Tunneling into a single magnetic atom: spectroscopic evidence of the kondo resonance

The Kondo effect arises from the quantum mechanical interplay between the electrons of a host metal and a magnetic impurity and is predicted to result in local charge and spin variations around the magnetic impurity. A cryogenic scanning tunneling microscope was used to spatially resolve the electronic properties of individual magnetic atoms displaying the Kondo effect. Spectroscopic measurements performed on individual cobalt atoms on the surface of gold show an energetically narrow feature that is identified as the Kondo resonance-the predicted response of a Kondo impurity. Unexpected structure in the Kondo resonance is shown to arise from quantum mechanical interference between the d orbital and conduction electron channels for an electron tunneling into a magnetic atom in a metallic host.     

The immobilization of nitrogen by straw decomposing in soil

Immobilization of nitrogen (N) in decomposing straw varies between soils, and the objective of this study was to identify the mechanisms responsible. Internode segments of wheat straw were incubated in Denmark and in Scotland in arable soils fertilized with NH₄NO₃, labelled with ¹⁵N, for periods up to 1 year. Straw was recovered from the soils periodically and analysed for microbial biomass and different forms of N using chemical methods and CPMAS ¹⁵N NMR spectroscopy. The total N content of the straw increased, as long as the soil was not too wet, such that there was overall immobilization. This was accompanied by a rapid increase in the content of amino acid N and to a lesser extent of glucosamine N and a concomitant decrease in the carbohydrate content of the straw. Using direct and plate counts for bacterial and ergosterol content for fungal estimation, we found that fungal biomass was much greater than that of bacteria. This correlated with the forms of N in the straw as determined by CPMAS ¹⁵N NMR, which showed spectra that were more typical of fungi than of bacteria. It seems that immobilization of N is primarily caused by fungi as they decompose the straw.     

Estimating soil C loss potentials from the C to N ratio

Thirty-seven arable soil samples (0–20 cm depth) ranging in clay content from 4 to 40% and in C to N ratio from 8 to 20 were incubated at ?100 hPa and 20 °C for 238 days. The CO₂-C evolved during days 105–147 and days 0–238 was related to the soil C to N ratio by a power function. We hypothesized the existence of a threshold value for the soil C to N ratio below which CO₂-C evolution remains constant. A soil C to N threshold value of 9.7 was estimated for both incubation periods and the function predicted that the CO₂-C evolution was halved when the soil C to N ratio approached 14. Data obtained by Springob and Kirchmann [Springob, G., Kirchmann, H., 2002. C-rich sandy Ap horizons of specific historical land-use contain large fractions of refractory organic matter. Soil Biology & Biochemistry 34, 1571–1581.] showed a similar relationship, but with a C to N threshold value of 10.8. We suggest that the relationship established between the C to N ratio of a soil and its CO₂-C loss during incubation represents a simple but useful measure for predicting soil C loss potentials on a more general level.     

Nitrous oxide evolution from structurally intact soil as influenced by tillage and soil water content

Among farmers there is a growing interest for adoption of reduced tillage practices, which has accentuated the need to understand the consequences for soil nutrient dynamics and losses. A laboratory study was conducted with structurally intact soil cores collected from two depths, 0–4 and 14–18 cm, within tillage experiments on contrasting soil types, both experiments with soil under mouldboard ploughing (MP) or shallow tillage (ST). The soil cores were adjusted to one of seven matric potentials ranging from ?1500 to ?15 hPa. The extent and regulation of nitrous oxide (N₂O) evolution as a function of tillage, depth and soil characteristics was studied by measurement of N₂O and CO₂ evolution rates, as well as nitrifying and denitrifying potentials, and subsequent data analysis by multiple linear regression models. At both sites, compaction of ST soil below the depth of tillage was significant. The vertical distribution of N₂O evolution was different in MP and ST soil, but no main effect of tillage on N₂O evolution was observed. Effects of soil variables on N₂O evolution were analysed using volumetric water content, water-filled pore space, or relative gas diffusivity (RD) to represent the effect of soil water. Using RD weakened interactions with tillage and C availability and strengthened main effects, suggesting that RD may provide a more general representation of the water effect. At 0–4 cm depth, N₂O evolution was related to NO₃^? availability in the soil with 5.1% C, but to C availability in the soil with 1.5% C. The contrasting patterns of dependencies in the different environments support the interpretation of reduced tillage and soil water content as indirect controls, via diffusional constraints, of N₂O evolution.     

Cover crop growth and impact on N leaching as affected by pre‐ and postharvest sowing and time of incorporation

In Northern Europe, cover crops are traditionally established before spring crops by undersowing, but some cover crops might also have an effect if preharvest sown before spring crops and even winter crops. The effects of cover crop sowing date, sowing technique and succeeding main crop on biomass production, N uptake, nitrate leaching and soil inorganic N were tested in lysimeters and in the field. Cruciferous cover crops (oil radish, white mustard) were sown preharvest by broadcasting into winter wheat in July and were allowed to grow until a following winter wheat was established in September. Other preharvest cover crops were left in place until late autumn. For comparison, the same cruciferous cover crops were established postharvest after light harrowing. Perennial ryegrass undersown in spring barley was also included. Aboveground N uptake in preharvest cover crops amounted to a maximum of 24 kg N/ha in September before sowing winter wheat. When left until late autumn, preharvest oil radish took up a maximum of 66 kg N/ha, and ryegrass and postharvest cover crops 35 kg N/ha. Preharvest establishment of cruciferous cover crops before a spring‐sown crop thus seems promising. The soil was depleted of inorganic N to the same extent in late autumn irrespective of cover crop type, sowing time and technique within winter wheat or spring barley. However, the reduction in nitrate leaching of preharvest cover crops incorporated after 2 months and followed by winter wheat was only half of that achieved by cover crops left until late autumn or spring.     

Genetic resistance to Hymenoscyphus pseudoalbidus limits fungal growth and symptom occurrence in Fraxinus excelsior

The European common ash (Fraxinus excelsior) is currently threatened by a pathogenic fungus, Hymenoscyphus pseudoalbidus, which seems to enter the trees through the leaves. Continuous assessments of 39 clones in Danish field trials have shown that there are significant differences in the susceptibility of clones to the new disease. Interestingly, clones that showed early leaf senescence in the autumn were in general less susceptible to the disease than late‐senescing clones. Thus, variation in susceptibility could be owing to phenological differences associated with the infection biology. To test whether differences in susceptibility are driven by genetically based factors other than phenology, we compared inoculations with H. pseudoalbidus on four highly susceptible clones with those of four less susceptible clones. Development of necrosis was hereafter followed regularly. The growth of the fungus in the inner bark was further detected with species‐specific PCR primers. The severity of the response to infection shows significant differentiation among clones and significant correlation with clone susceptibility, as assessed from natural infections in field trials. The fungus was detected in tissues immediately surrounding the necrosis but showed some signs of endophytic growth. The results suggest that healthier clones are able to limit the growth and spread of the fungus and thereby minimize the occurrence of symptoms. This gives hope for the future preservation of F. excelsior in Europe through selection and breeding.     

A study of duration of digital dermatitis lesions after treatment in a Danish dairy herd

