Effect of Root Density on Erosion and Erodibility of a Loamy Soil Under Simulated Rain Sheela Katuwal, 1 Jan Vermang, 2 Wim M. Cornelis, 2 Donald Gabriels, 3 Per Moldrup, 4 and Lis Wollesen de Jonge 1 Abstract: Although both aboveground and belowground components of vegetation act together in reducing soil erosion, mainly the above- ground component has received attention in past research. The aim of this study was to evaluate the contribution of roots in soil erosion con- trol and the effect of root density in soil erodibility and soil physical properties. Perennial ryegrass (Lolium perenne L. Hugo) was grown in soil pans, and laboratory rainfall simulation experiments were con- ducted after 4, 8, 12 weeks of their growth with seeding density of 50 kg ha j1 , after 4 weeks for seeding density of 100 kg ha j1 , and on a control. The experiments with ryegrass were done in the presence of complete plants and after clipping off the shoots. Roots of ryegrass grew rapidly, attaining densities of 0.614 kg m j2 and 2.280 kg m j2 in 4 and 12 weeks, respectively. With increasing root density, splash and wash decreased exponentially. There was positive correlation between soil shear strength and root density, but no influence of roots on bulk density and saturated hydraulic conductivity was observed. Key words: Rainfall simulation, root density, ryegrass, soil erodibility, soil erosion. (Soil Sci 2013;178: 29Y36) S oil erosion in agricultural lands is a major process respon- sible for soil loss. About 90% of agricultural land worldwide is affected by soil erosion to various extents (Lal and Stewart, 1990; Pimentel et al., 1995). Together with soil loss, it also causes loss in organic matter and vital plant nutrients such as nitrogen, phosphorus, potassium, and calcium (Lal and Stewart, 1990; Pimentel, 2006), affects the soil water-holding capacity (Murphree and Mcgregor, 1991; Wendt et al., 1986), and soil biota, that collectively influence soil productivity (Lal, 1998; Langdale et al., 1992; Pimentel, 2006). Vegetation has long been used in reducing soil erosion and restoring degraded lands. Where the aboveground com- ponent of vegetation is primarily responsible for providing shelter against the erosive forces of raindrops (Styczen and Morgan, 1995), it is the belowground biomass that is in direct interaction with the soil and microorganisms within. These interactions of roots with their environment can modify and stabilize soil aggregation and its structure (Gale et al., 2000; Ghidey and Alberts, 1997; Gyssels et al., 2005; Tisdall and Oades, 1982), modify the proportion and architecture of macropores by growing roots (Angers and Caron, 1998; Young, 1998), influence the infiltration rate (Bharati et al., 2002; Meek et al., 1992; Zhou and Shangguan, 2007), and in- crease the shear strength (Tengbeh, 1993; Waldron, 1977), depending on the root architecture (Carter et al., 1994), root functions, physiology of the roots (Chan and Heenan, 1996), and their stage of development. Although the importance of roots in soil erosion control was demonstrated long before by Kramer and Weaver (1936), it is only recently that studies have focused on the influence of roots of various annual and perennial crops on soil loss rate and soil erodibility. Root density (in kilograms per cubic meter or kilograms per square meter), root length density (in kilometers per cubic meter), and root surface area density (in square meters per cubic meters) are the common parameters to quantify the amount of roots in soil in relation to soil erodibility. In general, the erodibility and soil detachment rate have been found to de- crease with the presence of roots. Most of the studies conducted in loamy soils (sandy loam, silt loam, and silty clay loam) show an exponential decrease in erodibility and soil detachment rate with root density (Ghidey and Alberts, 1997) and/or root length density (De Baets and Poesen, 2010; De Baets et al., 2006; Mamo and Bubenzer, 2001b). Zhou and Shangguan (2007) reported a linear decrease in soil loss rate with an increase in root surface area density (in square centimeters per cubic centimeter) for a silty clay loam soil with ryegrass. However, most of these studies analyzed the effects of root density alone, and very limited information is available on the comparative effects of the aboveground and belowground components of vegetation simul- taneously on soil erosion and interrill erodibility. In addition, they lack the link on the effect of density of roots on different soil physical properties that influence the rate of soil detachment and interrill erodibility. The objectives of the study were (i) to compare the amount of soil erosion obtained with the total vegetation and with roots only, (ii) to assess the influence of root density on the erod- ibility of the soil, and (iii) to study the effect of varying root densities on soil physical properties that affect soil erosion (bulk density, saturated hydraulic conductivity, shear strength, soil surface strength, and aggregate stability). MATERIALS AND METHODS Preparation of the Soil Pans Grab samples were collected from an agricultural field in Nukerke (Belgium). The soil used was a silt loam soil, containing 191 g kg j1 clay (0Y2 Km), 511 g kg j1 silt (2Y50 Km), 298 g kg j1 sand (50Y1,000 Km), 2.1 g kg j1 organic matter, and 2 g kg j1 CaCO 3 . The soil was air-dried, crushed, and sieved to get TECHNICAL ARTICLE Soil Science & Volume 178, Number 1, January 2013 www.soilsci.com 29 1 Department of Agroecology, Aarhus University, Tjele, Denmark. 2 Department of Soil Management, Ghent University, Ghent, Belgium. 3 UNESCO Chair on Eremology, Department of Soil Management, Ghent University, Ghent, Belgium. 4 Department of Biotechnology, Chemistry, and Environmental Engineering, Aalborg University, Aalborg, Denmark. Address for correspondence: Sheela Katuwal, Department of Agroecology, Aarhus University, Blichers Alle ´ 20, DK-8830 Tjele, Denmark. E-mail: sheela.katuwal@agrsci.dk Financial Disclosures/Conflicts of Interest: This study was funded by a scholarship grant for Sheela Katuwal from the VLIR-UOS (Flemish Interuniversity Council-University Development Cooperation). The authors report no conflict of interest. Received June 26, 2012. Accepted for publication December 14, 2012. Copyright * 2013 by Lippincott Williams & Wilkins ISSN: 0038-075X DOI: 10.1097/SS.0b013e318285b052 Copyright © 2013 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.