Importance of surface sealing in the erosion of some soils from a mediterranean climate

Poor aggregate stability and high erodibility of soils found in Mediterranean climates are often due to low organic matter content. The low organic matter content is sometimes compensated for by iron oxides that stabilize soil structure. Low water stability of aggregates often determines the propensity for soils to form surface crusts and seals (wet crusts). Seals reduce soil infiltration rate and may increase erosion by increasing runoff. We conducted rainfall simulation experiments on 30 cm×30 cm plots inclined at 9% slope in which we measured soil splash and wash erosion, runoff and splash volume for seventeen California soils. The soils ranged in clay content from 8 to 36%, ESP from <1 to 56%, and organic carbon from 0.4 to 7.0%. Infiltration rate was used as a measure of seal formation. Soils that had 3.1 to 7.0% organic carbon and 2.4 to 10.7% CBD extractable iron plus aluminum did not form seals, infiltration rate remained high throughout the rainfall run, and neither runoff nor erosion occurred. Soils with <20% clay, <3% organic carbon, and <2.0% CBD extractable Fe+Al readily formed seals that reduced infiltration rate and total volume of water infiltrated and increased the volume of runoff water and mass of soil eroded. Initial soil water content was also important in determining crusting and subsequently the volume of runoff and amount of erosion. These data show that the formation of soil seals increases runoff and erosion through the early stages of seal development and that organic matter and iron oxides are critically important factors in determining soil surface stability and erodibility.