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+ The letters A, G, H, L stand for the initials of the terrace name. The following number in subscript stands for duration of cultivation in years. The subscript a stands for the cropped layer, and the subscript b stands for the 0.3- to 0.5-m soil layer. + CEC, cation-exchange capacity; ND, not determined. Table 3. Physicochemical characteristics of a selection of clay fractions. r? is the regression coefficient of variable with contact angle value. Non-crystalline Al and Fe were determined after oxalate extraction according to McKeague and Day (1996). In the soils of the present study organic matter con- tributed to decrease the wettability of aggregates in two ways: by lowering the wettability of the clay minerals and by the presence of particulate organic matter. Slaking of soil aggregates is due to the pressure devel- oped by air entrapped in pores upon sudden wetting. A reduced rate of entry of water into aggregates allows air to escape and minimizes slaking (Monnier, 1965). When water is drawn into aggregate pores by capillary action, the capillary pressure P is given by

Table 3 + The letters A, G, H, L stand for the initials of the terrace name. The following number in subscript stands for duration of cultivation in years. The subscript a stands for the cropped layer, and the subscript b stands for the 0.3- to 0.5-m soil layer. + CEC, cation-exchange capacity; ND, not determined. Table 3. Physicochemical characteristics of a selection of clay fractions. r? is the regression coefficient of variable with contact angle value. Non-crystalline Al and Fe were determined after oxalate extraction according to McKeague and Day (1996). In the soils of the present study organic matter con- tributed to decrease the wettability of aggregates in two ways: by lowering the wettability of the clay minerals and by the presence of particulate organic matter. Slaking of soil aggregates is due to the pressure devel- oped by air entrapped in pores upon sudden wetting. A reduced rate of entry of water into aggregates allows air to escape and minimizes slaking (Monnier, 1965). When water is drawn into aggregate pores by capillary action, the capillary pressure P is given by

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+ The letters A, G, H, L stand for the initials of the terrace name. The following number in subscript stands for duration of cultivation in years. The subscript a stands for the cropped layer, and the subscript b stands for the 0.3- to 0.5-m soil layer. + CEC, cation-exchange capacity; ND, not determined. Table 1. Soil location, cropping history, and main characteristics.
+ The letters A, G, H, L stand for the initials of the terrace name. The following number in subscript stands for duration of cultivation in years. The subscript a stands for the cropped layer, and the subscript b stands for the 0.3- to 0.5-m soil layer. + CEC, cation-exchange capacity; ND, not determined. Table 1. Soil location, cropping history, and main characteristics.
ig. 1. Aggregates size distributions after the water stability tests for four of the studied soils having decreasing C contents in relation to tim of cultivation: (a) Ly,, forest; (b) L;,, 7 yr cropping; (c) L3s,, 35 yr cropping; and (d) Gyoo,, 100 yr cropping.
ig. 1. Aggregates size distributions after the water stability tests for four of the studied soils having decreasing C contents in relation to tim of cultivation: (a) Ly,, forest; (b) L;,, 7 yr cropping; (c) L3s,, 35 yr cropping; and (d) Gyoo,, 100 yr cropping.
+ The letters A, G, H, L stand for the initials of the terrace name. The following number in subscript stands for duration of cultivation in years. The subscript a stands for the cropped layer, and the subscript b stands for the 0.3- to 0.5-m soil layer. Table 2. Mean weight diameters (MWD) after the water stability tests (mean of five replicates); water drop penetration times | aggregates (WDPT), and the determination coefficient of their linear correlation with soil C content. Letters in the WDPT colum: stand for significantly different values (P < 0.05).
+ The letters A, G, H, L stand for the initials of the terrace name. The following number in subscript stands for duration of cultivation in years. The subscript a stands for the cropped layer, and the subscript b stands for the 0.3- to 0.5-m soil layer. Table 2. Mean weight diameters (MWD) after the water stability tests (mean of five replicates); water drop penetration times | aggregates (WDPT), and the determination coefficient of their linear correlation with soil C content. Letters in the WDPT colum: stand for significantly different values (P < 0.05).
