD+model was capable of simulating observed soil pHand BS changes over time in the long-termliming experiments, except for a treatment in which sulphur (S) was added. NormalizedMean Absolute Errors (NMAE) and Normalized Root Mean Square Errors (NRMSE) of simulated and observed pH values, averaged over the observation periods varied between 0.02 and 0.08 (NMAE) and 0.01–0.05 (NRMSE). The acidity budget results for Park Grass suggest that nitrogen (N) transformations contributed most to acidity production, causing predominantly aluminium (Al) exchange in the topsoil (0–23 cm) followed by base cation (BC) release, but in the treatment with S addition, BC uptake had a nearly similar effect on acidity production.However, in Sawyers Field, the acidity budget suggested that BC uptakewas the dominant cause of soil acidification,while the impacts of N transformations were limited. Liming was found to sufficiently replenish BC and decrease Al exchange in the topsoil layer. Overall, the VSD+ model can adequately reconstruct the impacts of fertilizer and liming applications on acid neutralizing processes and related soil pH and BC changes at the soil exchange complex
Spectroscopic Analysis of H I Absorption Line Systems in 40 HIRES Quasars
We list and analyze H I absorption lines at redshifts 2 < z < 4 with column
density (12 < log(N_HI) < 19) in 40 high-resolutional (FWHM = 8.0 km/s) quasar
spectra obtained with the Keck+HIRES. We de-blend and fit all H I lines within
1,000 km/s of 86 strong H I lines whose column densities are log(N_HI/[cm^-2])
> 15. Unlike most prior studies, we use not only Lya but also all visible
higher Lyman series lines to improve the fitting accuracy. This reveals
components near to higher column density systems that can not be seen in Lya.
We list the Voigt profile fits to the 1339 H I components that we found. We
examined physical properties of H I lines after separating them into several
sub-samples according to their velocity separation from the quasars, their
redshift, column density and the S/N ratio of the spectrum. We found two
interesting trends for lines with 12 < log(N_HI) < 15 which are within 200-1000
km/s of systems with log(N_HI) > 15. First, their column density distribution
becomes steeper, meaning relatively fewer high column density lines, at z <
2.9. Second, their column density distribution also becomes steeper and their
line width becomes broader by about 2-3 km/s when they are within 5,000 km/s of
their quasar.Comment: 32 pages, 14 figures, accepted for publication in the Astronomical
Journal. A complete version with all tables and figures is available at
http://www.astro.psu.edu/users/misawa/pub/Paper/40hires.ps.g
Short-term biotic removal of dissolved organic nitrogen (DON) compounds from soil solution and subsequent mineralisation in contrasting grassland soils
Cycling of low molecular weight dissolved organic nitrogen compounds constitutes an important component of soil organic matter turnover in soils. We determined how rapidly grassland soils can cycle urea, compared to the amino acid l-alanine, and the peptide l-trialanine. Using naturally occurring concentrations of 14C-labelled compounds the rates of removal from soil solution and subsequent mineralisation were measured. Biotic removal of all three compounds and subsequent mineralisation to CO2 occurred within minutes. This research has demonstrated, for the first time, the potential for rapid removal of urea at low concentrations by the soil microbial biomass
The MACHO Project LMC Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo
The MACHO Project is a search for dark matter in the form of massive compact
halo objects (Machos). Photometric monitoring of millions of stars in the Large
Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is
used to search for gravitational microlensing events caused by these otherwise
invisible objects. Analysis of the first 2.1 years of photometry of 8.5 million
stars in the LMC reveals 8 candidate microlensing events. This is substantially
more than the number expected () from lensing by known stellar
populations. The timescales (\that) of the events range from 34 to 145 days.
We estimate the total microlensing optical depth towards the LMC from events
with 2 < \that < 200 days to be \tau_2^{200} = 2.9 ^{+1.4}_{-0.9} \ten{-7}
based upon our 8 event sample. This exceeds the optical depth, \tau_{\rm
backgnd} = 0.5 \ten{-7}, expected from known stars, and the difference is to
be compared with the optical depth predicted for a ``standard" halo composed
entirely of Machos: \tau_{halo} = 4.7\ten{-7}. Likelihood analysis gives a
fairly model independent estimate of the halo mass in Machos within 50 kpc of
2.0^{+1.2}_{-0.7} \ten{11} \msun, about half of the ``standard halo" value.
