Magnetic interaction of Co ions near the {10\bar{1}0} ZnO surface

Co-doped ZnO is the prototypical dilute magnetic oxide showing many of the characteristics of ferromagnetism. The microscopic origin of the long range order however remains elusive, since the conventional mechanisms for the magnetic interaction, such as super-exchange and double exchange, fail either at the fundamental or at a quantitative level. Intriguingly, there is a growing evidence that defects both in point-like or extended form play a fundamental role in driving the magnetic order. Here we explore one of such possibilities by performing {\it ab initio} density functional theory calculations for the magnetic interaction of Co ions at or near a ZnO \{10$\bar{1}$0\} surface. We find that extended surface states can hybridize with the $e$-levels of Co and efficiently mediate the magnetic order, although such a mechanism is effective only for ions placed in the first few atomic planes near the surface. We also find that the magnetic anisotropy changes at the surface from an hard-axis easy-plane to an easy axis, with an associated increase of its magnitude. We then conclude that clusters with high densities of surfacial Co ions may display blocking temperatures much higher than in the bulk

A distinct mechanism for the ABC transporter BtuCD–BtuF revealed by the dynamics of complex formation

ATP-binding cassette (ABC) transporters are integral membrane proteins that translocate a diverse array of substrates across cell membranes. We present here the dynamics of complex formation of three structurally characterized ABC transporters—the BtuCD vitamin B_(12) importer and MetNI d/l-methionine importer from Escherichia coli and the Hi1470/1 metal-chelate importer from Haemophilus influenzae—in complex with their cognate binding proteins. Similarly to other ABC importers, MetNI interacts with its binding protein with low affinity (K_d ~10^(−4) M). In contrast, BtuCD–BtuF and Hi1470/1–Hi1472 form stable, high-affinity complexes (K_d ~10^(−13) and 10^(−9) M, respectively). In BtuCD–BtuF, vitamin B_(12) accelerates the complex dissociation rate ~10^7-fold, with ATP having an additional destabilizing effect. The findings presented here highlight substantial mechanistic differences between BtuCD–BtuF, and likely Hi1470/1–Hi1472, and the better-characterized maltose and related ABC transport systems, indicating that there is considerable mechanistic diversity within this large protein super-family

Carbon Dioxide Dynamics During a Growing Season in Midwestern Cropping Systems

Daily and seasonal CO2-exchange dynamics between the boundary layer and biosphere is important to understanding Net Ecosystem Exchange of terrestrial ecosystems. Spatial and temporal variations of CO2 fluxes across midwestern cropping systems have not been well documented. This study was designed to monitor and evaluate spatial and temporal dynamics of CO2 exchange across a watershed region for typical production fields of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] in the Midwest by quantifying the above-canopy, within-canopy, and soil components of C balance for this cropping system. An energy-balance approach using eddy covariance was utilized across different fields making year-around measurements in both corn and soybean fields to quantify the exchange of CO2 and H2O between the crop canopy and the atmospheric boundary layer. Within-canopy concentrations of CO2 and H2O vapor were measured with an eight-port CO2/H2O infrared analyzer. Soil respiration was quantified using soil chambers at various landscape positions throughout the growing season. Fluxes of CO2 and H2O vapor throughout the day were dependent on net radiation and the stage of canopy development. Diurnal variations in CO2 and H2O vapor fluxes revealed that the magnitude of the fluxes is large and the variation of the fluxes among fields was consistent throughout the season. Integration of the daily fluxes into seasonal totals showed large differences among crops and fields. Flux differences were the result of the effect of varying soil types on water-holding capacity. Seasonal integrated values were lower than estimates derived from biomass samples collected within the fields and the measurement of the C content of the biomass. Within-canopy recycling of soil CO2 may provide insight to this discrepancy. The techniques are available to quantify the CO2 and H2O vapor fluxes across different management systems and landscapes to help refine our understanding of the magnitude of the CO2 and H2O dynamics in cropping systems

Do cover crops increase or decrease nitrous oxide emissions? A meta-analysis

There are many environmental benefits to incorporating cover crops into crop rotations, such as their potential to decrease soil erosion, reduce nitrate (NO3) leaching, and increase soil organic matter. Some of these benefits impact other agroecosystem processes, such as greenhouse gas emissions. In particular, there is not a consensus in the literature regarding the effect of cover crops on nitrous oxide (N2O) emissions. Compared to site-specific studies, meta-analysis can provide a more general investigation into these effects. Twenty-six peer-reviewed articles including 106 observations of cover crop effects on N2O emissions from the soil surface were analyzed according to their response ratio, the natural log of the N2O flux with a cover crop divided by the N2O flux without a cover crop (LRR). Forty percent of the observations had negative LRRs, indicating a cover crop treatment which decreased N2O, while 60% had positive LRRs indicating a cover crop treatment which increased N2O. There was a significant interaction between N rate and the type of cover crop where legumes had higher LRRs at lower N rates than nonlegume species. When cover crop residues were incorporated into the soil, LRRs were significantly higher than those where residue was not incorporated. Geographies with higher total precipitation and variability in precipitation tended to produce higher LRRs. Finally, data points measured during cover crop decomposition had large positive LRRs and were larger than those measured when the cover crop was alive. In contrast, those data points measuring for a full year had LRRs close to zero, indicating that there was a balance between periods when cover crops increased N2O and periods when cover crops decreased emissions. Therefore, N2O measurements over the entire year may be needed to determine the net effect of cover crops on N2O. The data included in this meta-analysis indicate some overarching crop management practices that reduce direct N2O emissions from the soil surface, such as no soil incorporation of residues and use of nonlegume cover crop species. However, our results demonstrate that cover crops do not always reduce direct N2O emissions from the soil surface in the short term and that more work is needed to understand the full global warming potential of cover crop management