Fig. 2. Frequency distribution of water drop penetration time (WDPT) on individual aggregates of Lacadee soils under forest, and cultivated for 7 and 35 yr: (a) soils from Lacadee terraces and (b) soils from other terraces.
Fig. 2. Frequency distribution of water drop penetration time (WDPT) on individual aggregates of Lacadee soils under forest, and cultivated for 7 and 35 yr: (a) soils from Lacadee terraces and (b) soils from other terraces.
Fig. 3. Carbon contents of <2-m fractions ys. the C content of soil. from the different soils the nature of exchangeable cat- ions and the abundance of poorly crystallized Al or Fe compounds differed (Table 3). In the soil under forest the cation exchange capacity of the bulk soil and of the clay fraction was partly saturated by aluminium (Tables 1 and 3). With cultivation and liming of the soil, the clay fraction shifted from AI saturation to Ca saturation, and poorly crystallized Al compounds decreased. How- ever, the nature of exchangeable cations or poorly crys- tallized Al compounds were not significantly correlated with the contact angle of the clay fractions (Table 3).
Fig. 3. Carbon contents of <2-m fractions ys. the C content of soil. from the different soils the nature of exchangeable cat- ions and the abundance of poorly crystallized Al or Fe compounds differed (Table 3). In the soil under forest the cation exchange capacity of the bulk soil and of the clay fraction was partly saturated by aluminium (Tables 1 and 3). With cultivation and liming of the soil, the clay fraction shifted from AI saturation to Ca saturation, and poorly crystallized Al compounds decreased. How- ever, the nature of exchangeable cations or poorly crys- tallized Al compounds were not significantly correlated with the contact angle of the clay fractions (Table 3).
Fig. 4. Contact angles of water on the clay fractions vs. their C content. Bars are standard deviations.
Fig. 4. Contact angles of water on the clay fractions vs. their C content. Bars are standard deviations.
Fig. 5. Difference between mean weight diameter (MWD) after fast and slow wetting as affected by C content. Among the three tests, the MWD after slow and rapid wetting were the best correlated with the C content of aggregates. Differential swelling of clay and slaking are responsible for aggregate breakdown in these tests. In order to isolate the effect of slaking the difference be- tween MWD values after slow and fast wetting was calculated (Fig. 5). When cultivated soils only were con- sidered, this difference increased with the C content of aggregates (r? = 0.57). Slaking of air-dried aggregates was thus C content-dependent for cultivated soils. The very small difference between MWD after slow or fast wetting observed for the forest soil (Lo,) could be as- cribed to the very high water stability of this soil (Fig. 1): nearly all aggregates were water-stable to either treatment, and the soil was thus given nearly the maxi- mum MWD value in both cases (i.e., 3.19 for fast wetting and 3.28 for slow wetting).
Fig. 5. Difference between mean weight diameter (MWD) after fast and slow wetting as affected by C content. Among the three tests, the MWD after slow and rapid wetting were the best correlated with the C content of aggregates. Differential swelling of clay and slaking are responsible for aggregate breakdown in these tests. In order to isolate the effect of slaking the difference be- tween MWD values after slow and fast wetting was calculated (Fig. 5). When cultivated soils only were con- sidered, this difference increased with the C content of aggregates (r? = 0.57). Slaking of air-dried aggregates was thus C content-dependent for cultivated soils. The very small difference between MWD after slow or fast wetting observed for the forest soil (Lo,) could be as- cribed to the very high water stability of this soil (Fig. 1): nearly all aggregates were water-stable to either treatment, and the soil was thus given nearly the maxi- mum MWD value in both cases (i.e., 3.19 for fast wetting and 3.28 for slow wetting).
Fig. 6. Relation between the wettability of the clay fraction (expressed by the contact angle value) and the stablity and wettability of aggregates. Bars are standard deviations.
Fig. 6. Relation between the wettability of the clay fraction (expressed by the contact angle value) and the stablity and wettability of aggregates. Bars are standard deviations.
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