Quantification of accelerated soil erosion using the environmental tracer caesium-137

The principal objective of this investigation was to quantify erosion rates for five agricultural fields in three separate study areas in Saskatchewan. The radionuclide tracer caesium-137 (¹³⁷Cs) was used to quantify net erosion and net deposition within the landscape over a 30-year period. Uneroded (native) sites were used to establish the mean background level of ¹³⁷Cs in each of the study areas. The assumption being that agricultural sites with ¹³⁷Cs areal activities greater than the native site were subject to deposition, and sites with ¹³⁷Cs less than the native control site were subject to erosion. A linear proportionality model was used to convert the loss or gain of ¹³⁷Cs to net soil erosion or deposition. Results have indicated that accelerated (anthropogenic) erosion has been commonplace on arable land in Saskatchewan, even on near-level fields (< 1.3 degrees). The net integrated sediment output from the five agricultural fields ranged from — 0.6 t th⁻¹ y⁻¹ to — 6.8 t ha⁻¹ y⁻¹ (where negative values represent erosion). What is more alarming is that between 40 and 75 per cent of all sites sampled within individual fields had erosion rates in excess of the generally accepted rate of soil formation (1.0 t ha⁻¹ y⁻¹). Also, in one highly eroded field (Crystal Springs medium sloping site) 65 per cent of the sites sampled exceeded the upper tolerable erosion rate of 11.0 t ha⁻¹ y⁻¹. These results indicate significant degradation of the non-renewable soil resource has occurred over the past 30 years and is still presently active. Land degradation by accelerated erosion would result in reductions in effective rooting depth, soil moisture holding capacity, essential nutrient stores, and would adversely effect the physical structure of the topsoil. The major reason for accelerated erosion on arable land in Saskatchewan is the practice of summer fallowing, where the field is left in a ‘bare’ state and repeatedly tilled every second or third year. During a fallow period, or prior to crop emergence during a cropping year, fields are subject to wind and water erosion. On near-level fields wind would be the dominant transport agent, while on sloping fields inter-rill and rill erosion would be the primary forces of erosion. It is suggested that the appropriate conservation farming response would be to increase application of surface mulches and possibly to decrease the frequency of summer fallowing. Without such efforts long-term sustainable agricultural production in the Prairies of Canada is considered to be a tenuous land use practice.