ABSTRACT Land-use change from herbaceous to woody cover -or vice versa- largely affects water fluxes, which in turn can mobilize existing salts dissolved in the soil, and thereby negatively affect soil and/or water quality and site productivity. Secondary salinization was observed after afforestation at the Northern and Eastern subregions of the Río de la Plata Grassland. Based on this, our objective was to determine the degree of association between the climatic (e.g. water balance), edaphic (e.g. texture, presence of petrocalcic horizons) and biological factors (age of plantation, stand density and stand basal area) with the secondary salinization processes under the particular environmental conditions of the Austral Pampas. The lowest electrical conductivity (EC, µS.cm-1) values were found in sites with sandy-textured soils and negative water balance, regardless of tree canopy cover. When EC differences did occur between land uses of the same site (50% of the sites), in most cases - but not in all of them - the highest EC was measured under tree cover. When salinization was significant, no pattern was observed in the salt distribution along the soil profile. An increase of EC was associated with the oldest-unmanaged plantations, with mean age of 40 years, a period much longer than the productive rotation recommended. Under similar edaphic conditions where salinization was observed (i.e. clayed soils, with naturally high salt level), salinization was not significant in relatively young plantations (<rotation period), demonstrating the key role of forest management to decrease its negative impacts on soils.

ABSTRACT Stand density management diagrams (SDMD) provide a guide for forest density management taking into account stands attributes such as trees´ diameter or volume. One of the most common species planted in Pampean plains of Argentina is Eucalyptus viminalis for multiple objectives: solid wood use or firewood in local markets, pulp for cellulose industry and to provide services for agriculture and cattle raising (windbreaks or cattle refuge). The objective of this study was to gather the available production information /inventory data and to develop a first SDMD for estimating standing volume, biomass and canopy cover of E. viminalis as a tool for forest managers aiming at different plantation purposes. Data to develop the SDMD were obtained from 161 plots, distributed along a climate and soil gradient. We also generated two predictive equations capable of estimating dominant height from the diameter of the trees as well as canopy cover from stand basal area. As an example of application, the SDMD was used to estimate the wood production of three alternative systems: a) an unmanaged plantation (simulating a common practice in the region), b) a mixed production system such as an agroforestry system, and c) a plantation that maximizes wood biomass or volume production.

The replacement of grasslands by forests is one of the most extreme changes in land use due to its impact on soil and vegetation functioning. On large plain ecosystems, this shift causes, in many cases, soil chemical changes such as salinization, sodification and alkalization/acidification. Although the hydrological, chemical and biological processes involved have been extensively characterized, due to their coupling and non-linearity, their quantification is a hard task, and until the present day, they were not addressed by any research work. The present work intends to capture though reactive transport modeling, the physical-chemical processes that take place in an afforested grassland with Eucalyptus camaldulensis located on the Flooding Pampas (Castelli, province of Buenos Aires), using hydrological and chemical information (water, soil and plants). The numerical model simulates the water flows and chemical reactions in the context of reproduce 1) the hydrological changes induced by forest, and 2) the intrusion of deep saline water, 3) the absorption and exudation of nutrients by the roots, and 4) the respiration and solutes recycling. Modeling results suggest that factors such as the water table chemical composition, the cation exchange processes with the soil solution, the mineral precipitation/dissolution, and the interaction of the solution with the rhizosphere (solutes uptake/ exclusion/exudation), are the dominant mechanisms that control the direction of chemical change in the soil. This change shows marked contrasts in the short distance (250 m) from grassland passes through the outer belt and reaches the core of the afforested plot. Finally, the change intensity and direction, and transformation of soil will depend both on the new ecosystem that humans impose and on the chemical and hydrological nature of the site.

El reemplazo de pastizales por forestaciones es uno de los cambios más extremos en el uso del suelo por su impacto en el suelo y en el funcionamiento de la vegetación. En los ecosistemas de llanuras, este cambio generó, en muchos casos, alteraciones químicas en los suelos como salinización, sodificación y alcalinización/ acidificación. Si bien los procesos hídricos, químicos y biológicos que tienen lugar fueron extensamente caracterizados, debido al acoplamiento entre estos procesos y su no-linealidad, la tarea de cuantificarlos es difícil y, hasta el momento, no existen estudios que los hayan evaluado. El presente trabajo intenta caracterizar, mediante la modelación del transporte reactivo, los procesos físico-químicos que tienen lugar en un pastizal forestado con Eucalyptus camaldulensis en la Pampa Deprimida (Castelli, provincia de Buenos Aires). Para ello, se utilizó información hidrológica y química (agua, suelo y plantas). El modelo numérico simula los flujos de agua y las potenciales reacciones químicas en el contexto de reproducir 1) las alteraciones hidrológicas que causa la forestación y por 2) la intrusión de agua salina profunda, 3) la absorción y la exudación de nutrientes por las raíces, y 4) la respiración y el reciclaje de solutos. Los resultados de la modelación sugieren que factores como la composición química de la napa freática, el intercambio catiónico de los materiales del suelo con la solución del suelo, la precipitación/disolución de minerales y la interacción con la rizósfera (toma/exclusión/ exudación de solutos) son los mecanismos dominantes que controlarían la dirección del cambio químico en el suelo (acidificación vs. alcalinización). Este cambio muestra contrastes marcados en distancias cortas (250 m) desde el pastizal, pasando por el margen, hasta el centro del sector forestado. Finalmente, la intensidad y la dirección de los cambios, y la transformación del suelo dependerá tanto del nuevo ecosistema que los humanos imponen como de la naturaleza química e hidrológica del sitio.