We also find a most probable Macho mass of 0.5^{+0.3}_{-0.2}\msun, although
this value is strongly model dependent. Additionally, the absence of short
duration events places stringent upper limits on the contribution of low-mass
Machos: objects from 10^{-4} \msun to 0.03 \msun contribute \simlt 20\%
of the ``standard" dark halo.Comment: Latex, 54 pages, uses aas2pp4.sty and astrobib.sty, with 24 out of 26
Postscript figures in gzipped tar file. 2 extra greyscale figures and/or full
paper available from ftp://igpp.llnl.gov/pub/macho/LMC2/ Submitted to ApJ,
June 199
Light Element Abundances From z=0 To z=5
Plausible ratios of deuterium to hydrogen D/H as a function of metallicity,
time, and redshift are investigated. Guided by the heavy element abundance
patterns observed locally in Galactic dwarf stars and at large redshift in
quasi-stellar object absorption line systems, empirical evolution of the
relative abundance ratios Li/D, B/D, N/D, O/D, and F/D for QSO absorption line
systems are given for the possible evolutionary patterns in D/H. Shortened
abstract.Comment: 30 pages including 8 figures, ApJ in pres
Limits on Active-Sterile Neutrino Mixing and the Primordial Deuterium Abundance
Studies of limits on active-sterile neutrino mixing derived from big bang
nucleosynthesis considerations are extended to consider the dependance of these
constraints on the primordial deuterium abundance. This study is motivated by
recent measurements of D/H in quasar absorption systems, which at present yield
discordant results. Limits on active-sterile mixing are somewhat relaxed for
high D/H. For low D/H (), no active-sterile neutrino
mixing is allowed by currently popular upper limits on the primordial He
abundance . For such low primordial D/H values, the observational inference
of active-sterile neutrino mixing by upcoming solar neutrino experiments would
imply that has been systematically underestimated, unless there is new
physics not included in standard BBN.Comment: 10 pages + 2 figures, uses revtex macros, submitted to Phys. Rev. D.
Corrected figure captions and an added referenc
Geocentrism reexamined
The universe is nearly isotropic on very large scales. It is much more
difficult to show that the universe is radially homogeneous (independent of
distance), or equivalently, that it is isotropic about distant points. This
taken as an axiom, since if it were not true, then we would occupy a preferred
position. This paper considers several empirical arguments for radial
homogeneity based on the cosmic microwave background (CMB). The tightest limits
on inhomogeneity on the scale of the horizon are of order ten percent but will
improve soon. These limits involve the Sunyaev-Zel'dovich effect in clusters of
galaxies, excitation of low-energy atomic transitions, and the accurately
thermal spectrum of the CMB. Weaker limits from primordial nucleosynthesis are
discussed briefly.Comment: RevTeX source, 14 pages, no figs. To appear Phys Rev
Large-scale structure in the Lyman-alpha forest II: analysis of a group of ten QSOs
The spatial distribution of Ly-alpha forest absorption systems towards ten
QSOs has been analysed to search for large-scale structure over the redshift
range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are
concentrated within a 1 deg^2 field. We have employed a technique based on the
first and second moments of the transmission probability density function which
is capable of identifying and assessing the significance of regions of over- or
underdense Ly-alpha absorption. We find evidence for large-scale structure in
the distribution of Ly-alpha forest absorption at the > 99 per cent confidence
level. In individual spectra we find overdense Ly-alpha absorption on scales of
up to 1200 km s^-1. There is also strong evidence for correlated absorption
across line of sight pairs separated by < 3 h^-1 proper Mpc (q_0 = 0.5). For
larger separations the cross-correlation signal becomes progressively less
significant.Comment: 15 pages, LaTeX, 6 Postscript figures, accepted for publication in
MNRA
Electromagnetic Cascades and Cascade Nucleosynthesis in the Early Universe
We describe a calculation of electromagnetic cascading in radiation and
matter in the early universe initiated by the decay of massive particles or by
some other process. We have used a combination of Monte Carlo and numerical
techniques which enables us to use exact cross sections, where known, for all
the relevant processes. In cascades initiated after the epoch of big bang
nucleosynthesis -rays in the cascades will photodisintegrate He,
producing He and deuterium. Using the observed He and deuterium
abundances we are able to place constraints on the cascade energy deposition as
a function of cosmic time. In the case of the decay of massive primordial
particles, we place limits on the density of massive primordial particles as a
function of their mean decay time, and on the expected intensity of decay
neutrinos.Comment: compressed and uuencoded postscript. We now include a comparison with
previous work of the photon spectrum in the cascade and the limits we
calculate for the density of massive particles. The method of calculation of
photon spectra at low energies has been improved. Most figures are revised.
Our conclusions are substantially unchange
CMB Observables and Their Cosmological Implications
We show that recent measurements of the power spectrum of cosmic microwave
background anisotropies by BOOMERanG and MAXIMA can be characterized by four
observables, the position of the first acoustic peak l_1= 206 pm 6, the height
of the first peak relative to COBE normalization H_1= 7.6 pm 1.4, the height of
the second peak relative to the first H_2 = 0.38 pm 0.04, and the height of the
third peak relative to the first H_3 = 0.43 pm 0.07. This phenomenological
representation of the measurements complements more detailed likelihood
analyses in multidimensional parameter space, clarifying the dependence on
prior assumptions and the specific aspects of the data leading to the
constraints. We illustrate their use in the flat LCDM family of models, where
we find Omega_m h^{3.8} > 0.079 (or nearly equivalently, the age of the
universe t_0 0.019, a
matter density Omega_m h^2 0.85 from the peak heights (95%
CL). With the aid of several external constraints, notably nucleosynthesis, the
age of the universe and the cluster abundance and baryon fraction, we construct
the allowed region in the (Omega_m,h) plane; it points to high h (0.6< h < 0.9)
and moderate Omega_m (0.25 < Omega_m < 0.6).Comment: 12pages, 11figs. Version accepted in ApJ. Several typos fixed
including eqns (13,A22
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