137Cs-based analysis of soil redistribution in the Integrated Watershed Management intervention area in northern Ethiopia

Abstract

Spatial patterns of trace isotope cesium-137 (137Cs) can be strongly correlated with soil movement in a landscape, thereby allowing assessments of soil erosion and deposition processes, based on these patterns. Our study assessed how soil organic carbon (SOC) stocks were related to the redistribution of 137Cs in prevalent forest and agricultural land-use systems in a north Ethiopian watershed undergoing land rehabilitation. Composite soil samples were collected from 0 to 5, 5–10, 10–20, and 20–30 cm layers of exclosures, croplands, rangelands, and bare lands at different slope positions. Reference samples were collected from a vegetated hilltop with no anthropogenic soil disturbance history. The use of 137Cs analysis was not effective in revealing SOC dynamics in croplands, which was probably a result of soil tillage and generally low SOC contents. Otherwise, 137Cs tracing was a suitable method for the soil redistribution assessment in the tropical environment, owing to sufficient amount of the isotope detected following the original fallout. Comparisons of 137Cs contents between the reference and sampling sites revealed a gap in 137Cs inventories in 87% of all locations, thereby indicating severe erosion, especially on the backslopes. Some of the examined samples, mostly from rangelands and exclosures, exhibited 137Cs activities that were higher than the reference, signifying these sites as depositional areas for soil material eroded elsewhere. Soil movements indicated by 137Cs profiles, were particularly substantial in rangelands, where 137Cs stock varied significantly according to the slope positions and soil depths. Notably, significant correlations between SOC stocks and 137Cs levels in exclosures suggested that both the prevention of soil loss and the capture of soil eroded from degraded sites fortified SOC accumulation. This suggests that not only the increase in vegetation cover but also the soil deposition was an attribute of the land rehabilitation process.