Determining the depth and rate of soil movement down the soil profile using an environmental tracer: a hillslope scale assessment

Abstract

Context Soil materials can be delivered to depth from both in situ and ex situ materials. Here, we examine a hillslope in an agricultural environment that has been used for cropping and cattle grazing for over 150 years and a parallel area where cattle have been excluded for approximately 20 years. The exclusion area is a shelterbelt and also provides ecological services. Aims To quantify the depth and rate of down profile soil movement using the environmental tracer 137Cs at points along a hillslope profile. Methods 137Cs concentration is measured to bedrock at regular intervals both inside and outside a fenced of ecological services area pre-drought (2015) and post-drought (2021). In Australia. 137Cs is the by-product of nuclear weapons use and testing from1945 to 1972. Therefore, this places an age constraint on any labelled soil. Key results Results show that soil materials can move down the soil profile to reach bedrock at decadal time scales. An important finding is that materials from the surface can reach depths of up to 80 cm near the hillslope crest and up to 2.2 m at the base of the hillslope. Conclusions This demonstrates a relatively rapid translocation of surface material. Implications The method provides the ability to quantify the rapid movement of soil components and demonstrates the potential for deep sequestration of soil organic carbon. The results demonstrate the potential for soil amendments and agrochemicals to be rapidly transported to depth. The findings suggest that cattle exclusion has no impact on the movement of soil materials down the profile over the 20-year exclusion period.