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. 2005. A method for measuring above-and below-ground C stocks in hillside landscapes. Can. J. Soil Sci. 85: 523-530. Information on C stocks in agriculture and forest ecosystems in hillside landscapes is limited. The objective of this study was to develop and test field methods to measure above-and below-ground C stocks in hillside landscapes. Above-ground biomass in agricultural system was determined by measuring weight of residues left after crop harvest. In degraded secondary forests, tree biomass was estimated using allometric equations developed from in situ measurements. Herbs + bushes and litter dry weight were measured in two 0.25-m 2 quadrats located within one 100-m 2 treed plots. Carbon stocks were determined after chemical analysis of plant tissue and soil samples by dry combustion. Geo-referenced cores were taken inside a 1-m-diameter soil sampling clock that allows for spatial and temporal monitoring of soil C changes. The clock was marked with 12 divisions to establish the exact location of present and future sampling points. The below-ground fraction of C (mineral soil and fine roots) amounted to nearly 95% of the total C stock in agricultural systems and between 57 and 82% in the case of forest systems. Soil C stocks in hillside agricultural soils were higher than those found in forested soils with 70% of the C stored below-ground residing in the 0-45 cm of soil. The field method detected differences in C stocks in pools associated with various vegetations and soils in hillside ecosystems. Les auteurs ont déterminé la biomasse aérienne des systèmes agricoles en pesant les débris végétaux après la récolte. Dans les forêts secondaires dégradées, la biomasse des arbres a été estimée au moyen d'équations allométriques élaborées à partir de relevés pris sur le terrain. On a mesuré le poids sec des herbacées et des broussailles ainsi que de la litière dans deux cadrats de 0,25 m 2 situés à l'intérieur d'une parcelle arborée de 100 m 2 . Les réserves de carbone ont été établies après analyse chimique des tissus végétaux et des échantillons de sol par combustion sèche. On a prélevé des carottes géoréférencées dans un cercle d'échantillonnage de un mètre permettant de suivre les variations spatiales et temporelles du C dans le sol. Le cercle a été divisé en 12 secteurs ce qui autorise la localisation exacte des points d'échantillonnage actuels et futurs. La fraction souterraine du C (sol minéral et radicelles) représente près de 95 % des stocks totaux de C des systèmes agricoles et 57 à 82 % des stocks des systèmes forestiers. Les systèmes agricoles en pente possèdent de plus grandes réserves de C que les sols forestiers, 70 % du C souterrain se retrouvant dans la couche de 0 à 45 cm. Cette méthode permet de déceler la fluctuation des réserves de C dans les puits associés aux divers types de végétation et de sol dans les écosystèmes en pente.

In this study, the effect of phosphate on Ni sorption by Mahan soil was investigated. In batch experiments, an initial solution concentration of 1 mM phosphate increased Ni sorption by 15.1%, and initial concentrations of 1 mM Ni increased phosphate sorption by 12.8%. The effect of phosphate on Ni sorption, and vice versa, was dependent on initial concentrations of either solute, with greater effects being observed with greater initial concentrations. Sequential extractions suggest that the increased sorption of either species in the presence of the other is due to a combination of electrostatic interactions and the formation of ternary complexes. A novel Freundlich-type multi-component isotherm (FMC) was developed and described the sorption of Ni both in the absence and presence of phosphate with a single set of parameters. The effect of phosphate on the sorption kinetics of Ni was evaluated using the stirred-flow approach. The presence of phosphate increased the maximum sorbed concentration of Ni by 7.3% and increased the rate of Ni sorption. A novel Freundlich-type multicomponent kinetic model (FKM) was developed and described the stirred-flow data well. The FKM was able to describe the sorption-desorption of both Ni and phosphate applied individually as well as simultaneously. The FMC and FKM offer advantages over previous modeling approaches in that they require far fewer parameters than surface complexation models and have greater descriptive capabilities than traditional empirical models. However, both models are empirical, and their ability to describe the observed data should not be taken to imply specific molecular-level mechanisms.

The [Zn4O]6+ cluster is well-known to form the archetypal MOF-5 topology with dicarboxylate ligands. Here we report two new materials (CPM-300 and -301) that show dramatic alteration of topological and chemical behaviors of [Zn4O]6+ clusters. In CPM-300, [Zn4O]6+ untypically forms the MIL-88/MOF-235 type framework with a small pentane-ring-based chiral dicarboxylate. In contrast, in CPM-301, when mediated by [Zn9(btz)12]6+ clusters (btz = benzotriazolate), the MOF-5 topology is regenerated with the same chiral ligand, albeit with alternating [Zn4O]6+ and [Zn9(btz)12]6+ clusters. Importantly, both CPM-300 and CPM-301 are homochiral, hydrothermally stable in boiling water and alcohol, and thermally stable to 440 °C or higher. It is concluded that small methyl groups on the chiral ligand is sufficiently powerful to shield [Zn4O]6+ clusters from degradation by water, even at high temperatures. These results reveal a promising platform for the development of a new class of cluster-based homochiral and hydrothermally stable porous materials.

The mobility and fate of heavy metals in soils is of environmental concern. Stirredflow experiments were conducted to study the competitive sorption and desorption of Zn, Ni, and Pb in binary systems in acidic Olivier loam soil. For all three binary systems, consecutive pulse applications demonstrated that the metal of highest affinity displaced sorbed amounts of metals of low affinity, whereas sorbed concentrations of the ion of highest affinity were unaffected by the addition of the metal of lower affinity. The displacement of Ni by Zn appeared irreversible, whereas the displacement of Zn by Pb and Ni by Pb was reversible. A two-site competitive kinetic model consisting of common and ion-specific sites successfully described the experimental data. Specifically, the model was able to accurately predict lower sorbed concentrations in concurrent pulse studies, as well as the displacement of Ni by Zn and Zn by Pb in the consecutive pulse studies. The presented model provided significant improvement over single-ion models that have been previously used to describe competitive interactions between species in stirred flow experiments. How to cite this article: Padilla JT, Selim HM. Modeling the kinetics of competitive sorption and desorption of Zn(II), Ni(II), and Pb(II) in an acidic soil: Stirred-flow experiments.