A Review of REE Tracer Method Used in Soil Erosion Studies

Agricultural Sciences in China 2010, 9(8): 1167-1174 August 2010 A Review of REE Tracer Method Used in Soil Erosion Studies ZHU Ming-yong 1, 3, TAN Shu-duan 1, 2, LIU Wen-zhi 1 and ZHANG Quan-fa 1 1 Key Laboratory of Aquatic Botany and Watershed Ecology, Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, P.R.China 2 College of Bioscience and Biotechnology, Hunan Agricultural University, Changsha 410128, P.R.China 3 Graduate University, Chinese Academy of Sciences, Beijing 100049, P.R.China Abstract Rare earth elements (REEs) have been proved ideal tracers for soil erosion and aggregation. REE tracer method (REETM) used to study soil erosion, as a new technique, has been developed in recent 20 yr. It is able to quantitatively monitor the temporal and spatial variation of erosion intensity, compared with traditional approaches. The applications of REETM in studying of sediment differentiation law, erosion process evolution, determination of sediment sources and sedimentation investigations, and determination of prediction parameters, were reviewed. Some application limitations were summarized. Key words: soil erosion, rare earth element (REE), REE tracer method (REETM), research progress INTRODUCTION Soil erosion and its adverse impacts on soil quality, land productivity, water quality, and environments have attracted extensive global attention (Pimentel et al. 1995). Traditional methods of soil erosion research such as runoff plots and field survey can be used to study erosion intensity as a whole, but they are difficult to adopt to study the erosion processes or spatial distributions, while effective erosion control requires a thorough understanding of these. Tracer method as an emerging approach used to track soil mass movement can be used to gain this aim. Rare earth element (REE) tracer generated much attention within the field of geochemistry (Henderson 1984). It has been used in environmental science (Krezoski 1989) and geology (Mahler et al. 1998). REEs can be placed on different topographical positions according to the research requirement, erosion intensity and sediment sources can be estimated by analyzing REE concentrations in sediments. Over the last two decades, great progress has been made in soil erosion studies with REE tracer method (REETM). THE FEASIBILITY OF USING REETM FOR SOIL EROSION STUDIES In studying soil erosion and sedimentation investigations, Knaus and van Gent (1989) indicated that elements as tracers should have four characteristics: well integrated with soil particles, insoluble or low-soluble in water, low plant uptake and no harm to eco-environment, and low background concentration in soils. Most REEs of the lanthanide series can meet these requirements (Markert 1987; Zhu and Xing 1992; Liu et al. 1997b; Zhu et al. 1997). The most important of REEs as tracers is their ability to bind with soil materials (Zhang et al. 2001). Motisoff et al. (2001) determined that the soil tagged with REEs behaved in the same manner as untagged ones. REEs showed strong-binding ability Received 22 September, 2009 Accepted 13 April, 2010 Correspondence ZHANG Quan-fa, Professor, Ph D, Tel: +86-27-87510702, Fax: +86-27-87510251, E-mail: qzhang@wbgcas.cn doi:10.1016/s1671-2927(09)60204-2

1168 ZHU Ming-yong et al. with soil particles and were eroded at the same rate, not interfere with sediment transportation, no obvious vertical movement of REEs in the soil profile, low mobility, and evenly incorporated into soil aggregates of the various size groups (Zhu et al. 1993; Zhang et al. 2001, 2003; Polyakov and Nearing 2004; Kimoto et al. 2006b; Stevens and Quinton 2008). What s more, direct mixing of a trace amount of REEs does not substantially alter physiochemical properties of soil particles and aggregates (Zhang et al. 2001; Kimoto et al. 2006b). Though non-uniform binding issue exists (Mahler et al. 1998), the workers have found effective ways to deal with it (Kimoto et al. 2006b). The main advantage of using REEs as tracers is that REEs provide multiple tracers. The use of multiple tracers excels a single tracer