Using 137Cs and 210Pbex to assess soil redistribution on slopes at different temporal scales

Increasing risk of soil loss as a result of climate change, has generated a need for reliable information on erosion rates at different temporal scales. Use of the fallout radionuclides 137Cs, 210Pbex and 7Be as tracers of sediment mobilization and redistribution makes it possible to obtain estimates of soil redistribution rates within both undisturbed and cultivated landscapes over a range of timescales. Mediterranean landscapes are characterized by a great diversity of physiography and land use, and as a consequence erosion and deposition patterns are highly variable spatially. To document such spatial variability, a slope transect located in the subhumid Pre-Pyrenean mountains (NE Spain) was selected to use 137Cs and 210Pbex to assess medium- and longer-term soil redistribution rates. A total of 23 sectioned soil cores spaced 50 m apart were collected along the slope transect, where 7Be had been previously used to document soil redistribution resulting from an individual storm event. The inventories of both radionuclides varied markedly, between 409 and 6080 Bq m− 2 for 137Cs, and between 0 and 6734 Bq m− 2 for 210Pbex. Estimates of soil redistribution, derived from the 137Cs depth profiles, using appropriate conversion models, show that erosion rates along the transect vary between 2.6 and 31.9 Mg ha− 1 year− 1, and that sedimentation rates vary between 0.2 and 24.5 Mg ha− 1 year− 1. The highest soil losses occur in cultivated fields, within the midslope zone of the transect, while the highest deposition rates are found in tilled fields within the lower part of the transect. Erosion rates from 210Pbex varied widely between 0.1 and 83.7 Mg ha− 1 year− 1 on the lower slope, whereas sedimentation rates ranged between 0.8 and 110 Mg ha− 1 year− 1 also at the bottom slope. The spatial distribution of the radionuclides along the transect reflects the effects of different land use and slope gradient on water erosion. The results obtained confirm the potential for using 137Cs and 210Pbex measurements for assessing soil redistribution on slopes in the Mediterranean environment over different temporal scales.

Research Highlights
► 137Cs and 210Pbexsoil redistribution rates reflect different temporal sensitivity.
► Land use and slope gradient exert important controls on soil erosion.
► Dense forest protects the soil surface from erosion on steeper slopes.
► 137Cs and 210Pbex provide evidences of the complex patterns of soil redistribution.