Plutonium concentrations link soil organic matter decline to wind erosion in ploughed soils of South Africa

Loss of soil organic matter (SOM) from arable land poses a serious threat to soil fertility and crop yields, and it thwarts efforts to conserve soils as carbon sinks to mitigate global warming. Wind erosion can be a major factor in the redistribution of soil fines including SOM, but assessments of its impact have typically been limited by short observation periods of a few years at most. Longer time frames, extending back to the mid-20th century, may however be probed using the concentrations of radionuclides that were globally distributed by nuclear weapon tests conducted during the 1950s and early 1960s. The basic concept is that differences in fallout radionuclide (FRN) activities between undisturbed and arable soils can be used to infer soil particle redistribution. In the present work, we have measured activities of 137Cs and 239+240Pu in soils from three agricultural regions of the plains of the South African Highveld. The three regions represent distinct agroecosystems, and within each region the temporal length of cultivation varies from 0 (i.e. native grassland) to almost 100 years. Wind erosion has previously been shown to play a dominant role in soil particle loss from agricultural sites in the Highveld, and the level plots we investigate here did not show any evidence of fluvial erosion. Hence, we interpret the fate of soil fines, including SOM, to be governed by wind erosion. For the cultivated soils, radionuclide activities are found to be less than in adjacent native grassland, and the magnitude of the reduction is strongly correlated with the duration of cultivation. Specifically, the original inventories of both 137Cs and 239+240Pu are approximately halved after ∼20–40 years of cropping. The initial rate loss relative to the undisturbed soils is, however, considerably higher, with ∼6%yr-1 recorded during the first year after native grassland is converted to arable land. We correlate our radionuclide data with previously published SOM contents from the same sampled material and find that the radionuclides are an excellent indicator of SOM decline at the sites we investigate. We conclude that wind erosion can exert a dominant control on SOM loss in arable land of South Africa.