Modeling soil chemical changes induced by grassland afforestation in a sedimentary plain with shallow groundwater

Abstract

Chemical changes produced by the establishment of tree plantations on grasslands occupying sedimentary plains could potentially impact their highly productive soils through multiple mechanisms. Some of the reported chemical changes following this vegetation shift include soil salinization, sodification, alkalization and acidification. While the associated hydrological, chemical and biological processes have been characterized in most cases, their quantitative integration remains as a complex challenge. The present work explored the simultaneous occurrence of the four mentioned soil chemical changes using reactive transport modeling and hydrological and chemical datasets (water, soil and plants) for a well described afforested grassland in the Pampas of Argentina. Processes included in the numerical model were unsaturated water flow, chemical reactions (soil cation exchange, mineral dissolution-precipitation), the intrusion of salty deep water induced by tree groundwater consumption, rhizosphere-plant processes (respiration, selective solute uptake/exclusion, root mediated electric charge compensation, and elemental recycling and uplift). Modeling results suggest that the dominant soil chemical changes depend on the interplay of all the aforementioned processes and help explain strong soil variation across very short distances (250 m) along edge-core gradients within the tree plantation. This complexity is hard to expect and explain from an isolated treatment of each of the aforementioned processes, calling for the application of numerical tools.