LaCrO3 heteroepitaxy on SrTiO3(001) by molecular beam epitaxy

Stoichiometric, epitaxial LaCrO3 films have been grown on TiO2-terminated SrTiO3(001) substrates by molecular beam epitaxy using O2 as the oxidant. Film growth occurred in a layer-by-layer fashion, giving rise to structurally excellent films and surfaces which preserve the step-terrace structure of the substrate. The critical thickness is in excess of 500 {\AA}. Near-surface Cr(III) is highly susceptible to further oxidation to Cr(V), leading to the formation of a disordered phase upon exposure to atomic oxygen. Recovery of the original epitaxial LaCrO3 phase is readily achieved by vacuum annealing.Comment: 10 pages, 4 figure

Testing a nitrogen fertilizer applicator designed to reduce leaching losses

Conventional practices for nitrogen fertilization of corn produce soil conditions that are conducive to preferential water flow and nitrate leaching. A new fertilizer applicator is proposed that will more effectively protect the fertilizer from infiltrating water and thus reduce the potential for leaching. The device forms a small compacted layer of soil above the subsurface fertilizer band and then mounds soil into a surface dome directly above the fertilizer band. This new localized compaction and doming (LCD) method is evaluated by measuring soil physical properties around the fertilizer band and comparing them with measurements made within the conventional knifing system. The LCD applicator increased penetration resistance from 0.50 to 0.75 MPa at the fertilizer band. As the knife slit above the fertilizer band was closed by the LCD applicator, soil bulk density was increased from 1.2 to 1.4 g/cm3 in the region. The ponded infiltration rate through the fertilizer band was reduced from 19.7 cm/h at the conventional knife slit to 10.1 cm/h at the LCD surface. Reduced water flow through the fertilizer band will result in reduced NO3-N movement. Nitrate movement was measured during a growing season in a corn field, and NO3-N applied by the LCD applicator moved approximately 60% as deep as NO3-N applied by a conventional knife applicator. The ability to restrict NO3-N movement by modifying the surface soil at N application represents a simple yet effective strategy to reduce NO3-N leaching losses and possible impacts on groundwater quality

Time Interval Between Cover Crop Termination and Planting Influences Corn Seedling Disease, Plant Growth, and Yield

Experiments were established in a controlled-growth chamber and in the field to evaluate the effect of the length of time intervals between winter rye cover crop termination and corn planting on corn seedling disease, corn growth, and grain yield in 2014 and 2015. Rye termination dates ranged from 25 days before planting (DBP) to 2 days after planting (DAP) corn in the field and from 21 DBP to 1 DAP in controlled studies. Results were similar in both environments. In general, shorter intervals increased seedling disease and reduced corn emergence, shoot growth, and grain yield of corn following winter rye compared with corn planted 10 or more days after rye termination or without rye. Incidence of Pythium spp. increased with shorter intervals (less than 8 DBP); incidence of Fusarium spp. was not consistent between runs and experiments. In 2014, in the 1-DAP treatment, number of ears and grain yield were reduced (P = 0.05 and 0.02, respectively). In 2015, all termination intervals reduced plant population, number of ears, and yield (P = 0.01), with the 2-DBP treatment causing the biggest decrease. A 10- to 14-day interval between rye termination and corn planting should be followed to improve corn yield following a rye cover crop

Are Coronae of Magnetically Active Stars Heated by Flares? III. Analytical Distribution of Superimposed Flares

(abridged) We study the hypothesis that observed X-ray/extreme ultraviolet emission from coronae of magnetically active stars is entirely (or to a large part) due to the superposition of flares, using an analytic approach to determine the amplitude distribution of flares in light curves. The flare-heating hypothesis is motivated by time series that show continuous variability suggesting the presence of a large number of superimposed flares with similar rise and decay time scales. We rigorously relate the amplitude distribution of stellar flares to the observed histograms of binned counts and photon waiting times, under the assumption that the flares occur at random and have similar shapes. Applying these results to EUVE/DS observations of the flaring star AD Leo, we find that the flare amplitude distribution can be represented by a truncated power law with a power law index of 2.3 +/- 0.1. Our analytical results agree with existing Monte Carlo results of Kashyap et al. (2002) and Guedel et al. (2003). The method is applicable to a wide range of further stochastically bursting astrophysical sources such as cataclysmic variables, Gamma Ray Burst substructures, X-ray binaries, and spatially resolved observations of solar flares.Comment: accepted for publication in Ap