Organic matter influence on clay wettability and soil aggregate stability

Soil organic matter is thought to increase aggregate stability by lowering the wettability and increasing the cohesion of aggregates. In southwest France, thick humic loamy soils (Vermic Haplubrepts) have been intensively cropped for 40 yr, decreasing the soil organic pool and lowering the soil agregate stability. This study assessed (i) the contribution of organic matter to aggregate stability by decreasing aggregate wettability and (ii) the specific role of clay-associated organic matter. Soil samples with a C content of 4 to 53 g kg(-1) were sampled and soil aggregate stability was measured. Aggregate wettability was assessed by measuring water drop penetration times on individual 3- to 5-mm aggregates. The <2-mu m fractions were extracted without organic matter destruction and their wettability was determined by measuring contact angles of water on clay deposits. Aggregate stability against slaking was correlated to soil C content (r(2) = 0.71 for fast wetting). 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 (r(2) = 0.86). This change in clay wettability could partly explain the higher water stability of soils rich in C.