Changes in structural stability with soil surface crusting: Consequences for erodibility estimation
Changements de stabilité structurale avec l'encroûtement de la surface du sol : conséquences pour l'estimation de l'érodibilité.
Résumé
Soil erodibility is a key parameter in soil erosion prediction. Stability tests are used to assess erodibility. Because samples used in stability tests are usually taken in the plough layer, estimation of erodibility may not be valid for the soil surface material. Through soil surface degradation under rain impact, soil material at the surface undergoes numerous changes that lead to crust formation. Biological activity is another factor that may induce seasonal change in structural stability. From a loamy soil material, structural crusts and sedimentary crusts were prepared using a laboratory rainfall simulator. Nutrients were added to part of the samples to stimulate biological activity. The structural stability of crusts and seedbed were compared. Without adding nutrients, the initial soil, the seedbed and the structural crust had similar structural stabilities, with a mean weight diameter of 500 mu m for the slow-wetting test and 250 mu m for the fast-wetting test. By comparison, the sedimentary crust was significantly less stable, with a mean weight diameter of 100 mu m for both tests. The addition of nutrients altered this ranking: the seedbed was more stable than the structural crust and the sedimentary crust remained the least stable. Considering that structural stability can vary by more than 50% depending on the crusting stage, erodibility changes are expected to be of the same order of magnitude. In consequence, estimation of soil erodibility can be significantly improved by applying stability tests on the exact soil material that undergoes erosion processes, i.e. the soil at the surface.