Comparison of two methodologies used to estimate erosion rates in Mediterranean ecosystems: (137)~Cs and exposed tree roots

摘要

The (137)~Cs deposited in soil and exposed tree roots have been widely applied to estimate medium-term soil erosion rates. However, comparative studies between these methods are scarce. For this purpose, three hillsides in two Mediterranean dehesas (rangeland with disperse tree cover) were selected. Regarding the (137)~Cs technique, a reference site close to the study areas and with similar altitude and rainfall was selected. In order to reduce uncertainties related to the use of point soil profiles, all those collected in an area were combined to form a representative composite profile. The total inventory was 2790 ± 50 Bq/m~2, and the relaxation coefficient indicated it was an undisturbed soil. The radiocaesium inventory in the study areas was 14-23% lower than in the reference area. The erosion rates for (137)~Cs were in the range 20.9-38.1 ha~(-1) y~(-1). The exposed root technique was applied to holm oak trees (age about 90 years), and the erosion rates were in the range 22-341 ha~(-1) y~(-1) The ratio between exposed root and (137)~Cs techniques was 1.02 ± 0.11 (S.D.) within the range 0.89-12. Both methods produced very similar results equally with respect to the mean erosion rate as well as the relative difference between the hillslope sections, i.e. displaying the same spatial variation in the study areas. As the accounting time for these two techniques is different, 50 and 90 y for (137)~Cs and exposed roots respectively, results suggest that no change in mid-term erosion rates was implied for these areas for almost a century. The use of (137)~Cs and exposed roots methodology for the determination of mean erosion rates can be reproduced in other ecosystems, but a careful selection of the reference site for (137)~Cs is essential.