286 research outputs found
Thermodynamic Constraints on Nitrogen Transformations and Other Biogeochemical Processes at Soil-Stream Interfaces
There is much interest in biogeochemical processes that occur at the interface between soils and streams since, at the scale of landscapes, these habitats may function as control points for fluxes of nitrogen (N) and other nutrients from terrestrial to aquatic ecosystems. Here we examine whether a thermodynamic perspective can enhance our mechanistic and predictive understanding of the biogeochemical function of soil-stream interfaces, by considering how microbial communities interact with variations in supplies of electron donors and acceptors. Over a two-year period we analyzed \u3e1400 individual samples of subsurface waters from networks of sample wells in riparian wetlands along Smith Creek, a first-order stream draining a mixed forested-agricultural landscape in southwestern Michigan, USA. We focused on areas where soil water and ground water emerged into the stream, and where we could characterize subsurface flow paths by measures of hydraulic head and/or by in situ additions of hydrologic tracers. We found strong support for the idea that the biogeochemical function of soil-stream interfaces is a predictable outcome of the interaction between microbial communities and supplies of electron donors and acceptors. Variations in key electron donors and acceptors (NO3â,N2O,NH4+,SO42â,CH4 role= presentation \u3eNO3â,N2O,NH4+,SO42â,CH4
,, and dissolved organic carbon [DOC]) closely followed predictions from thermodynamic theory. Transformations of N and other elements resulted from the response of microbial communities to two dominant hydrologic flow paths: (1) horizontal flow of shallow subsurface waters with high levels of electron donors (i.e., DOC, CH4, and NH4+),, and (2) near-stream vertical upwelling of deep subsurface waters with high levels of energetically favorable electron acceptors (i.e., NO3-,N2O, and SO42-).. Our results support the popular notion that soil-stream interfaces can possess strong potential for removing dissolved N by denitrification. Yet in contrast to prevailing ideas, we found that denitrification did not consume all NO3- that reached the soil-stream interface via subsurface flow paths. Analyses of subsurface N chemistry and natural abundances of Ύ 15N in NO3- and NH4+ suggested a narrow near-stream region as functionally the most important location for NO3- consumption by denitrification. This region was characterized by high throughput of terrestrially derived water, by accumulation of dissolved NO3- and N2O, and by low levels of DOC. Field experiments supported our hypothesis that the sustained ability for removal of dissolved NO3- and N2O should be limited by supplies of oxidizable carbon via shallow flowpaths. In situ additions of acetate, succinate, and propionate induced rates of NO3- removal (⌠1.8 g N· m-2· d-1) that were orders of magnitude greater than typically reported from riparian habitats. We propose that the immediate near-stream region may be especially important for determining the landscape-level function of many riparian wetlands. Management efforts to optimize the removal of NO3- by denitrification ought to consider promoting natural inputs of oxidizable carbon to this near-stream region
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Land use change and nitrogen feedbacks constrain the trajectory of the land carbon sink
Our understanding of Earth's carbon climate system depends critically upon interactions between rising atmospheric CO2, changing land use, and nitrogen limitation on vegetation growth. Using a global land model, we show how these factors interact locally to generate the global land carbon sink over the past 200âyears. Nitrogen constraints were alleviated by N2 fixation in the tropics and by atmospheric nitrogen deposition in extratropical regions. Nonlinear interactions between land use change and land carbon and nitrogen cycling originated from three major mechanisms: (i) a sink foregone that would have occurred without land use conversion; (ii) an accelerated response of secondary vegetation to CO2 and nitrogen, and (iii) a compounded clearance loss from deforestation. Over time, these nonlinear effects have become increasingly important and reduce the present-day net carbon sink by ~40% or 0.4 PgC yrâ1
Global Patterns of Terrestrial Biological Nitrogen (N2) Fixation in Natural Ecosystems
Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized \u3e100 preexisting published estimates of BNF to generate biome- and global-level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the model\u27s simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our âbest estimateâ of potential nitrogen fixation by natural ecosystems is âŒ195 Tg N yrâ1, with a range of 100â290 Tg N yrâ1. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux
Magnetic phases near the Van Hove singularity in s- and d-band Hubbard model
We investigate the magnetic instabilities of the nondegenerate (s-band) and a
degenerate (d-band) Hubbard model in two dimensions using many-body effects due
to the particle-particle diagrams and Hund's rule local correlations. The
density of states and the position of Van Hove singularity change depending on
the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly
reduced in the s-band case, and ferromagnetism survives only if electron
density is small, and the band is almost flat at small momenta due to
next-nearest neighbor hopping. In contrast, for the d-band case the reduction
of the Stoner parameter which follows from particle-particle correlations is
much smaller and ferromagnetism survives to a large extent. Inclusion of local
spin-spin correlations has a limited destabilizing effect on the magnetic
states.Comment: 8 pages, 7 figure
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Molybdenum and Phosphorus Interact to Constrain Asymbiotic Nitrogen Fixation in Tropical Forests
