O. A. Bachmeier

Environmental Dynamics in Salinas Grandes, Catamarca, Argentina

Saline systems are open systems where energy and matter fluxes produce modifications in the ecosystem, especially in plant communities, defined by species abundance and plant cover. Based on these changes, it is possible to define vectors that show the tendencies of such communities to reach mature or stationary states and their modifications by natural or anthropic disturbances. Salinity and topsoil thickness dominate the system dynamics. The decrease in soil salinity and the increase in the sandy topsoil horizon thickness would allow installation of new plant populations, a higher biodiversity, ecological resilience, and productive potential. Plant and soil surveys were conducted, establishing several relationships among the studied variables. Lyapunov coefficients (L) were calculated as the Euclidean distance of each site from a reference condition, used as exergy of the system. These values were correlated with the electrical conductivity data. The presence of soluble salts in the soils generates adverse conditions for unadapted species, reducing the total amount of biomass of a plant community, and therefore its exergy. When L decreases in time, it is expected that the system goes towards a steady state. If the contrary occurs, the equilibrium is unstable and becomes less resilient. While primary succession evolves, minimum rangeland requirements increase, depending on the dynamics of the system. According to the relationship between climate, plant cover, and erosive processes, dunes development would increase total biomass and biodiversity. Anthropic degradation, through fire, deforestation, and overgrazing, can produce retrogression of primary or secondary successions through erosive processes and salinization.

Phosphorus availability in the central area of the Argentine Pampean region. 1: Relationship between soil parameters, adsorption processes and wheat, soybean and corn yields in different soil and management environments .

Retention of phosphorus in the soil solid phase is a complex process, caused by a combination of physical and chemical mechanisms that determine pools of compounds with different degrees of solubility. The amount of P available in a given pool can be associated with the adsorption maximum proposed by the Langmuir isotherm model, and with the energy with which it is retained. The aims of this work were: i) to evaluate the impact of the fertilization history and soil type on phosphorus adsorption parameters, and ii) to analyze the effect of soil properties, mainly the phosphorus adsorption parameters, on the yield of wheat, soybean and corn crops. In this study we established that the Bray & Kurtz 1 extractant is insufficiently sensitive for assessing changes in the nutrient availability when phosphorous concentrations are relatively low. The amount of phosphorus retained in the labile pool and its retention energy, however, are sensitive indicators of the availability of this nutrient. These indexes explain most of the variations in crop yields, and are determined more by P fertilization management than by soil type.

Available phosphorus in the central area of the Argentinean Pampas. 2: Kinetics of adsorption and desorption of phosphorus under different soil and management environments

The concentration of phosphate ions in solution is critical in defining the ability of a soil to adequately meet crop demand. That concentration is regulated by adsorption/desorption and precipitation/dissolution processes, particularly its kinetics of reaction. This work was developed in order to find the kinetic model describing the sorption/desorption of phosphate in soils of the Argentinean Pampas region, to describe the processes associated with these reactions and to define the effect of temperature on them. A bi-linear model that adequately describes the data obtained in the adsorption and desorption experiments is proposed. Thus, it was possible to clearly differentiate two different kinetic mechanisms, each characterized by different reaction rates. Two very labile pools of phosphorus were determined in these soils. The rate at which these processes occur indicates the occurrence of chemical reactions of precipitation in highly soluble compounds and surface adsorption reactions with very low binding energy, as P is quickly released into the soil solution in the desorption process. However, since the amounts of P released in the desorption process are very much less than the adsorbed, is possible to infer that both processes occur at non-equilibrium conditions and there is a lack of reversibility of the reaction P-Soil – P-Solution in the soils studied. The intensity of these processes is differentially affected by temperature and, in general, there is a positive effect of phosphorus fertilization history, although the effect of this is not clearly seen in the amounts of extractable phosphorus with the classic Bray & Kurtz N°1 method.

Mineralización de nitrógeno in situ en un cultivo estival: efecto de la humedad edáfica y forma nitrogenada presente

Mediante incubaciones in situ de muestras de suelo sin disturbar, se estudio el efecto de la forma nitrogenada presente y la humedad edafica sobre los procesos de mineralizacion e inmovilizacion de nitrogeno en un suelo Haplustol entico del centro de la provincia de Cordoba (Argentina). A lo largo del ciclo de un cultivo de soja (Glycine max L. Merr.) se evaluaron las cantidades de nitrato y amonio producidas durante 14 dias de incubacion, determinandose, paralelamente,la humedad edafica. Se establecio la existencia de una relacion negativa entreel contenido de N mineral inicial (nitrato + amonio) y el producido durante la incubacion, cuando el contenido hidrico edafico no fue limitante para el proceso.Los fenomenos de mineralizacion e inmovilizacion se relacionaron estrechamente con el estado energetico del agua edafica: tuvo lugar mineralizacion cuando el potencial matrico vario entre -17 y -39 kPa, mientras que a potenciales menores se produjo inmovilizacion de nitrogeno.