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a Unité de Science du Sol, INRA, 78026 Versailles, France
b Unité de Science du Sol, Service d'Etudes des Sols et de la Carte Pédologique de France, INRA, 45160 Olivet, France
chenu{at}versailles.inra.fr
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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. Water drop penetration time increased with C contents from 1 to 32 s and was very heterogeneous among individual aggregates from a given soil. The contact angle of water on the clay fraction increased linearly with the C content
. This change in clay wettability could partly explain the higher water stability of soils rich in C.
Abbreviations: MWD, mean weight diameter • POM, particulate organic matter • SOM, soil organic matter • WDPT, water drop penetration time
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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The main processes by which soils aggregates are disrupted upon rainfall are (i) slaking, that is, the disruption of aggregates due to the forces exerted by compressed air entrapped during rewetting; (ii) differential swelling of clays; (iii) mechanical dispersion due to the kinetic energy of rain drops; and (iv) physicochemical dispersion (Le Bissonnais, 1996). Soil organic matter (SOM) is assumed to stabilize aggregates against these disruptive processes by two major actions. First, organic matter increases the cohesion of aggregates, through the binding of mineral particles by organic polymers, or through the physical enmeshment of particles by fine roots or fungi (Tisdall and Oades, 1982; Chenu and Guérif, 1991; Dorioz et al., 1993; Chenu et al., 1994). Second, organic matter may decrease the wettability of aggregates, slowing their rates of wetting and thus the extent of slaking (Monnier, 1965; Chassin, 1979; Sullivan, 1990). The second mechanism has received far less attention than the first one.
In some soils, organic substances induce very severe water repellency, especially in sandy soils (Bond, 1969; Wallis and Horne, 1992) but also in heavy textured ones (MacGhie and Posner, 1980). Strongly hydrophobic organic coatings can prevent water from entering the aggregates or the horizon, restrict infiltration, and cause intense surface runoff (Wallis and Horne, 1992).
Apart from the case of strongly hydrophobic soils, SOM may impart partial repellency to soil aggregates and thereby contribute to their stability. Haynes and Swift (1990) reported that dried aggregates from a pasture soil rich in organic matter, were more stable than field moist ones, and that it was the opposite for arable soils with low C content. The slower rewetting of pasture aggregates as compared to arable counterparts was ascribed to hydrophobic properties of SOM (Sullivan, 1990). Furthermore, Capriel et al. (1990) reported good correlations between the aliphatic fraction of a soil extracted with supercritical hexane, and its aggregate stability.
Several organic fractions were shown to be responsible for the hydrophobicity of soils or to be partly hydrophobic: humic acids (Roberts and Carbon, 1972; Tschapek et al., 1973; Giovannini et al., 1983; Jouany and Chassin, 1987b), aliphatic fractions (MacGhie and Posner, 1980; Ma'shum et al., 1988), or plant litter debris (MacGhie and Posner, 1981). Using model organic molecules and reference clays it was shown that organic substances can render clays hydrophobic (Jouany and Chassin, 1987a; Jañczuk et al., 1990; Jouany, 1991). However, it has not been established to which extent natural clay–organic matter associations have hydrophobic properties, nor whether they contribute to soil aggregate stability.
In southwest France, thick humic soils developed on loams have been deforested and converted to intensive arable cropping during the last century. This conversion led to a rapid decrease of the organic pool (Arrouays and Pélissier, 1994) and to an associated decrease in aggregate stability, infiltration and increase of sealing (Le Bissonnais and Arrouays, 1997). These soils then provide a unique sequence of soils with the same texture and mineralogy but differing organic matter contents and physical properties.
The present work aimed (i) to analyze in this soil sequence the possible contribution of SOM to aggregate stability by decreasing their wettability and (ii) to evaluate the role of clay-associated organic matter in soil aggregate wettability and water stability.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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The soil clods, while moist, were broken apart by hand into aggregates <10 mm by following the planes of least resistance, being careful to break them in traction rather than in compression, and then air dried. The 3- to 5-mm aggregates were then separated by dry sieving, and coarse plant debris retained on the 3 mm sieve were discarded. The 3- to 5-mm aggregates represented 20 to 50% of the mass of the soil sieved. The C content of the 3- to 5-mm aggregates was measured by dry combustion and expressed on a 105°C oven-dry weight basis.
