Effect of Floods and Grazing on Flooding Pampa Soils

The combined effect of flooding and grazing was investigated in a field 2 % year study, in which the variation of soil physical properties was compared in nearby grazing enclosured and a continuously grazed (about 0.5 cow ha-1 year-1) areas (Taboada and Lavado, 1993; Taboada et al., 1999). It was found that soil water content and not grazing was the primary cause of changes in soil total porosity. Because of the continuous trampling by cattle, the applied stress never ceased during the study period. Despite this, soil pores damaged in summer were regenerated during the subsequent period of surface ponding, when the soil swelled and both total porosity and macroporosity > 30 ^m increased significantly in the grazed area. The regeneration of pores could be related to the appearance of microcracks (Dexter 1988), which are induced by air entrapment when the surface is flooded (Taboada et al., 2001). Parker et al. (1977) suggested that the magnitude of volume change on swelling can be increased by compaction and disruption of aggregates. In Flooding Pampas soils pore damage by trampling during the summer could have led to the increased soil expansion when it rewetted in winter.

Soil water content was the primary cause of changes in total porosity, because of shrink-swell processes (Taboada and Lavado, 1993). Contrary to what was expected, trampling caused macropores larger than 60 ^m to collapse and decreased the size of water-stable aggregates in dry soil. Because of this collapse and the greater shrinkage, the soil under grazing tended to have significantly less total porosity during summer than during winter. The damaged soil pores and the aggregates both increased in size some months later, during the period of surface ponding. Trampling accentuated soil swelling during these periods, and no poaching damage was observed.

Grazing effects on soil structural stability were superimposed on the environmental effects. Aggregate mean weight diameter (MWD) was often lower in the soil under grazing (Taboada and Lavado, 1993; Taboada et al., 1999). This aggregate size reduction is attributable to the fracturing and pulverizing of dry soil caused by the mechanical action of trampling, as shown by Warren et al. (1986). This increased the percentage of water-stable microaggregates (< 0.3 mm) both in dry summers in the soil under grazing. A highly significant decrease in wet MWD was also found in both grazed and old exclosure areas at the end of the dry summer. The low soil matric potential during that period (Lavado and Taboada 1988) brought about significant shrinkage. It was demonstrated that planes of weakness are generated within the dried and shrunken mass of soils with a certain proportion of active clay, as a consequence of water stress (Utomo and Dexter 1982). Likewise, in smectitic clay soil, most disaggregation could be caused by macroscopic and microscopic shrinkage planes developed when the soil was dry. When the soil was rewetted during the fall, the aggregate size increased again in both treatments.

Aggregate stability under grazing increased when the soil was wet and decreased when the soil was dry (Taboada and Lavado, 1993; Taboada et al., 1999). Results on aggregate MWD and on soil porosity agree well. Those results showed detrimental effects by trampling during the summer when the soil was dry. Figure 4 shows a conceptual model that postulates decreases in structural stability resulting from crushing air-filled pores by cattle hooves. This yields smaller water-stable aggregates, as shown by the higher proportion of aggregates < 0.3 mm usually found in the soil of the grazed area compared to the soil in the enclosure area. Only at low water contents was the structure of the topsoil destabilized by grazing. The recovery of structural stability began in the fall and was completed in the winter, when the soil was ponded (Figure 4). The structural recovery results from swelling, when the smaller aggregates created by trampling of dry soil are bound again into larger structural units.

The ultimate cause of this unusual behaviour was studied by Taboada et al. (2001). Topsoil structural improvement found during flooding appears to be a consequence of soil inflation. In a laboratory experiment, Gath and Frede (1995) found a process of soil "inflation", under quick saturation of a rigid material. Our field results showed the same process. A conceptual model describing the changes taking place in the soil profile was proposed (Figure 5). The unusually high entrapped air volumes at the maximum swollen condition can be regarded as the result of two coincident wetting fronts during winter - spring periods (Lavado and Taboada, 1988). Air bubbles entrapped between the ponded soil surface and the rising water table could not escape. This explains why air entrapment exerted so great influence on the swelling of our soils.

Larger water-stable -

aggregates » Structural

Stability recovery

Creation of pores

Structural Stability decrease

Trampling

Trampling

Figure 4. Conceptual model showing the process of soil structural destabilization when the soil dries and the process of structural recovery when soil wets.

Figure 4. Conceptual model showing the process of soil structural destabilization when the soil dries and the process of structural recovery when soil wets.

Ah horizon BA horizon

Bt horizon Bik horizon

Percolation of rain water

Groundwater

Ponding water

{ Soil swelling Ah ho ■■■■■■■■■■■■■■

BA horizon

Bt horizon Bik horizon

Groundwater ttttt

Air release 3f

Ah horizon

BA horizon

Traped Air bubles

Bt horizon

Bik horizo

Table rise

Groundwater

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