Impact of Sampling Approach and Hydraulic Topsoil Properties on Soil Erosion Estimates by the Fallout Radionuclide Be

There is growing interest in the application of the natural fallout radionuclide Be as a soil erosion and sediment tracer. Development of robust datasets is, however, hampered by unquantified variability in its vertical distribution within surface soil. Models that convert Be inventory measurements to soil erosion estimates are all based on the observed depth distribution of Be, described by the relaxation mass depth (h0) parameter. Previous work, however, has not considered potential spatial variation in h0 linked to variability in soil physical properties, which could have major implications for the reliability of soil erosion estimates. Two complementary experiments were designed to study the variability in depth distribution within and between potential reference sites. First, a field sampling program was carried out on two reference sites with variable degrees of compaction as visually observed in the field, defined as the compacted reference site (ref C) and a non-compacted reference site (ref NC). These sites were sampled using two different sectioning techniques, i.e. by use of a fine increment soil collector (FISC) and the scraping methodology. In this field experiment, variation between both reference sites in the Be depth distribution, and thus in h0, was limited (13 to 16%). In contrast, the impact of the sectioning technique was remarkable, with scraping resulting in a higher h0 (up to 60%) compared to the estimates based on the use of a FISC (Figure 1). The observed h0 were used in a model sensitivity analysis, with differences in h0 proportionally propagating in the obtained average erosion budgets (ranging between 38 ton ha and 62 ton ha). During a laboratory rainfall simulation experiment, water spiked with stable Be was used to study the variability in Be depth distribution, whereby Be was used as a substitute for Be. Eight undisturbed soil cores were collected at both the compacted and the non-compacted reference sites. X-ray Computed Tomography (CT) scans were used to characterize the porosity of the soil cores, showing significant lower, strongly horizontally oriented, total porosity of the compacted ones (Figure 2B). The average saturated hydraulic conductivity was 0.89 m day and 17 m day for the compacted and the non-compacted samples, respectively. This compaction resulted in a clear distinction in the Be depth distribution (Figure 2A). With an average h0 of 4.66 ± 1.1 kg m, Be penetrated deeper in the non-compacted soil cores, while the compacted cores showed an average h0 of 2.42 ± 0.26 kg m . These results indicate a strong relation between the infiltration rate and the Be depth distribution. The reported h0 values at the former site were also characterized by a larger coefficient of variation (24%) than those at the latter site (11%), similar to the variations in soil structure observed by the CT-scans. The results indicate the importance of selecting appropriate reference sites and of ensuring an adequate sampling strategy to encompass local variability in soil physical properties. Hydraulic conductivity assessment can be applied as a useful tool to properly assess suitable reference sites and the number of samples needed to determine the reference inventory in a