in that it may provide information on sediment redistribution. Researcher can discharge multiple REE tracers in a test. Extra errors resulting from different chemical characteristics can be eliminated, because the REEs have extremely similar chemical properties and geochemical activities in soil. Furthermore, instrument neutron activation analysis or inductively coupled plasma mass spectrometry has a high sensitivity for most REEs and the analysis procedure provides a reliable guarantee for the application of tracers (Liu et al. 1997a; Zhang et al. 2001). Compared with actual soil loss data, the average relative error with REETM is less than 15% (Tian et al. 1994; Zhang et al. 2003, 2005; Xue et al. 2004a; Lei et al. 2006; Wang N et al. 2008), which is considered satisfactory in the study of soil erosion. All these described above proved the feasibility and effectiveness of the direct use of REEs for tracing soil erosion. SOIL EROSION PROCESS STUDIES WITH REETM Limited by research means, sediment distribution and sedimentation investigations are rarely concerned in soil erosion processes, and these directly affect the erosion mechanism research and erosion prediction. So exploring new methods with less investment and short period for determination of erosion intensity under natural conditions becomes one focus of soil erosion scientists. Knaus and van Gent (1989) firstly used REEs as a horizon marker, rather than as a tracer, to investigate accretion in a wetland habitat. Using REETM to study soil erosion mechanisms was initially conducted by Tian et al. (1994), who studied the slope distribution of erosion intensities under simulated rainfall. The principles and operating techniques (such as the selection rules of tracers, the calculation of tracer quantity added, the marking method, and some attention should be paid in application, etc.) were put forward in detail by Tian et al. (1994) and Liu et al. (1997a, 2004). REETM was also used in another pioneer studies for monitoring soil erosion (Riebe 1995; Plante et al. 1999). Traces should be able to represent the translocation of tracer vector, and discharging method must be ensured for it. Initially three discharging methods (i.e., spot, band-tracing and section methods) were proposed. Now the advanced ones have been suggested by Song et al. (2003) and Xue et al. (2004a, b). They placed different REEs in different soil depths across a slope to investigate the rill development along the erosion depth, or placed different REEs in different soil depths and different sections in a plot to investigate the erosion processes in three-dimensional space. Experimental studies applying the REETM method under both rainfall and scouring conditions were summarized in Tables 1 and 2. Sediment differentiation law of erosion The sediment differentiation law in erosion processes provides important basis for tracing the origins of sediment yield, rational treatment of farmland and the associated soil conservation measures in eroded areas. The pioneer work conducted by Tian et al. (1994) (Table 1) showed that REETM could be used not only to determine the distribution of erosion intensities accurately, but also to reveal the variation tendency during the erosion processes. They used the section method in the experiment, which has high precision and large workload, so the method is fit for applying in laboratory or field plot scale research. They also proved that the band-tracing method has a relatively higher accuracy compared with the spot method, as well as smaller workload compared with the section method. Therefore, this method has been used widely from then

