Theoretical considerations based on nutrition experiments suggest that nutrient ratios of terrestrial plants are similar to the Redfield ratio found in marine phytoplankton. Laboratory experiments have shown that seedlings of many different plant species have similar nutrient concentration ratios when supplied with nutrients at free access. However, at free access, nutrients are likely to be taken up in amounts in excess of a plant's requirements for growth. In further experiments, therefore, the supply rate of each nutrient was reduced so that excessive uptake did not occur. Again, similar nutrient ratios were found among the plant species tested, although the ratios differed from those found in plants given free access to nutrients. Based on the law of the minimum, we suggest that optimum nutrient ratios be defined as the ratios found in plants when all nutrients are limiting growth simultaneously. The literature on nutrient concentrations was surveyed to investigate nutrient ratios in terrestrial ecosystems. Nutrients taken into consideration were nitrogen, phosphorus, potassium, calcium and magnesium. Based on the assumption that nitrogen is either the limiting nutrient or, when not limiting, is taken up only in small excess amounts, we calculated nutrient ratios from published data. The calculated ratios corresponded closely to the ratios determined in laboratory and field experiments.

Volume transition of covalently cross-linked sodium polyacrylate gels (micrometer-sized) due to the absorption of dodecyltrimethylammonium bromide from bulk aqueous solution is studied by means of light and fluorescence microscopy. The volume transition occurs in a narrow range of surfactant concentrations. In the presence of 0.01 M sodium bromide in the solution, the equilibrium volume is reduced typically 50 times for surfactant concentrations larger that 4 × 10-4 M. During the transition, a collapsed, surfactant-rich, surface phase is formed, enclosing the swollen gel core. Evidence is found for equilibrium between the core and surface phase in the collapsed state of the gel. The relation to surfactant self-assembly in solutions of linear polyion and in centimeter-sized gels is discussed. The kinetics of diffusion-controlled shrinking is analyzed theoretically. The model considers the transport of surfactant from the bulk, through the “stagnant” liquid layer and the surface phase, to the gel core. During shrinking, the osmotic swelling of the gel is assumed to be in quasi-equilibrium with the bulk and calculated using a recent model for equilibrium swelling of phase-separated gels (Hansson et al. J. Phys. Chem. B 2002, 106, 9777). The kinetic model is used to analyze time-resolved experimental shrinking curves. The result suggests that the shrinking is controlled by stagnant layer diffusion of surfactant, and that the relative swelling at intermediate stages during the collapse is the same as for slab gels of the same composition. The lag time measured before shrinking starts is longer than the time expected for the surfactant concentration to exceed the critical aggregation concentration in the gels, suggesting that the gels are arrested for some time in a metastable state before the surface phase starts to form.

In a series of experiments, growth of small birch plants (Betula pendula Roth) was controlled by the relative addition rate of manganese, R(Mn) (day(-1)). The R(Mn) treatments were 0.05, 0.10, 0.15 and 0.20 day(-1) with free access to all other nutrients. In an additional treatment, FA, there was free access to all nutrients including Mn. The pH of the nutrient solution ranged between 3.9 and 4.1, and the conductivity was between 100 and 200 micro S cm(-1). After an adjustment phase to steady-state growth, there was a one-to-one relationship between the relative growth rate, R(G) (day(-1)), and the supply of manganese, R(Mn) (day(-1)). The Mn concentration of the plants ranged from 6 to 13 micro g g(DW) (-1) in all treatments with limiting R(Mn) and was approximately 200 micro g g(DW) (-1) in the FA treatment. At steady-state growth, the plants showed specific Mn deficiency symptoms, including leaf mortality, that were more pronounced at severe Mn limitation. Total nonstructural carbohydrate concentrations were low, less than 7.5% of dry weight at Mn limitation, and the fraction of plant dry matter partitioned to roots was much less at Mn limitation than has previously been reported for equivalent rates of N or P supply. Manganese uptake rate per unit root growth rate, dMn/dW(r) ( micro mol g(DW) (-1)) was unaffected by the supply of Mn. At Mn limitation, low rates of plant growth were associated with high values of specific leaf area (37 versus 36 m(2) kg(DW) (-1)), and lower values of leaf weight ratio (40 versus 61%) and net assimilation rate (3 versus 10 kg(DW) (-1) m(-2) day(-1)) than were found at higher R(Mn).

International audienceUtilising energy efficiency to lower energy demand in buildings is a key policy goal of the European Commission. This paper presents the results of bottom-up modelling to elucidate the impact of energy efficiency on the EU building stock up to 2050 under three different scenarios. The modelling is performed for eight individual EU countries and a ninth hypothetical entity that represents the remaining nineteen EU countries. The scenarios highlight the roles of different levels of efficiency improvements in the context of increasing floor area and the demand for energy services. From the results it can be concluded that the EC 2020 goals for primary energy savings can be met by focusing on a combination of minimum efficiency construction standards, improved conversion efficiency standards for final energy to useful energy, and a ≥2% annual improvement in end-use efficiency applied at the useful energy level. A comparison of the results obtained in the present study for Spain with the estimates of savings documented in the Spanish Energy Efficiency Action Plan indicate that the plan could lead to the closing of the energy efficiency gap for buildings in that country by 2020

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