Adsorption—desorption and/or precipitation—dissolution processes of zinc in soils

The concentrations of trace and toxic metals in soil solutions are explained by several authors either in terms of adsorption—desorption or precipitation—dissolution reactions in soils. Data have been given for zinc to test the applicability of both concepts. The results show that the concentrations of zinc in equilibrium solutions with soil clay fractions and whole soil samples at pH values below 7 are determined exclusively by adsorption—desorption reactions for various pH's, contents of bound zinc and compositions of soils. At neutral to alkaline pH values precipitation—dissolution reactions of zinc may take place. There is some evidence that formation of zinc silicates may control the zinc concentration in solution provided natural complexing agents are absent, the affinity of the soil for zinc is low and the content of reaching zinc is high (> ～ 100 ppm). Even at pH values above 7, the formation of other zinc compounds is unlikely in most soils because additions of large amounts of zinc are required to ensure saturation of the adsorption sites of different soil components before the zinc concentration in the soil solution can increase sufficiently to bring about the precipitation of definite compounds. Model experiments in CaCO₃-buffered systems showed that the adsorption capacity for specifically adsorbed zinc (in μmole/g) by the following components increased in the order CaCO₃ (0.44), bentonite (44), humic acid (842), amorphous Fe- and Al-oxides (1190, 1310) and δ -MnO₂ (1540) and demonstrated the importance of Mn-, Fe-, and Al-oxides and humic substances for the binding of zinc in soils containing carbonates, and thus indicate the special role of these components in limiting precipitation reactions.     

Nitrous oxide emissions from fertilised grassland: A 2-year study of the effects of N fertiliser form and environmental conditions

The aim was to investigate the effects of different N fertilisers on nitrous oxide (N₂O) flux from agricultural grassland, with a view to suggesting fertiliser practices least likely to cause substantial N₂O emissions, and to assess the influence of soil and environmental factors on the emissions. Replicate plots on a clay loam grassland were fertilised with ammonium sulphate (AS), urea (U), calcium nitrate (CN), ammonium nitrate (AN), or cattle slurry supplemented with AN on three occasions in each of 2 years. Frequent measurements were made of N₂O flux and soil and environmental variables. The loss of N₂O-N as a percentage of N fertiliser applied was highest from the supplemented slurry (SS) treatment and U, and lowest from AS. The temporal pattern of losses was different for the different fertilisers and between years. Losses from U were lower than those from AN and CN in the spring, but higher in the summer. The high summer fluxes were associated with high water-filled pore space (WFPS) values. Fluxes also rose steeply with temperature where WFPS or mineral N values were not limiting. Total annual loss was higher in the 2nd year, probably because of the rainfall pattern: the percentage losses were 2.2, 1.4, 1.2, 1.1 and 0.4 from SS, U, AN, CN and AS, respectively. Application of U in the spring and AN twice in the summer in the 2nd year gave an average emission factor of 0.8% – lower than from application of either individual fertiliser. We suggest that similar varied fertilisation practices, modified according to soil and crop type and climatic conditions, might be employed to minimise N₂O emissions from agricultural land. Received: 30 August 1996     

A potential second messenger role for unsaturated fatty acids: activation of Ca2+-dependent protein kinase

Arachidonate and other unsaturated long-chain fatty acids were found to activate protein kinase C from human neutrophils. Kinase activation by arachidonate required calcium and was enhanced by diolein but did not require exogenous phosphatidylserine. Submaximal levels of arachidonate also enhanced the affinity of the kinase for calcium during activation by phosphatidylserine. Thus the release of arachidonate, which is triggered in many cell types by ligand-receptor interactions, could play a second messenger role in the regulation of cellular function by activation of protein kinase C.     

The mentality of crows: convergent evolution of intelligence in corvids and apes

Discussions of the evolution of intelligence have focused on monkeys and apes because of their close evolutionary relationship to humans. Other large-brained social animals, such as corvids, also understand their physical and social worlds. Here we review recent studies of tool manufacture, mental time travel, and social cognition in corvids, and suggest that complex cognition depends on a "tool kit" consisting of causal reasoning, flexibility, imagination, and prospection. Because corvids and apes share these cognitive tools, we argue that complex cognitive abilities evolved multiple times in distantly related species with vastly different brain structures in order to solve similar socioecological problems.