|Van Pelt, Robert - Scott|
Submitted to: World Congress of Soil Science
Publication Type: Proceedings
Publication Acceptance Date: 5/30/2002
Publication Date: 7/15/2002
Interpretive Summary: Wind erosion robs the soil of essential plant nutrients and water-holding capacity and negatively affects soil productivity. The release of radioactive isotopes from atmospheric testing of nuclear weapons during the 1950's and 1960's has created a convenient way of measuring soil redistribution that occurred during the last 40 years. Many of these isotopes, particularly 137Cesium (137Cs), are strongly adsorbed into the finer soil particles and the fallout resulting from the nuclear testing is contained in the upper layers of the soil. The activity of 137Cs in a soil profile relative to that from an uneroded reference site has been used by water erosion researchers to estimate long-term erosion and deposition rates. Models of different forms have been developed to equate changes in 137Cs activity with soil loss and accretion in a given soil profile. Compared to water erosion, wind erosion is a relatively selective process that winnows and removes the finer soil particles and leaves the coarser particles behind. This study investigated whether or not wind erosion and deposition estimates based on 137Cs activities would agree with measured erosion and deposition. We found that 137Cs activities tended to overestimate long-term rates of soil loss and either under- or over- estimated long-term deposition rates, depending on the nature of the depositional environment. The results indicate that models having a particle size correction factor are more appropriate for wind erosion and deposition estimates than simple proportional or mass balance type models.
Technical Abstract: Wind erosion has degraded over one-half billion hectares of land worldwide. 137Cs has been used as a tracer to study long-term water erosion and sedimentation trends and, to a lesser extent, it has been used as an idealized tracer to study wind erosion rates as well. These studies assume that the decline in 137Cs activity for a potentially eroded soil relative to that for a soil from an uneroded soil is proportional to soil loss. While this may be the case for relatively non-selective rill erosion caused by water, wind erosion is a selective process that tends to remove the finer portions of the soil that are the sites of 137Cs binding. Therefore, 137Cs concentration may tend to overestimate soil loss due to wind erosion. We investigated the partitioning of 137Cs in wind eroded sediments and with soil surface samples sieved into contiguous ranges of particle sizes. We also compared the 137Cs activities and stratification of several adjacent soils with known wind erosion and deposition histories. 137Cs activities and mean diameters of samples captured at different heights above the surface during an aeolian event indicated increasing 137Cs activity and decreasing particle size with increasing height above the surface and agreed well with the 137Cs activities and respective mean diameters of the sieved surface soil samples. While 137Cs activities of wind eroded soil profiles decreased with increases in erosion histories, analysis of depositional profiles indicated that particle size and 137Cs stratification are more important in determining depositional histories than are the total 137Cs activities alone.