Aggregate Stability
Aggregate stability was measured according to Le Bissonnais (1996) on 3- to 5-mm air-dried aggregates. The method separates between the various mechanisms of breakdown: slaking due to fast wetting (Treatment 1), microcracking due to slow wetting (Treatment 2) and mechanical breakdown of prewetted aggregates (Treatment 3) (Amezketa et al., 1996). Treatment 1: 5 g of aggregates were immersed in deionized water for 10 min. After sucking off the water with a pipette, the soil material was gently transferred on a 0.05-mm sieve previously immersed in ethanol. The fraction <50 µm was recovered after gentle sieving and oven dried. The fraction >0.05 mm was oven dried and its size distribution was measured by dry sieving with sieves of 2, 1, 0.5, 0.2, 0.1, and 0.05 mm. Treatment 2: the aggregates were capillary rewetted on a tension table at 3-cm tension for 30 min before immersion in water. The procedure was then continued as above. Treatment 3: the aggregates were rewetted with ethanol, which was nondestructive. The ethanol was sucked off with a pipette, 200 cm3 of deionized water were added and the flask was agitated end over end 20 times. The procedure was then pursued as above. The results are expressed as the resulting fragment size distribution and as the mean weight diameter (MWD), which is the sum of the mass fraction remaining on each sieve after sieving, multiplied by the mean aperture of the adjacent sieves. Five replicates were performed for each treatment. Calculated MWD values range between 0.025 and 3.5 mm for the initial size of aggregates and mesh of sieves used.
Wettability Measurement on Aggregates
The wettability of 3- to 5-mm aggregates was assessed with the water drop penetration time (WDPT) method of Letey (1969). Results obtained by this method are fairly well correlated with other methods to determine the repellency of soils (King, 1981). It is better suited to soils with low degrees of repellency, than the Molarity of Ethanol Droplet method (King, 1981). It is simple, rapid and requires only small amounts of samples.
De-ionized water drops (0.1 ± 0.005 mL) were deposited with a micro-syringe on the surface of individual air-dried aggregates (3–5 mm diam.), and the time required for the drop to penetrate the aggregate was recorded. Times less or equal to 1 s were given a value of 1 s. Measurements were replicated on 100 to 200 individual aggregates for each soil. We performed a one-way variance analysis (ANOVA) with soil as the main effect. Then we tested the least significant difference between all WDTP mean values, one to each other, or by grouping them into two groups of organic C contents on the basis of a threshold value of 15 g kg-1. This threshold value comes from a previous study (Le Bissonnais and Arrouays, 1997), which indicated a threshold effect for infiltration.
Extraction and Wettability Measurement on the Clay Fraction of Soils
The clay fraction (<2 µm) of soils was extracted without organic matter destruction, by mechanical dispersion of the soil and sedimentation according to Balesdent et al. (1991). The C content of the clay fractions was determined by dry combustion and expressed on a 105°C oven-dry weight basis. Total C could be equated with organic carbon (OC) because the soils contained no carbonates. For one soil for each terrace, an aliquot of the clay fraction was treated with H2O2 to remove the organic matter. The mineralogy of the clay fraction of these samples was determined by x-ray diffractometry using conventional methods.
Oriented deposits were prepared by allowing a 1 mL drop of a 20 g L-1 suspension of the clay fraction to evaporate on glass slides and to dry over silica gel. For two soils, particulate organic matter >50 µm (POM) was also separated (Balesdent et al., 1991; Besnard et al. 1996), air-dried, finely ground and pressed into pellets (Jouany and Chassin, 1987b). Contact angles of water were measured with a Ramé Hart telegoniometer, by depositing de-ionized water drops on the clay deposits or on the POM pellets with a micro-syringe according to Chassin et al. (1986). Contact angles values are an average of 25 determinations.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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15 g kg-1).
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Characteristics and Wettability of the Clay and Particulate Organic Matter Fractions
The fractions <2 µm of all the soils from all the four terraces exhibited the same mineralogy. It was a mixture of predominantly illites, kaolinite and chlorites, with some quartz (results not shown). There was no smectites. The clay fractions separated from the different soil samples exhibited a wide range of C contents. The C content of <2-µm fractions increased proportionally to the variations of organic carbon content of the bulk soils
(Fig. 3)
and was always higher, as generally found for other silty soils (Balesdent et al., 1991).
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> 90°). Contact angles increased with the C content of the fraction. The curve was fitted with a linear regression
, and the contact angle of water on clay without organic matter was then extrapolated to be of
. We could not measure contact angles directly on clays in which the organic matter had been oxidized by H2O2 because the clays then lost their cohesion and it was then not possible to make deposits coherent enough to measure contact angles. Among the <2-µm fractions from the different soils the nature of exchangeable cations 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 Al saturation to Ca saturation, and poorly crystallized Al compounds decreased. However, the nature of exchangeable cations or poorly crystallized Al compounds were not significantly correlated with the contact angle of the clay fractions (Table 3).
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and that from the forest soils
. These values are close to those for the clay fractions from the forest soil (L0a) (Fig 4). |
Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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The surface energy of clay minerals depends on their mineralogy and on the nature of their exchangeable cations (Jouany and Chassin, 1987b; Jouany, 1991). Coatings of amorphous Fe and Al compounds decrease the wettability of clay minerals (Le Souder, 1990). In the present study, all the clay fractions had the same mineralogy, and the nature of exchangeable cations or the presence of noncrystalline Al were not significantly related to hydrophobicity (Table 3). Hence, increased hydrophobicity of <2 µm clay fractions was mostly due to their organic constituents.
