Pore shape and organic compounds drive major changes in the hydrological characteristics of agricultural soils

A small increase in soil organic matter (SOM) content can change soil hydrological properties from a completely wettable to a partially water-repellent state. Although considerable research describes hydrophobic compounds as a primary driver of this shift, the influence of pore shape has only been considered in a few studies and none of these has emphasized the role of different carbon compounds. Using a capillary bundle model of non-cylindrical (wavy) capillaries, we described measured hydrological properties of five agricultural soils that have a small degree of water repellency and textures ranging from coarse sand to heavy clay. To isolate the influence of SOM, it was removed by combustion to provide an SOM-free treatment. Water and methanol sorptivities quantified infiltration rates and soil-water wetting angles in packed soil cores. Different cores were sectioned to measure wetting profiles and calculate diffusivity. The results from natural soils were supplemented by measurements carried out on model soils' consisting of quartz particles (50-200 mu m) with four different hydrophobic states. Soil organic matter removal increased water sorptivity from about 60% for a coarse sandy soil (Haplic Arenosol) to about 290% for a heavy clay soil (Haplic Leptosol), corresponding to a decreased apparent wetting angle of 20-30 degrees. Application of the wavy pore model suggests that the apparent wetting angle resulting from SOM removal can be several times smaller than its Young value. Generally, SOM removal increased water diffusivity values by one to two orders of magnitudes. The SOM components having the greatest impact on contact angle were hexanedioic acid and heneicosanoic acid (both hydrophilic) and docosane (hydrophobic).