Alejandra A. Jara

UREASE ACTIVITY AND NITROGEN MINERALIZATION KINETICS AS AFFECTED BY TEMPERATURE AND UREA INPUT RATE IN SOUTHERN CHILEAN ANDISOLS

Se realizaron estudios de incubacion para evaluar el efecto de la temperatura y niveles de suministro de urea sobre la actividad ureasa y la cinetica de mineralizacion de N en dos Andisoles del Sur de Chile. Nuestros resultados mostraron que la actividad ureasa fue mas alta en el suelo Piedras Negras (PNS) que en el suelo Freire (FS) a 24 y 48 h, y la enzima fue activada cuando la temperatura incremento desde 5 a 21 °C. La actividad ureasa siguio una cinetica simple de Michaelis-Menten. En PNS, la constante de Michaelis-Menten (Km) disminuyo, mientras que la velocidad maxima de reaccion de la enzima (Vmax) y la eficiencia catalitica aumentaron cuando la temperatura fue elevada hasta 21 °C a 24 y 48 h de incubacion del suelo. En FS, la actividad ureasa fue la mas alta a las 24 h, y esta disminuyo a las 48 h como consecuencia del agotamiento del sustrato. Por otra parte, los datos de cinetica de mineralizacion de N estrechamente se ajustaron a un modelo exponencial simple. A etapas tempranas de la incubacion, cantidades similares de N mineralizado (Nmin) fueron encontradas en ambos suelos. Sin embargo, Nmin fue 27 a 64 % mas alto en PNS que FS al final del experimento. Estos resultados tuvieron directa relacion con el contenido de materia organica del suelo, la temperatura y el nivel suministro de urea. En PNS, nosotros observamos que 47 a 80 % de Nmin permanecio como NH4+-N, mientras que en FS N03--N represento entre 80 y 96 %. Tales diferencias en la dinamica de transformacion de N estuvieron de acuerdo con la variacion del pH registrada durante el periodo experimental. Estos hechos tienen implicancias practicas para el manejo de los sistemas agricolas en una localidad en particular, debido a que la dinamica de la mineralizacion de N en Andisoles es principalmente controlada por el contenido de materia organica y las propiedades biologicas de suelos, asi como por la temperatura y el nivel de suministro de urea al sistema suelo-planta.

Kinetic and thermodynamic study of chlorophenol sorption in an allophanic soil.

The sorption of 2,4-dichlorophenol, 2,4,6-trichlorophenol, and pentachlorophenol by a allophanic soil was studied in a series of batch experiments. Chlorophenol sorption behavior was evaluated as a function of reaction time (0-96h) and input concentration at a fixed ionic strength (0.1mol L(-1) KCl) at 25, 35, and 45 degrees C. Sorption results for the various reaction temperatures were used in calculating thermodynamic parameters. Chlorophenol sorption increased with temperature, suggesting an endothermic process. The Elovich equation was used to describe the kinetic data. Data from the isotherm experiments were described by the Triple-Layer Model in which monodentate outer- and inner-sphere complexes were formed between deprotonated organic molecules and active sites on the variable-charge soil. The calculated thermodynamic parameters suggest that chlorophenol sorption is a spontaneous (DeltaG<0), endothermic (DeltaH>0) and entropy-driven reaction (DeltaS>0).

Description of mutual interactions between silicon and phosphorus in Andisols by mathematical and mechanistic models

The Freundlich model and the Constant Capacitance Model (CCM) were used to describe silicon (Si) and phosphorus (P) sorption, both individually and for binary P-Si systems, on two Andisols with different chemical properties: Freire soil (FS) and Piedras Negras soil (PNS). Silicon sorption kinetics were examined through the Elovich equation, revealing that the initial sorption rate was 16 times greater in PNS. The Freundlich equation provides a good fit to the sorption data for both Andisols. When compared with FS, larger Si sorption capacity and lower Si affinity for the surface sites were observed in PNS; nevertheless, Si sorption decreased in both soils as P sorption increased. Slight reductions in P sorption capacity due to the presence of Si were found, whereas there was no apparent effect on P bonding intensity. The CCM was able to describe Si adsorption, and potentiometric titrations support that Si seems to be specifically sorbed mainly onto sites of negative charge. Comparable log KSiint values were obtained for both soils, indicating that Si was bound on similar sites. Phosphorus sorption was well described by the CCM, and log KPint denoted strong interactions of P with the surface sites. For binary systems, log KPint did not vary with increasing Si concentration; comparatively, log KSiint scarcely decreased with increasing P concentration in PNS, but a 28% reduction was found in FS at the highest initial P concentration.

Effect Of pH, Phosphate and/or Malate on Sulfate Sorption on Andisols

Key words: Sulfate, adsorption, Andisol, mineralogical composition, phosphate, malate. ABSTRACTThe sulfate sorption was studied on Andisols with variable mineralogical composition and low organic matter content in presence and absence of phosphate or malate ligands at different pH. Crystalline clay mineralogy was similar for all the studied samples, except for the 2C horizon of the pedon 2, which did not show crystalline minerals. The soil samples showed content of allophane ranging from 16% to 42%. The sulfate adsorption decreased when descending the first 3 horizons (A1, 2A2 and 3Bw1) of the pedon 1. This behavior was attributed to the decreasing organic carbon content. At different pH sulfate sorption was drastically decreased by increasing the pH from 4.0 to 7.0, precisely by 80% for sample 5 to 100% for sample 1. Experiments on the competitive sorption of phosphate and sulfate on volcanic soils were carried out. Sulfate sorption was reduced even in the presence of low concentrations of phosphate ions (phosphate/sulfate molar ratio << 1). Sulfate sorption was influenced by many factors as the organic carbon content, pH, the mineralogical composition of soil samples and the order of sulfate and phosphate addition into the soils. Sulfate competes with phosphate particularly when added before phosphate and at low pH values (pH < 4.5). Phosphate more than malate affected the kinetics of sulfate sorption onto a volcanic soil containing a large amount of allophanic materials.Palabras claves: Sulfato, adsorcion, Andisol, composicion mineralogica, fosfato, malate