Water and Solute Transport in a Cultivated Silt Loam Soil: 1. Field Observations

ated with soil type, vadose zone flow and transport processes are also affected by such external factors as tillage Vadose zone flow and transport processes are known to be strongly practices. For example, Petersen et al. (1999, 2001) affected by both soil structure and soil texture. We conducted a field experiment to explore the impact of heterogeneity in soil structure showed that surface tillage may strongly affect the dycreated by agricultural operations such as wheel traffic, plowing, and namics of dye tracers in soils. Trafficking by agricultural surface tillage on water and solute transport. The experiment was or other machinery has also been shown to modify the performed on a 4 by 2 m2 field plot perpendicular to the path of a hydraulic properties and porosity of soils (Meek et al., tractor that had pulled a harrow for seedbed preparation. The plot 1989; Ankeny et al., 1990; Mohanty et al., 1994; Gysi et was irrigated with a rainfall simulator at a rate of 21 mm h 1 for 2 h al., 1999). A major challenge is to find useful proxy and 20 min. An 850 mg L 1 bromide solution was subsequently applied variables to assess soil structure heterogeneity (Young at a rate of 26 mm h 1 for 2 h. Soil water contents and pressure heads et al., 2001). during the experiment were monitored with time domain reflectomeThe structure of tilled soils is known to be strongly try (TDR) probes and tensiometers. The soil was sampled for resident affected by agricultural practices to which the field is bromide (Br ) concentrations at the end of the experiment. Water and bromide fronts were found to be highly heterogeneous. The hetersubjected. For instance, heavy compaction by the wheels ogeneity could be explained by the particular soil structural features of farm machinery will produce an abundance of comcreated by the agricultural practices, in particular by the locations pacted clods within the tilled layer once the soil is and sizes of compacted soil zones. Very little water and bromide plowed (Roger-Estrade et al., 2000), especially for relahad penetrated the large compacted zones under the wheel tracks. tively fine-textured soils. On the other hand, surface Bromide, TDR, and tensiometer measurements all indicated the prestillage may lead to the destruction of compacted clods ence of preferential flow of water and bromide along paths immedithat were initially present in the plow layer. This disinteately bordering the wheel tracks. The compacted clods in the plow gration of clods is generally limited to the surface few layer furthermore acted as low-permeability barriers that diverted centimeters of the plow layer (i.e., the seed bed). water and bromide flow around them. The highly heterogeneous plow To better describe the effects of agricultural machinlayer between the wheel tracks produced a much higher solute dispersivity as compared to the compacted soil below the wheel tracks. ery on soil structure and compaction, Manichon (1982) proposed a field method for characterizing soil structure. The method pays particular attention to visible porosity and the compaction status of the soil. The tilled N studies have shown that water and solute soil for this purpose was divided vertically into several transport in soils can be highly heterogeneous at layers or compartments consistent with the depth of the field scale (e.g., Butters et al., 1989; Roth et al., various tillage operations. For instance, for cropping 1991; Flury et al., 1994, 1995; Hammel et al., 1999). This systems that include plowing, the vertical compartments fact makes it difficult to define and estimate equivalent immediately after sowing may include a seed bed, a or effective parameters for use in relatively simple modplow layer that is unaffected by seed bed preparation, els describing field behavior (Ellsworth and Jury, 1991; and the underlying undisturbed soil. Compartments were Mayer et al., 1999). Recently, Vanderborght et al. (1997, similarly separated laterally according to the locations 2001) highlighted the importance of soil morphology in of wheel tracks of tractors and other farm machinery explaining prevailing solute transport properties. The that may have compacted the underlying soil. These presence of different horizons or layers within a soil compacted zones would alternate within the plow layer can be a major factor causing flow heterogeneity (van with noncompacted zones located between the wheel Wesenbeck and Kachanoski, 1994; Deurer et al., 2001). tracks. Manichon (1982) identified three different types In addition to the intrinsic structural properties associof soil structure ( , , and ), each having unique internal macroscopic porosities. These types of soil structure Y. Coquet, C. Coutadeur, C. Labat, and P. Vachier, UMR INRA/ may hold for entire compartments if they are homogeINAPG Environment and Arable Crops, Institut National de la Reneous (e.g., the relatively large compacted zone in the cherche Agronomique/Institut National Agronomique Paris-Grignon, B.P. 01, 78850 Thiverval-Grignon, France; J. Roger-Estrade, UMR plow layer below the wheel tracks), or for particular INRA/INAPG Agronomy, Institut National de la Recherche Agroclods within a more heterogeneous compartment (e.g., nomique/Institut National Agronomique Paris-Grignon, B.P. 01, the plow layer between wheel tracks containing com78850 Thiverval-Grignon, France; M.Th. van Genuchten, USDApacted clods originating from former wheel tracks, as ARS, George E. Brown, Jr. Salinity Lab., 450 West Big Springs Road, well as noncompacted macroporous clods). Several Riverside, CA 92507; J. Šimůnek, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521. Received 7 Oct. 2004. *Corstudies have shown that these different compartments responding author ([email protected]). or clods can have significantly different saturated and unsaturated hydraulic properties (Curmi, 1987; CoutaPublished in Vadose Zone Journal 4:573–586 (2005). deur et al., 2002). Original Research Paper doi:10.2136/vzj2004.0152 © Soil Science Society of America Abbreviations: HPLC, high performance liquid chromatography; TDR, time domain reflectometry. 677 S. Segoe Rd., Madison, WI 53711 USA 573 Published online July 18, 2005