Leibniz University of Hanover

About: Leibniz University of Hanover is a education organization based out in Hanover, Niedersachsen, Germany. It is known for research contribution in the topics: Finite element method & Computer science. The organization has 14283 authors who have published 29845 publications receiving 682152 citations.

Soil phosphorus supply controls P nutrition strategies of beech forest ecosystems in Central Europe

Abstract: Phosphorus availability may shape plant–microorganism–soil interactions in forest ecosystems. Our aim was to quantify the interactions between soil P availability and P nutrition strategies of European beech (Fagus sylvatica) forests. We assumed that plants and microorganisms of P-rich forests carry over mineral-bound P into the biogeochemical P cycle (acquiring strategy). In contrast, P-poor ecosystems establish tight P cycles to sustain their P demand (recycling strategy). We tested if this conceptual model on supply-controlled P nutrition strategies was consistent with data from five European beech forest ecosystems with different parent materials (geosequence), covering a wide range of total soil P stocks (160–900 g P m−2; <1 m depth). We analyzed numerous soil chemical and biological properties. Especially P-rich beech ecosystems accumulated P in topsoil horizons in moderately labile forms. Forest floor turnover rates decreased with decreasing total P stocks (from 1/5 to 1/40 per year) while ratios between organic carbon and organic phosphorus (C:Porg) increased from 110 to 984 (A horizons). High proportions of fine-root biomass in forest floors seemed to favor tight P recycling. Phosphorus in fine-root biomass increased relative to microbial P with decreasing P stocks. Concomitantly, phosphodiesterase activity decreased, which might explain increasing proportions of diester-P remaining in the soil organic matter. With decreasing P supply indicator values for P acquisition decreased and those for recycling increased, implying adjustment of plant–microorganism–soil feedbacks to soil P availability. Intense recycling improves the P use efficiency of beech forests.

Uranium Isotope Fractionation

Abstract: This review focuses on the rapidly growing field of natural 238U/235U variability, largely driven by the technical advances in the measurement of U isotope ratios by mass spectrometry with increasing precision over the last decade. A thorough review on the application of the U-decay series systems within Earth sciences was published in Reviews in Mineralogy and Geochemistry (RiMG) volume 52 in 2003, and will not be discussed further within this review. Instead, this article will first focus on the basic chemical properties of U and the evolution of 238U/235U measurement techniques, before discussing the latest findings and use of this isotopic system to address questions within geochronology, cosmochemistry and Earth sciences.

Oriented Crystallisation on Supports and Anisotropic Mass Transport of the Metal‐Organic Framework Manganese Formate

Abstract: The oriented in situ crystallisation of microporous manganese(II) formate [Mn(HCO2)2] on different porous supports (e.g. porous alumina and graphite) has been investigated. The anisotropic growth of Mn(HCO2)2 was examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The intracrystalline diffusion of methanol in Mn(HCO2)2 was studied by observation of the sorption kinetics by interference microscopy (IFM). It was found that untreated discs of porous alumina and graphite supports exhibit poor densities of Mn(HCO2)2 crystals per supported area. Even lower crystal densities are found for activated supports such as for alumina after basic treatment and for oxidized graphite supports. Improved results have been achieved by replacing formic acid by sodium formate in the synthesis route. Crystal growth on graphite gives results superior to those on alumina with respect to the crystal density as well as the relative orientation of the 1D channel network to the support surface. Methanol uptake from the vapour phase is similar at 25 and 40 °C and gives an adsorption capacity of about 95 mL g–1. The BET surface area was found to be 280 m2 g–1. IFM indicates that diffusion occurs only along one crystallographic axis, thus proving the presence of a 1D channel system. Diffusion coefficients of about 10–12 m2 s–1 for methanol in microporous Mn(HCO2)2 were found. These results can be used to prepare supported metal–organic framework (MOF) membranes for molecular sieving. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Eliciting security requirements and tracing them to design: an integration of Common Criteria, heuristics, and UMLsec

Abstract: Building secure systems is difficult for many reasons. This paper deals with two of the main challenges: (i) the lack of security expertise in development teams and (ii) the inadequacy of existing methodologies to support developers who are not security experts. The security standard ISO 14508 Common Criteria (CC) together with secure design techniques such as UMLsec can provide the security expertise, knowledge, and guidelines that are needed. However, security expertise and guidelines are not stated explicitly in the CC. They are rather phrased in security domain terminology and difficult to understand for developers. This means that some general security and secure design expertise are required to fully take advantage of the CC and UMLsec. In addition, there is the problem of tracing security requirements and objectives into solution design, which is needed for proof of requirements fulfilment. This paper describes a security requirements engineering methodology called SecReq. SecReq combines three techniques: the CC, the heuristic requirements editor HeRA, and UMLsec. SecReq makes systematic use of the security engineering knowledge contained in the CC and UMLsec, as well as security-related heuristics in the HeRA tool. The integrated SecReq method supports early detection of security-related issues (HeRA), their systematic refinement guided by the CC, and the ability to trace security requirements into UML design models. A feedback loop helps reusing experience within SecReq and turns the approach into an iterative process for the secure system life-cycle, also in the presence of system evolution.