A Review of REE Tracer Method Used in Soil Erosion Studies 1169 Table 1 Basic information about some experiment setup with REETM under rainfall conditions Researchers Length Width ( Depth) (m) Runoff plot Slope (degree) Rainfall inten- sity (I, mm h -1 ) 2) Soil Discharging method REE selection ISWC 1) 113.0 5.0 0.2 27 Loess Band-tracing method La, Ce, Nd, Sm, Eu, Yb (Tian et al. 1997; Shi et al. 1997; Wu C L et al. 1997; Wu P T et al. 1997) Tian et al. (1994) 6.5 2.0 15 68.40, 96.60 Loess Section method, La, Ce, Nd, Sm, Eu, Yb band-tracing method Song et al. (2003) 5.0 1.0 0.5 15 63.39, 60.13 Loess Section method in La, Ce, Nd, Sm, Eu different layers Xue et al. (2004a, b) 5.0 1.0 0.2 22 90.00, 85.00 Loess Section method in different La, Ce, Nd, Sm, Eu, Yb, Tb layers and different band Shen et al. (2007) 16.0 2.0 0.2 22 Loess Section method in different La, Ce, Nd, Sm, Yb layers and different band Tang et al. (2004, 2006) 8.0 3.0 5, 10, 15, 20 50, 100, 150 Loess Section method La, Ce, Nd, Sm, Eu, Dy, Tb, Yb Wang N et al. (2008) 10 2.0 0.1 12 109.2 Black soil Section method La, Ce, Nd, Sm, Eu 10 2.0 0.1 7 99 Ma et al. (2003) 2.8 1.0 15 24-188.4 Red soil Spot method Eu 20 10 15 Zhang et al. (2003) 4.0 4.0 0.2 5.73 (10%) 60, 90 A silt soil Band-tracing method La, Nd, Sm, Pr, Gd Polyakov et al. (2004) 4.0 4.0 5.73 (10%) 63 A silt loam Band-tracing method La, Nd, Sm, Pr, Gd 1) ISWC, Institute of Soil and Water Conservation, Chinese Academy of Sciences. 2) Natural rainfall. The I 30 (the maximum 30 min intensity in a rainfall event) is between 25.8 and 41.4 mm h -1 in the experiment of ISWC (1997), and 35.2, 35.9 and 39.4 mm h -1 in the experiment of Shen et al. (2007). All of the other experiments in this Table were conducted with simulated rainfall. Table 2 Basic information about some scouring experiment setup on soil erosion studies with REETM Researchers Length Width ( Depth) (m) Runoff plot Slope (degree) Inflow rate (L min -1 ) Soil Discharging method REE selection Lei et al. (2006) Zhang et al. (2008) 8.0 1.0 5, 10, 15, 20, 25 2, 4, 8 Loess Band-tracing method La, Ce, Nd, Sm, Eu, Yb, Dy, Tm, Ho, Tb Wang et al. (2004) 5.0 0.33 0.5 3, 6, 9 2.5, 3.5, 4.5, 5.5, 6.5 Loess Band-tracing method Ce, Nd, Sm, Dy Wang X et al. (2008) 5.0 1.0 0.05 15, 18, 21 3.5, 4.5, 5.5 Loess Section mthod and La, Ce, Nd, Sm, Eu, band-tracing method Yb, Tb, Pr, Dy Li et al. (2006) 5.0 0.3 0.2 6, 9, 12 2.5, 3.5, 4.5, 5.5, 6.5 Loess Band-tracing method Ce, Nd, Sm, Dy Ding et al. (2003) 5.0 1.0 6, 9, 12 2.5, 3.5, 4.5, 5.5, 6.5 Loess Band-tracing method Ce, Nd, Sm, Dy Zhang et al. (2005) 5.0 0.3 0.2 6, 9, 12 2.5, 3.5, 4.5, 5.5, 6.5 Loess Band-tracing method Ce, Nd, Sm, Dy on. Apparently, there is an internal assumption in this experiment that no deposition occurred on the slope surface or the deposition can be neglected. Thus the method applied in the complex terrain region for erosion distribution research is inappropriate. After that, they spread the band-tracing method in a field slope in Loess Plateau, and the relationship between slope length and sediment yield was obtained (Wu C L et al. 1997; Shi et al. 1997) (Table 1). Generally sediment tended to increase as slope length increased in individual rainfall event (Li et al. 1997; Wu and Liu 1997). Because of the increasing of confluence area, the runoff increased and then its erosive ability enhanced. The slopegully test displayed that discharging REE with spot method can deal with the difficult problem of quantitatively monitoring sediment origin (Wu and Liu 1997). Yang et al. (2003, 2008) applied a positioning soil core Eu tracer (PSCET) to investigate spatial distribution of erosion intensity in red soil and loess sloping fields. The authors found that slope morphology had great effect on soil erosion and deposition. Ma et al. (2003) further examined the PSCET results with actual soil loss data in their tests (Table 1). Actually PSCET is a spot discharging method. A point selected is assumed to represent the average erosion in that area where the REE applied. The accuracy and reliability of this method should be verified, and the selection problem of appropriate discharging spots should be resolved. Due to

