Using Rare-Earth Oxide Tracers for Studying Soil Erosion Dynamics

these advantages will not be fully recognized unless the spatial and temporal predictability of process-based Spatially averaged soil erosion data provide little information on models are validated. Thus, spatially distributed erosion soil erosion dynamics. Dynamically varied, spatially distributed erosion data are needed to better understand erosion processes and data are of great importance in advancing process-based thoroughly evaluate process-based erosion prediction models. The erosion prediction models. objectives of this study are to examine the feasibility of direct mixing Various types of tracers have been developed and rare-earth element (REE) oxides with soil materials to trace soil used to obtain spatially distributed data. Commonly used erosion at a plot scale and to explore further the potential of using tracers include atomic bomb fallout radionuclide 137Cs this technique to study soil erosion dynamics. Five REE oxide powders (Ritchie and McHenry, 1990; Walling and He, 1999), were mixed with a Camden silt soil (fine, silty, mixed, mesic Typic naturally occurring radionuclides 210Pb, 7Be, and 234Th Hapludalt) at five slope positions. Six rainstorms were applied to a (Wallbrink and Murray, 1993), deliberately introduced 4 by 4 m soil bed at a 10% slope. Runoff was collected during the radionuclides such as 56Fe and 60Co (Wooldridge, 1965; rains and surface soil was sampled after each rain. All samples were Toth and Alderfer, 1960), noble element (e.g., Au or extracted for REE with a quick acid-leaching method and extracts were analyzed by inductively coupled plasma mass spectrometry Ag)-labeled natural particles (Olmez et al., 1994), REE(ICPMS). Mean error of REE-derived sediment discharge relative to tagged particles (Riebe, 1995; Plante et al., 1999), and measured total across six rainstorms was 14.5%. Our results demonexotic materials including fluorescent dye-coated partistrate that the REE-tracing technique is feasible and sound, being cles (Young and Holt, 1968), glass beads, and magnetic capable of producing both spatially and temporally distributed erosion plastic beads (Ventura et al., 2001). Although these tracdata. Results indicate that the most severe erosion occurred in the ers have proven useful, each has limitations. upper middle section of a uniform slope, and total sediment delivery Bomb-produced 137Cs (half-life of 30 yr) and naturally from each segment was positively related to amounts of sediment occurring 210Pb (half-life of 20.2 yr) are widely used deposition on the entire slope from that segment. This technique has to estimate long-term soil erosion rates and erosion the potential to bring new perspectives to soil erosion research. patterns across a landscape. However, these methods are quantitatively challenged by inconsistencies and uncertainties because of assumptions stemming from indeS averaged soil erosion data are useful for terminate input functions and indefinite spatial and developing empirical erosion prediction models, undepth distributions of the original deposition at the locaderstanding erosion principles, and deriving optimal tion of interest. For example, estimates of soil erosion erosion control management practices. However, such rates associated with a particular level of 137Cs depletion data are of limited value for understanding soil erosion can vary by more than one order of magnitude when dynamics such as temporal and spatial variations of soil using calibration relationships derived under different detachment, sediment transport, deposition, and redeassumptions (Walling and Quine, 1990). Above all, since tachment along a hillslope profile. Spatially and tempouniform spatial distributions are always assumed, 137Cs rally distributed erosion data are needed not only to and 210Pb provide little information on sediment origin. better understand soil erosion dynamics, but also for Unlike bomb-produced and cosmogenic radionuthorough validation of process-based erosion models. clides, radioactive elements can be manipulated and Spatially distributed soil erosion data are also useful deliberately introduced to study soil erosion. A major for evaluating on-site impacts such as effects of soil concern with this type of tracer is the radiological risk redistribution on crop yields and off-site impacts includto researchers and the environment. Exotic particles ing sediment origin and destination. Many processsuch as fluorescent glass or magnetic plastic beads have based erosion prediction models have been developed been used to overcome this concern, but suffer other in the past several decades to predict soil erosion at serious shortcomings. Major criticisms for exotic partivarious temporal and spatial scales (Nearing et al., 1989; cles are that (i) these particles may not bind with soil Hairsine and Rose, 1992; Borah, 1989). Since erosion particles or soil aggregates and therefore are transprocesses constantly change in time and space, processported separately; (ii) exotic particles differ in size distribased models have advantages over spatially and tempobution, particle density, shape, surface morphology, and rally lumped empirical prediction models. However, surface chemical properties from soil particles and aggregates; and (iii) sensitivity of quantitative analysis is X.-C. (John) Zhang, USDA-ARS, Grazinglands Research Lab., 7207 normally low, which means that a large quantity of tracW. Cheyenne St., El Reno, OK 73036; M.A. Nearing and V. Polyakov, ers are needed to trace soil and sediment movement on USDA-ARS, National Soil Erosion Research Lab., Purdue Univ., a hill slope or in a small watershed. West Lafayette, IN 47907; and J.M. Friedrich, Dep. of Chemistry, Purdue Univ., West Lafayette, IN 47907. Received 6 Nov. 2001. *Corresponding author ([email protected]). Abbreviations: REE, rare-earth element; DEM, digital elevation map; ICPMS, inductively coupled plasma mass spectrometry. Published in Soil Sci. Soc. Am. J. 67:279–288 (2003).