The observation of <2-µm fractions from other silty soils with electron microscopy (Robert and Chenu, 1992), as well as preliminary observations of the clay fractions from this study have shown that the organic constituents are small plant or microbial debris, bacteria, free amorphous organic matter and organic matter strongly associated with clay particles, that is, clay coatings. Increasing hydrophobicity with increasing SOM content in the clay fractions from this study may thus correspond to (i) an increasing proportion of organic particles with hydrophobic character, among mineral hydrophilic ones, (ii) or an increasing coverage of clay mineral particles by hydrophobic organic coatings. With cultivation, the organic inputs to soils change from forest vegetation remnants to maize. Changes in the nature of SOM in the <2-µm fractions are then expected and could also affect the wettability of the clay fractions.
Plant debris could also reduce the wettability of soil aggregates. We found that particulate organic matter were partly hydrophobic. Several authors demonstrated the hydrophobicity of plant debris (MacGhie and Posner, 1980; MacGhie and Posner, 1981; Valat et al., 1991; Franco et al., 1995) and MacGhie and Posner (1981) showed that material derived from cereal crop aerial parts was more wettable than that derived from forest or pasture.
In the soils of the present study organic matter contributed 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.
Relation between Wettability and Aggregate Stability
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 between MWD values after slow and fast wetting was calculated (Fig. 5)
. When cultivated soils only were considered, this difference increased with the C content of aggregates
. 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 (L0a) could be ascribed 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 maximum MWD value in both cases (i.e., 3.19 for fast wetting and 3.28 for slow wetting).
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 | (1) |
is the surface tension of water (dynes cm-1) is the contact angle of water (°)
The rate of water entry into aggregate pores is expressed by Poiseuille's law
 | (2) |
is the viscosity of water (P)
Combining Eq. [1] and [2] gives
 | (3) |
An increase of the the contact angle of water from 16 to 58°, as shown in this study makes cos and thus the rate of water entry into aggregates to decrease by 45%, which would reduce slaking. In this soil sequence, we found that the rate of water entry into agregates (WDPT) increased with C contents. High WDPT may be ascribed to changes in the contact angle of water on pore surfaces, to the presence of slightly repellent particulate organic matter, or to changes in the pore diameters. The latter were not investigated in the present study. An increased hydrophobicity of the clay fractions should also reduce the extent of clay swelling, and thereby reduce the extent of aggregate disruption by microfissuration.
On the other hand, the resistance of aggregates to mechanical disruption after rewetting with ethanol (Treatment 3) was also related to SOM contents. It implies that organic matter also acted in this sequence by increasing the internal cohesion of aggregates. This would also increase the resistance of aggregates to slaking and to differential swelling of clays.
Difference of Properties between the Clay Fraction and the Aggregates
The water stability of aggregates and their water drop penetration time generally increased with the hydrophobicity of the clay fractions (Fig. 6)
. However, there was no significant linear relationship between clays wettability and aggregates stability or WDPT. This may be explained by several processes. First, soil fractions other than <2 µm contribute to aggregate properties. For example, POM had contact angles of about 55 to 58° and will contribute to rates of aggregate rewetting. However, POM represents a small proportion of the soil mass: about 6% in the forest L0a soil and 1 to 5% in the cultivated ones (Besnard et al., 1996).
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We found that aggregate stability and WDPT were very variable among individual millimetric aggregates (Fig. 2 and 3). The samples consisted in populations of 3- to 5-mm aggregates with very different water stabilities and wettabilities. As shown on Fig. 1, 
90% aggregates from the forest soil did not slake and remained in the >2-mm class. After 7, 35, and 100 yr of cultivation respectively, only 35, 3, and 1% of the soil mass were aggregates that resisted slaking. Similarly the proportion of aggregates having a WDPT > 10 s was 61% for the soil under forest (L0a) and it was of 17% after 7 yr of cultivation, 15% after 35 yr of cultivation and 0% after 100 yr of cultivation. As the C content decreased in the sequence, the properties of the soil did not change homogeneously, but rather the proportion of individual aggregates having a high water stability and a high WDPT decreased.
In silty cultivated soils from the Paris basin, the distribution of organic matter was found to be heterogeneous as stable aggregates were richer in C and POM than unstable aggregates (Puget et al., 1995; Puget, 1997). We hypothesize that organic matter has an uneven spatial distribution in these soils also and that this explains pro-parte the formation of aggregates with such a range of stability and water uptake rates. With cropping and tillage, SOM contents generally decrease due to increased SOM mineralization (Balesdent et al., 2000). Aggregate stability under different land uses may be viewed, as suggested by Haynes and Swift (1990), as changes in the proportion of aggregates with enough C to be stable.
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Conclusion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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Aggregate properties were very heterogeneous among individual aggregates and were not always related to the properties of the <2-µm fraction extracted from bulk soil. This shows that the spatial distribution of organic matter at the scale of individual aggregates is of major importance for soil physical properties and should be analyzed.Hairsine Rose 1991
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ACKNOWLEDGMENTS |
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Received for publication July 27, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResultsDiscussionConclusionREFERENCES |
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