The Flooding Pampa grasslands cover a plain of about 6 Mha in the Buenos Aires province, in Argentina, including the lowlands known as the Rio Salado Basin (Figure 1) (Soriano et al., 1991). Natural grasslands are the main forage resource that supports the cattle activity of the region and continuous grazing is one of the main determinants of the structure and functioning of the vegetation (León et al., 1984; Sala et al., 1986; Rusch and Oesterheld, 1997).
As previously mentioned, periodical floods are a typical environmental event in the region. Floods are determined by a climatic, soil properties, and topographical factors, and their duration and intensity vary from year to year. Floods are partially associated to the seasonal cyclic behavior of the water table and to the occurrence of intense precipitation. Two kind of floods can be distinguished. The ones are marked by a lower duration and intensity, take place at the end of the winter and the onset of spring. The others are more intense and prolongued, and do not occur in a precise period of the year (Paruelo and Sala, 1990; Taboada and Lavado, 2003).
As a natural disturbance, flood consequences vary according to the system upon which they impact and the way this impact is evaluated (Chaneton and Facelli, 1991). Traditionally, floods have been considered disturbances that alter the vegetation structure (White, 1979) and, owing to the fact that flood tolerance varies significantly among the different species, great changes in the floristic composition of the community can be expected. Thus, the disturbed ecosystems may experience major changes in the dominant species, an important part of their dynamics (Odum, 1985). Similarly, floods are often believed to have degradative effects on soils (Taboada y Lavado. 2003)
Flooding generates gaps in the vegetal canopy and modifies the physical and biotic environment explored by the different species, thus determining changes in the quantity and time in which resources are available (Tilman, 1982; Insausti et al., 1995). Natural grasslands are systems that generally show remarkable flood tolerance (Holberg and Bischoff, 1980). This differs from many other cultures, including sown grasslands, which may be significantly damaged by flooding (Jackson and Drew, 1984; Pezeshki, 1994; 2001). Responses to poor soil aeration depend upon the species. Many experience severe limitations for root growth, with soil oxygen diffusion rates below 20 ^gcm-2min-1. However, other species such as Agrostis palustris and Eleusine indica experience solid radical growth in soils with an oxygen diffusion rate below the aforementioned value (Meek and Stolzy, 1978). Depending on the species, the ecological, physiological, morphological and anatomical flood responses may vary, but with only two biological results: tolerance or susceptibility to flooding.
At the individual level and depending on the behavior of each species, the restriction of the soil oxygen caused by flooding reduces the respiration rate and water and nutrient absorption through plant roots. This occurs when plants are not adapted for internal oxygen transfer from the aerial part (e.g. aerenchyma) (Ponnamperuma, 1984; Voesenek et al., 2006). In the short term, physiological stress induced by soil flooding reduces the productivity of species sensitive to flooding, whereas in the long term, flooding may increase the mortality rate in the population, thus decreasing the presence of those species (Wehier and Keddy, 1995). Nevertheless, in the Flooding Pampa grasslands, species from flooded sites like Paspalum dilatatum grow during spring floods and present a higher photosynthesis rate and water potential during flooding than at field capacity (Insausti et al., 1999; 2001).
The Flooding Pampa is one of the areas in the world with extensive areas covered by Solonetz type soils (Bui et al., 1998). These soils were developed in a large scale by pedogenetic factors that are well known (Lavado and Taboada, 1988; Moscatelli and Scoppa 1984; Tricart 1973). Most soils (60%) in the region under study are included in the aquic regime (Salazar Lea Plaza and Moscatelli, 1989); most of them show a natric horizon and excess of soluble salts. The main Great Groups of Soil Taxonomy in this region are Natraquolls (28 000 km2) and Natraqualfs (11 000 km2). These soils do not occupy patches of different size, as in other regions in the world, but exist in large areas: the same soil Series may cover a continuous area of a hundred thousands hectares (Moscatelli and Salazar Lea Plaza, 1989). Natralbolls, Argialbolls, etc. are also present, but in a smaller extent. Most soils belong to loamy or loamy-fine texture families. They show a strong textural contrast between horizons, with a tough Bt (natric) horizon limiting soil water movements through the profile (Lavado and Taboada, 1988; Taboada et al., 2001).
In the region cattle grazing was found to be a major factor causing topsoil salinization. This process was associated to the decovering action of grazing that favors upward salt movements from deep soil layers (Lavado and Taboada, 1987; 1988).
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