1538 Abstract The mobilization and transport of colloid particles in soils can have negative agronomic and environmental efects. This work investigates the controls of particle release and transport from undisturbed soil columns sampled from an agricultural, loamy feld with clay and silt contents of 0.05 to 0.14 and 0.07 to 0.16 kg kg −1 , respectively. Forty-fve soil columns (20 ´ 20 cm) were collected from the feld and exposed to a constant irrigation of 10 mm h −1 for 8 h. The accumulated mass of particles in the outfow from each column was highly correlated (r = 0.88) with the volumetric mass of fnes (MF vol ). The MF vol is defned as the sum of clay and fne silt (<20 mm) multiplied by the soil bulk density and divided by the particle density of the mineral fnes. Thereby, MF vol represents both the particle source available for mobilization and leaching and an indicator of soil structure. The particle release process showed two linear particle release rates. Although the two particle release rates were distinctly diferent, both were strongly correlated with MF vol . The diference between the two rates was related to the degree of preferential fow characterized by the 5% arrival time of an applied tracer pulse. Soil columns with a longer 5% arrival time (less preferential fow) showed a distinct diference between the two rates, whereas soil columns with a short 5% arrival time and fast water transport showed resemblance between the two particle release rates. Thus, the combined efects of particle source, type, and pathways (via soil structure and compaction) need consideration to understand and predict particle transport dynamics through intact topsoil. Particle Leaching Rates from a Loamy Soil Are Controlled by the Mineral Fines Content and the Degree of Preferential Flow Trine Norgaard,* Marcos Paradelo, Per Moldrup, Sheela Katuwal, and Lis W. de Jonge O ur ability to estimate the mobilization and leach- ing of colloid soil particles is essential for the environ- ment, as colloids have a strong afnity for adsorption and transport of specifc chemicals (McCarthy and Zachara, 1989; Liu et al., 2013; Norgaard et al., 2014). Colloids are defned as soil particles measuring approximately 10 nm to 10 mm in diameter (DeNovio et al., 2004), and due to their large surface area and high sorption capacity, they may act as carriers of, for example, pesticides (de Jonge et al., 2000; Gjettermann et al., 2009; Villholth et al., 2000; Kjær et al., 2011), P (Vendelboe et al., 2011; Ulén et al., 2012), heavy metals (Pontoni et al., 2016), bacteria (Vasiliadou and Chrysikopoulos, 2011; Yang et al., 2012), and viruses (Syngouna and Chrysikopoulos, 2013, 2015). Failure to consider this colloid-facilitated transport will result in an underestimation of the transport range of contami- nants (Knappenberger et al., 2014). Additionally, the migration of colloids can exacerbate ongoing detrimental soil physical pro- cesses (e.g., soil erosion, hard setting, and soil clogging) and, in some cases, negatively afect the ability of soils to transmit fuids and solutes (Dexter, 1988; Mays and Hunt, 2005). Te colloid mobilization processes in porous soil are com- plex, and multiple factors have been reported to infuence the mobilization process. Soil water content is decisive for the extent of colloid mobilization (Michel et al., 2010; Dexter et al., 2011; Shang et al., 2008), and at higher water contents, the capillary forces between the colloids and the soil surface become less dominant until a critical water content where the capillary forces become repulsive and the colloids mobilize (Shang et al., 2008). Besides the soil water content, the fractions of mineral fnes in the soil defne the pool of colloids available for mobiliza- tion. Te efect of clay alone has been emphasized in numerous studies, and colloid leaching has thus been reported to increase with increasing clay content (de Jonge et al., 2004; Glæsner et al., 2011b; Vendelboe et al., 2011; Torkzaban et al., 2015). Organic C (OC) has been reported to exert a stabilizing efect on soil, and Lekfeldt et al. (2017) reported that soils treated with large Abbreviations: Al ox , oxalate-extractable aluminum; APM, accumulated particle mass leached between 15- and 70-mm outfow; BSR, best subset regression; BTC, breakthrough curve; CT, computed tomography; EC, electrical conductivity; Fe ox , oxalate-extractable iron; MF vol , volumetric content of mineral fnes; NTU, nephelometric turbidity; OC, organic carbon; P ox , oxalate-extractable phosphorus; R1, Rate 1; R2, Rate 2. T. Norgaard, M. Paradelo, S. Katuwal, and L.W. de Jonge, Dep. of Agroecology, Faculty of Science and Technology, Aarhus Univ., Blichers Allé 20, PO Box 50, DK- 8830 Tjele, Denmark; M. Paradelo, Sustainable Agricultural Sciences, Rothamsted Research, Harpenden, UK; P. Moldrup, Dep. of Civil Engineering, Aalborg Univ., Thomas Manns Vej 23, DK-9220 Aalborg Ø, Denmark. Assigned to Associate Editor Hyunjung Kim. Copyright © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. J. Environ. Qual. 47:1538–1545 (2018) doi:10.2134/jeq2018.02.0065 Supplemental material is available online for this article. Received 1 May 2018. Accepted 20 Aug 2018. *Corresponding author (trine.norgaard@agro.au.dk). Journal of Environmental Quality VADOSE ZONE PROCESSES AND CHEMICAL TRANSPORT TECHNICAL REPORTS Core Ideas • Particle leaching from intact soil columns depends on both texture and structure. • Volumetric content of mineral fnes well predicts particle mobili- zation and transport. • During preferential fow conditions, the particle leaching rate is relatively constant. • Varying particle mobilization rates occur mainly during matrix- dominated fow. Published September 27, 2018