Digital dermatitis (DD) is a contagious disease of cattle affecting the skin adjacent to the claws. Disease dynamics of DD have been described to some extend, but we still need to quantify the duration of lesions and look into non-treatment factors affecting this. The aim of this study was to estimate the duration of lesions due to DD and to evaluate parity and lactation stage as potential risk factors for longer duration of such lesions. An estimate of the duration of lesions will be a valuable parameter in the evaluation of the economic impact of the disease and will additionally allow estimates of incidence based on prevalence figures. From May 2007 until November 2008, lesions associated with DD in the hind legs of 151 Danish Holstein cows at the Danish Cattle Research Centre were clinically scored on fifteen occasions. The mean interval between recordings was 39 days. Onset and end of each new case were estimated as midway between recordings prior to a change in the presence or absence of a lesion. Kaplan-Meier survival functions and Cox proportional hazard regression were performed to estimate the duration and analyse differences in the duration of lesions between primi- and multiparous cows and between different stages in lactation at onset of the lesion. The median duration of lesions were estimated to be 42 days, less than most previous published estimates. The relatively aggressive regime of topical treatment in the study herd might have shortened the duration of the lesions. Furthermore the comparatively long interval between recordings introduced an element of uncertainty in this estimate. No significant effects of parity or days in milk at lesion onset on the duration of DD were found using these data though lesions developed earlier in the lactation may have a longer duration. Further data would be needed to confirm the latter.     

The effect of grazing on cow mortality in Danish dairy herds

The effect of summer grazing in large Danish dairy herds and certain management characteristics of grazing were studied for their impact on dairy cow mortality. Mortality data (from the Danish Cattle Database) from 391 Danish dairy herds (>100 cows) were combined with information from a questionnaire survey of grazing procedures on these herds in 2008. In all, 131 of the herds were identified as summer grazing and 260 as zero-grazing herds. The mortality was affected by an interaction of summer grazing and milking system. The risk of a cow dying was reduced to 46% in a grazing compared to a zero-grazing herd having automatic milking system. In traditional milking system, mortality was reduced to 75% in grazing compared to zero-grazing herds. Within the grazing herds, the risk of mortality decreased with increasing number of hours on pasture during the season. Free access between barn and pasture was associated with increased cow mortality.     

Leaching and plant offtake of N in field pea/cereal cropping sequences with incorporation of 15N-labelled pea harvest residues

Abstract. Field peas (Pisum sativum L.) were grown in sequence with winter wheat (Triticum aestivum L.) or spring barley (Hordeum vulgare L.) in large outdoor lysimeters. The pea crop was harvested either in a green immature state or at physiological maturity and residues returned to the lysimeters after pea harvest. After harvest of the pea crop in 1993, pea crop residues (pods and straw) were replaced with corresponding amounts of ¹⁵N‐labelled pea residues grown in an adjacent field plot. Reference lysimeters grew sequences of cereals (spring barley/spring barley and spring barley/winter wheat) with the straw removed. Leaching and crop offtake of ¹⁵N and total N were measured for the following two years. These treatments were tested on two soils: a coarse sand and a sandy loam. Nitrate concentrations were greatest in percolate from lysimeters with immature peas. Peas harvested at maturity also raised the nitrate concentrations above those recorded for continuous cereal growing. The cumulative nitrate loss was 9–12 g NO₃‐N m^–2 after immature peas and 5–7 g NO₃‐N m^–2 after mature peas. Autumn sown winter wheat did not significantly reduce leaching losses after field peas compared with spring sown barley. ¹⁵N derived from above‐ground pea residues accounted for 18–25% of the total nitrate leaching losses after immature peas and 12–17% after mature peas. When compared with leaching losses from the cereals, the extra leaching loss of N from roots and rhizodeposits of mature peas were estimated to be similar to losses of ¹⁵N from the above‐ground pea residues. Only winter wheat yield on the coarse sand was increased by a previous crop of peas compared to wheat following barley. Differences between barley grown after peas and after barley were not statistically significant. ¹⁵N lost by leaching in the first winter after incorporation accounted for 11–19% of ¹⁵N applied in immature pea residues and 10–15% of ¹⁵N in mature residues. Another 2–5% were lost in the second winter. The ¹⁵N recovery in the two crops succeeding the peas was 3–6% in the first crop and 1–3% in the second crop. The winter wheat did not significantly improve the utilization of ¹⁵N from the pea residues compared with spring barley.