Using 137Cs and 210Pbex measurements to explore the effectiveness of soil conservation measures in semi-arid lands: a case study in the Kouhin region of Iran

Abstract

One of the most fragile agro-ecosystems in Iran is represented by dry farming lands on steep hillslopes that occupy ca. 54% of the national agricultural lands. Therefore, in order to reduce loss of fertile soil from these lands, it is important to apply effective soil conservation strategies. This study used cesium-137 (137Cs) and excess lead-210 (210Pbex) measurements to assess the effectiveness of soil conservation practices in controlling soil erosion in Kouhin, Qazvin Province of Iran. Soil samples were collected from two adjacent hillslopes with and without conservation agriculture practices. The managed site benefited from conservation practices, including controlled grazing, terraces, and contour farming for 50 years. The unmanaged site was under cultivation without agricultural conservation practices. At the managed site, both 137Cs and 210Pbex activities were measured. Only 137Cs activity was measured for the unmanaged site. Conservation practices performed at the managed site were effective in minimizing soil erosion, i.e., net soil erosion rates provided by 137Cs measurements were only 4.6 Mg ha−1 year−1 at the managed site, compared to 12 Mg ha−1 year−1 for the unmanaged site. By using the 210Pbex approach, net soil loss and sediment delivery ratio in the managed site were 12.3 Mg ha−1 year−1 and 60%, respectively. Soil redistribution magnitudes established from 137Cs inventories are associated with a period extending from 1963 to the time of sampling, while those obtained from 210Pbex measurements are related to the past 100 years. However, 210Pbex measurements are expected to be more sensitive to erosive events that occurred during the last 15 to 20 years and this could reflect the higher estimate of erosion rate provided by this radionuclide for the managed site. The increasing trend seems to suggest a possibility that the higher soil losses estimated by the 210Pbex measurements are a result of increasing climate variability during the last two decades. However, further investigation would be needed to confirm this hypothesis.