Previous studies about the effect of antecedent moisture content (AMC) on seal formation have shown contradictory results. We hypothesize that this controversy is related to differences in slaking during wetting. The objectives were to analyze the effects of: (i) clay content on aggregate stability and slaking; (ii) clay content and slaking on seal formation and interrill erosion under various wetting rates (WR) and AMC under simulated rain. Aggregate stability was determined on six smectitic soils from Israel with clay content from 80 to 630 g kg−1 In the rain simulator, soils with 230, 410 and 620 g kg−1 clay were prewetted with WR = 1 and 5 mm h−1 to AMC = 0.25 and 0.5 of field capacity (FC), prior to the application of 80 mm of rain. Aggregate stability and slaking by fast WR increased with increase in clay content. In soils with 230 and 410 g kg−1 clay, raindrop impact was enough to disintegrate the aggregates and sealing was not affected by WR and AMC. Conversely, in the soil with 620 g kg−1 clay, seal formation increased with slaking caused by fast wetting. Thus, final infiltration rate of the clay soil with AMC = 0.5 FC and WR = 1 mm h−1 was 11.1 mm h−1 compared with 6.0 mm h−1 in the air‐dry soil (fast wetting by rain). The effects of WR and AMC on soil loss were similar to their effect on runoff but more pronounced. The relation between wetting process and clay content should be considered when predicting soil erosion in smectitic soils.

The objective of this study was to investigate the effect of the interaction between organic matter (OM) content and aggregate size on the mechanisms that degrade the soil structure and on the saturated hydraulic conductivity (Ks). Two samples of sandy loam (Humic Dystrudept) containing 2.5 and 3.5% OM, referred to as low‐OM soil and high‐OM soil, respectively, were collected from adjacent fields. Dry samples were sieved to obtain aggregate sizes of <2, 2 to 4, and 4 to 6 mm. Slaking, swelling, and dispersion values were measured for each soil and aggregate size. The saturated hydraulic conductivity was determined in disturbed soil columns by means of a constant‐head device. For the <2‐ and 2‐ to 4‐mm aggregate sizes, Ks of the high‐OM soil was, in general, significantly higher than that of the low‐OM soil. The average Ks for the entire leaching run in the <2‐mm and 2‐ to 4‐mm aggregate sizes was 7.7 and 183 mm h−1, respectively, for the low‐OM soil and 13 and 412 mm h−1, respectively, for the high‐OM soil. Moreover, there was a significant interaction between aggregate size and OM content in their effects on Ks For the low‐ and high‐OM soils, the slaking values were >93 and <6.7% respectively, and the clay dispersion values in deionized water were >2.9% and <2%, respectively. This suggests that the larger decrease in Ks of the low‐OM soil than in the high‐OM soil during wetting and leaching was mainly a result of more intense aggregate slaking and dispersion in the former soil.

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The objectives of this study were to investigate (i) the effect of soil organic matter (OM) content on the mechanisms that form a seal, and (ii) the OM content and aggregate size interactions in seal properties, infiltration rate (IR), and soil loss. Two samples of sandy loam (Humic Dystrudept) designated low‐OM soil (2.3% by weight OM) and high‐OM soil (3.5% OM) were studied. Aggregate sizes <2, 2 to 4, and 4 to 6 mm of each soil were exposed to 80 mm of simulated rainfall with an intensity of 42 mm h−1 The final IR increased with increasing aggregate size from 4.2 to 5.2 mm h−1 in the low‐OM soil, and from 5.8 to 10.8 mm h−1 in the high‐OM soil. There was a significant interaction between OM content and aggregate size in seal formation and final IR. The low aggregate stability and the high dispersivity of the low‐OM soil allowed the development of a dense and thick crust for all the aggregate sizes. Conversely, the high aggregate stability and the low dispersivity of the high‐OM soil limited the seal formation. Consequently, the IR values were high and differences in IR among the three aggregate sizes in this soil were relatively high. The soil losses increased with increasing aggregate size, ranging from 4.5 × 10−3 to 6.6 × 10−3 kg m−2 mm−1 in the low‐OM soil, and from 1.2 to 3.8 × 10−3 kg m−2 mm−1 in the high‐OM soil. In this case, no significant interaction was found between soil OM content and aggregate size.

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