Quantifying the spatial variation of 7 Be depth distributions towards improved erosion rate estimations N. Ryken a, ⁎, B. Al-Barri a , A. Taylor b , W. Blake b , P. Maenhout a , S. Sleutel a , F.M.G. Tack c , M. Dierick d , S. Bodé c , P. Boeckx c , A. Verdoodt a a Department of Soil Management, Ghent University, Gent, Belgium b School of Geography, Earth and Environmental Sciences, Plymouth University, United Kingdom c Department of Applied Analytical and Physical Chemistry, Ghent University, Gent, Belgium d Department of Subatomic and Radiation Physics, Ghent University, Gent, Belgium abstract article info Article history: Received 5 November 2015 Received in revised form 18 January 2016 Accepted 20 January 2016 Available online 1 February 2016 There is growing interest in the application of the natural fallout radionuclide 7 Be as a soil erosion and sediment tracer. Development of robust datasets is, however, hampered by unquantified variability in its vertical distribu- tion within surface soil. Models that convert 7 Be inventory measurements to soil erosion estimates are all based on the observed depth distribution of 7 Be, described by the relaxation mass depth (h 0 ) parameter. Previous work, however, has not considered potential spatial variation in h 0 linked to natural variability in soil physical proper- ties, 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 programme was carried out whereby two reference sites with variable degree of compaction were sampled using two different sectioning techniques, i.e. by use of a fine increment soil collector (FISC) and the scraping methodology. During a laboratory rainfall simulation experiment, water spiked with stable 9 Be was used to study the variability in 9 Be soil depth distribution within and between the two reference sites. In the field experiment, variations in the 7 Be depth distribution, and thus in h 0 , were limited between both reference sites (13 to 16%). In contrast, the impact of the sectioning technique was remarkable, with scraping resulting in a higher h 0 (up to 60%) compared to the estimates based on the use of a FISC. The rainfall simulation experiment offered the opportunity to study the variation in 9 Be depth distribution in more detail. With an average h 0 of 4.66 kg m -2 , 9 Be penetrated deeper in the non-compacted (NC) reference site cores, while the compacted (C) cores showed an average h 0 of 2.42 kg m -2 . The reported h 0 values at the former site were also characterized by a larger coefficient of variation (24%) than those at the latter site (11%). Lower bulk density, higher infiltration rates and a pore network characterized by a higher macroporosity and connectivity, as revealed by the X-ray Computed Tomography (CT) scans, explained the deeper penetration of Be into the topsoil of reference site NC. The results indicate the importance of selecting appropriate reference sites and for ensuring an adequate sam- pling strategy to encompass local variability in soil physical properties. Hydraulic conductivity assessment could be a useful tool to properly assess suitable reference sites and the number of samples needed to assess the reference inventory. © 2016 Elsevier B.V. All rights reserved. Keywords: Beryllium-7 Depth distribution Rainfall simulations Relaxation mass depth CT-scans 1. Introduction Reliable quantitative data on the extent of soil erosion is vital to sup- port sustainable land use management. Soil erosion is expected to increase in many areas of the world in the 21st century with increasing population pressure and consequent problems of deforestation and land management issues (Nearing et al., 2004; Nearing and Hairsine, 2011). In this context, both on-site and off-site impacts of soil loss could influ- ence food security and livelihood in the affected areas (e.g. Miller, 1997; Correll, 1998; Montgomery, 2007; Bilotta and Brazier, 2008; Weijters et al., 2009; Lal, 2011). However, obtaining accurate data with regard to spatial and temporal variability in soil redistribution can be problem- atic (Jetten et al., 2003). Traditional monitoring methods, although expensive, are unable to pinpoint the spatial distribution of sediment sources and require infrastructure investment that often disturbs the natural process (Walling, 2006). Fallout radionuclides (FRN) can pro- vide an effective means of quantifying and assessing spatial variations in soil redistribution rates. The FRNs Cesium-137, 137 Cs, and Lead-210, Geoderma 269 (2016) 10–18 ⁎ Corresponding author at: Department of Soil Management, Coupure Links 653, 9000 Gent, Belgium. E-mail address: nick.ryken@ugent.be (N. Ryken). http://dx.doi.org/10.1016/j.geoderma.2016.01.032 0016-7061/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma