Using 137Cs and 210Pbex to assess soil redistribution at different temporal scales along with lithogenic radionuclides to evaluate contrasted watersheds in the Uruguayan Pampa grassland

Abstract

The use of environmental radionuclides in soil monitoring has made it possible to establish erosion rates. The combined use of 137Cs and 210Pbex is a common approach for determining mean annual erosion rates retrospectively in two different time windows; 137Cs in the past 50–60 years whereas 210Pbex provide a longer period up to 100 years. However, these radionuclides have contrasting sensitive behaviors, 210Pbex is able to be more sensitive to recent redistribution events of its last period of disintegration (22.3 yr) than 137Cs, being more sensitive to the past erosion and depositional events.This work studies soil redistribution rates at different temporal scales and the variability of lithogenic radionuclides by contrasting two watersheds with different land uses. The watersheds are Arroyo del Arbolito (AAW) and Cañada del Horno (CHW), both located near the Baygorria hydroelectric dam reservoir in Uruguay. Erosion rates obtained with 137Cs and 210Pbex using the MODERN conversion model for both watersheds showed that there are no significant differences for 210Pbex estimates, which suggests that the land management practices for both watersheds in the last twenty years do not differ and the erosion rates remained the same as the earlier period. However, significant differences were found for erosion estimated by using 137Cs in CHW vs AAW (p ≤ 0.05) and cultivated vs. natural pasture (p ≤ 0.001), which indicates that the main erosion events occurred further in the past. Furthermore, these changes in erosion estimated using both isotopes coincide with changes in land use and land management, from tillage-based practices in the past to the predominance of no-tillage since the early 2000 s. This change in soil management resulted in a reduction of approximately 50% in soil loss in those cultivated areas since 1965. In AAW, 51.7% of the sampled points exceeded the tolerable erosion rate (7 Mg.ha−1.yr−1) estimated using 137Cs, whereas for 210Pbex, 37% of the sampled points exceeded this limit. For CHW, 63.6% of the sampled points exceeded the tolerable erosion limit (9 Mg.ha−1.yr−1) estimated by using 137Cs, whereas for 210Pbex, 34.2% exceeded this limit. Our results suggest that the variation of lithogenic radionuclides by erosion is dependent on soil type and erosion magnitude. This study, which describes the use of two fallout soil radiotracers, provides new insights into the impact of land use conversion on soil degradation in the Pampas grassland ecosystem.