Biological di-nitrogen fixation (N2) is the dominant natural source of new nitrogen to land ecosystems. Phosphorus (P) is thought to limit N2 fixation in many tropical soils, yet both molybdenum (Mo) and P are crucial for the nitrogenase reaction (which catalyzes N2 conversion to ammonia) and cell growth. We have limited understanding of how and when fixation is constrained by these nutrients in nature. Here we show in tropical forests of lowland Panama that the limiting element on asymbiotic N2 fixation shifts along a broad landscape gradient in soil P, where Mo limits fixation in P-rich soils while Mo and P co-limit in P-poor soils. In no circumstance did P alone limit fixation. We provide and experimentally test a mechanism that explains how Mo and P can interact to constrain asymbiotic N2 fixation. Fixation is uniformly favored in surface organic soil horizons - a niche characterized by exceedingly low levels of available Mo relative to P. We show that soil organic matter acts to reduce molybdate over phosphate bioavailability, which, in turn, promotes Mo limitation in sites where P is sufficient. Our findings show that asymbiotic N2 fixation is constrained by the relative availability and dynamics of Mo and P in soils. This conceptual framework can explain shifts in limitation status across broad landscape gradients in soil fertility and implies that fixation depends on Mo and P in ways that are more complex than previously thought
Neil3-dependent base excision repair regulates lipid metabolism and prevents atherosclerosis in Apoe-deficient mice
Increasing evidence suggests that oxidative DNA damage accumulates in atherosclerosis. Recently, we showed that a genetic variant in the human DNA repair enzyme NEIL3 was associated with increased risk of myocardial infarction. Here, we explored the role of Neil3/NEIL3 in atherogenesis by both clinical and experimental approaches. Human carotid plaques revealed increased NEIL3 mRNA expression which significantly correlated with mRNA levels of the macrophage marker CD68. Apoeâ/âNeil3â/â mice on high-fat diet showed accelerated plaque formation as compared to Apoeâ/â mice, reflecting an atherogenic lipid profile, increased hepatic triglyceride levels and attenuated macrophage cholesterol efflux capacity. Apoeâ/âNeil3â/â mice showed marked alterations in several pathways affecting hepatic lipid metabolism, but no genotypic alterations in genome integrity or genome-wide accumulation of oxidative DNA damage. These results suggest a novel role for the DNA glycosylase Neil3 in atherogenesis in balancing lipid metabolism and macrophage function, potentially independently of genome-wide canonical base excision repair of oxidative DNA damage
Search for right-handed W bosons in top quark decay
We present a measurement of the fraction f+ of right-handed W bosons produced
in top quark decays, based on a candidate sample of events in the
lepton+jets decay mode. These data correspond to an integrated luminosity of
230pb^-1, collected by the DO detector at the Fermilab Tevatron
Collider at sqrt(s)=1.96 TeV. We use a constrained fit to reconstruct the
kinematics of the and decay products, which allows for the
measurement of the leptonic decay angle for each event. By comparing
the distribution from the data with those for the expected
background and signal for various values of f+, we find
f+=0.00+-0.13(stat)+-0.07(syst). This measurement is consistent with the
standard model prediction of f+=3.6x10^-4.Comment: Submitted to Physical Review D Rapid Communications 7 pages, 3
figure
Dependence of the production cross section on the transverse momentum of the top quark
We present a measurement of the differential cross section for
events produced in collisions at TeV as a function
of the transverse momentum () of the top quark. The selected events
contain a high- lepton (), four or more jets, and a large imbalance
in , and correspond to 1 fb of integrated luminosity recorded
with the D0 detector. Each event must have at least one candidate for a
jet. Objects in the event are associated through a constrained kinematic fit to
the process. Results
from next-to-leading-order perturbative QCD calculations agree with the
measured differential cross section. Comparisons are also provided to
predictions from Monte Carlo event generators using QCD calculations at
different levels of precision.Comment: 8 pages, 6 figures, 4 tables, updated to reflect the published
versio
Search for first generation leptoquark pair production in the electron + missing energy + jets final state
We present a search for the pair production of first generation scalar
leptoquarks (LQ) in data corresponding to an integrated luminosity of 5.4
fb collected with the D0 detector at the Fermilab Tevatron Collider in
ppbar collisions at TeV. In the channel , where q, q' are u or d quarks, no significant excess
of data over background is observed, and we set a 95% C.L. lower limit of 326
GeV on the leptoquark mass, assuming equal probabilities of leptoquark decays
to eq and .Comment: 7 pages, 6 figures, submitted to PRD-R
Zgamma production and limits on anomalous ZZgamma and Zgammagamma couplings in ppbar collisions at sqrt(s)=1.96 TeV
We present a measurement of ppbar->Zgamma->ll+gamma (l = e, mu) production
with a data sample corresponding to an integrated luminosity of 6.2 fb^{-1}
collected by the D0 detector at the Fermilab Tevatron ppbar Collider. The
results of the electron and muon channels are combined, and we measure the
total production cross section and the differential cross section
dsigma/dp_T^gamma, where p_T^gamma is the momentum of the photon in the plane
transverse to the beamline. The results obtained are consistent with the
standard model predictions from next-to-leading order calculations. We use the
transverse momentum spectrum of the photon to place limits on anomalous ZZgamma
and Zgammagamma couplings
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