How the root zone modifies soil wettability: Model experiments with alfalfa and wheat

To investigate the effect of plants on soil water repellency (SWR), two column experiments with wheat (Triticum aestivum) and alfalfa (Medicago sativa) with a growing period of three months had been carried out under constant and near-natural climatic conditions. Model soils with defined wettability were created by mixing a natural sandy loam subsoil with different proportions of a wettable and a hydrophobized pure quartz sand, resulting in a wettable model soil and three model soils with increasing level of subcritical SWR (initial contact angle CA > 0 degrees and < 90 degrees). Results showed a significant decrease of the mean CA after the experiments compared to the initial CA while the mean CA was constant for plant free columns used as a reference. CA as a function of depth in some cases showed a depth dependent variation with decreased CA at the bottom or as well at top and bottom. The deviation from the initial CA was most pronounced for wheat under constant climatic conditions. Changes in CA could be related to changes in pH, i.e., CA was decreased and pH increased. Subcritical WR at the beginning of the growth period affected significantly the moisture content profiles during the entire growing season as well as plant dry mass production. We expect that plant root exudates of plants widely used for foot production cause directly or indirectly pH-related modifications of the WR level in the root zone dependent on plant species and the ambient climatic conditions.