Soil volumetric changes in natric soils caused by air entrapment followingseasonal ponding and water table rises

Soil volumetric changes have been seldom studied in seasonally ponded soils subjected to periodic water table rises. In the Flooding Pampa of Argentin a the topsoils develop significant swelling and shrinkage, despite their lo w percentages of total and expansible clay. We tested the hypothesis that: (a) the swelling of a Natraquoll and a Natraqualf of this region is caused by the wide change in water contents during pending-drying cycles; and (b) soil swelling is accentuated by the effect of air entrapment ahead of the a dvance of soil wetting fronts. The relationship between the reciprocal of b ulk density (i.e. soil specific volume), v, and water content, theta, was d etermined in the laboratory (clod shrinkage curves) and in the field (repea ted core sampling). Soil clods behaved in accordance to their inherent soil properties, with zero and residual shrinkage (slope n = delta theta/delta theta < 1) in both top horizons, and normal shrinkage (slope n = <delta>the ta/delta theta approximate to 1) throughout the water content range of Bt h orizons. Unlike the clods, in the field the slope, n, was as high as 1.47-1 .48 in top horizons, and 1.93-1.98 in both Bt horizons, showing the occurre nce of abnormal soil swelling processes. Taking into account the narrow vol umetric water content range found in the field (i.e. 0.25 v/v in both Bt ho rizons), this rejects our first proposed hypothesis. Soil air became trappe d ahead of the advance of two field wetting fronts: (a) water table rises f rom depth and (b) surface ponded water. As a result, pore air volume increa sed during soil wetting, and was as high as 0.24-0.34 v/v, and 0.35 v/v at the maximum swelling limit of top and Bt horizons, respectively. Results sh ow that air entrapment caused the swelling or "inflation" of soils, which a grees with our second hypothesis. However, the influence of air entrapment was more pronounced than a simple accentuation of swelling in Bt horizons. Air entrapment caused the whole soil to a depth of about 0.4 m to expand. ( C) 2001 Elsevier Science B.V. All rights reserved.