Measuring fallout plutonium activities in soils to trace the fate of soil organic matter in arable land, Republic of South Africa

The Free State Province comprises one of the major agricultural regions in the Republic of South Africa. Here, the loss of soil organic matter (SOM) from arable land bears the risk to pose a serious threat to regional crop production yields. For agricultural land located within the Free State Province, previous studies have found a strong relationship between (decreasing) SOM contents and the total time period soils were subject to cropping practices. While wind erosion and mineralisation processes have been identified to be the dominant factors causing this pattern, their individual contribution to SOM loss remained difficult to quantify. Datasets obtained from monitoring stations measuring soil redistribution by wind rarely cover timespans exceeding a few years of observation. To overcome this problem, we measure fallout 239+240Pu inventories in topsoils sampled from agricultural plots with different cropping histories. Plutonium inventories are frequently quantified to infer fluvial soil redistribution patterns and rates during the Anthropocene, but their application to investigate wind erosion is still at an early stage of development. The approach we follow allows us to identify strong correlations between SOM loss and decreasing radionuclide inventories during the second half of the 20th century. Furthermore, cropped soils lost about 50% of their initial 239+240Pu, indicating that the decrease of SOM contents over time is predominantly caused by wind erosion. Our results, which are in good agreement with 137Cs data, underscore the necessity to tackle unsustainable cropping practices in the Republic of South Africa and in comparable settings on Earth.