1170 ZHU Ming-yong et al. small workload of this method, it is suitable for the application in a large scale area (Matisoff et al. 2001). There is a relatively agreement conclusion that the most intensive soil erosion (MISE) occurred in the lower section of loess slope (Tian et al. 1994; Yang et al. 2003; Liu et al. 2004; Xue et al. 2004a, b; Shen et al. 2007). This relates to the loose structure of loess. Of course, there is different viewpoint. Wu P T et al. (1997) and Tang et al. (2006) found MISE area located in the middle section of loess slope. But with rainfall lasting, disagreement took place between them. Wu P T et al. (1997) and Wang N et al. (2008) considered that MISE area tend to move downward; but Tang et al. (2006) accounted that MISE tended to move upward, especially under high rainfall intensity, or steep slope conditions (Table 1). Perhaps different experimental slope leads to their disagreement. There are other studies demonstrated that MISE area occurred in the middle or lower section of a slope (Wang N et al. 2008), or in the upper-middle part of a slope (Zhang et al. 2003; Polyakov and Nearing 2004). It is closely connected with the development of rills during the erosion processes and has been proved by the simulation experiment conducted by Shi et al. (1996). The erosion rate increased along the slope from the upper to the bottom with the natural rain lasting in black soil area (Wang N et al. 2008), while it tended to decrease in red soil region (Yang et al. 2008) and in loess area (Yang et al. 2003), strong and weak erosion intensity interleaving occurred on a slope (Li et al. 2006). In almost all of the scouring experiments, MISE area occurred in the upper part of a plot (Lei et al. 2006; Li et al. 2006; Wang X et al. 2008; Wei et al. 2008). The result is directly associated with the clear water scouring from the inlet of the plot. This challenges the wisdom that MISE area takes place in the lower section of a uniform slope. With erosion progressing, the rill head area will be MISE area (Shi et al. 1996; Zhang et al. 2003; Xue et al. 2004a, b). In the flow scouring experiment, rainfall similitude is very poor, thus it is improper for the study of erosion distribution on a uniform slope. But this does not mean that REETM is not suitable for the study of rill development with scouring method since rill development mainly relies on runoff rate. Slope-gully erosion is a peculiar problem of Loess Plateau, while the research results stayed at qualitative description phase due to lacking sediment source identification means. Under different experiment set-up conditions, Wei et al. (2008) gained exactly the opposite conclusion with Wu and Liu (1997) (Fig.1). By using the scouring method, Wei et al. (2008) demonstrated that slope erosion was more serious than gully erosion, and erosion intensity in the upper section was more serious than that in the lower section, whether it was in the sloping area or in the gully area. Wu and Liu (1997) used simulated rainfall in a small runoff plot, and this might lead to the boundary effect. The energy gathered by the runoff might be insufficient to overcome the critical shear stress of soil when the slope length is too short (Tang et al. 2004). Shi et al. (1996) pointed out that the relationship between slope erosion and gully erosion was not constant, it varied with the development stages of the watershed. Fig. 1 Slope-gully system erosion experiment setup (left, Wu and Liu 1997; right, Wei et al. 2008).

A Review of REE Tracer Method Used in Soil Erosion Studies 1171 The other reason of the different conclusion between Wei et al. (2008) and Wu and Liu (1997) might be that they used different experimental slopes. In light of their experimental experience, the following research on the slope-gully erosion should adopt simulated rainfall, section REE discharging method, a long slope, such as 16-20 m, and appropriate slope degree. The differentiation law of soil erosion is affected by many factors, such as rainfall intensity and duration, slope degree and length, soil properties, development of rills, and even the randomness of rills occurrence, etc. Soil erosion process Effective erosion control requires a thorough understanding of soil erosion processed, which is how soil is detached, transported and deposited. This is necessary requirement for the development of process-based prediction models. REEs reflect the soil movement which they are tagged on, so it provides a very useful tool for quantification study of erosion processes. Rills are critical components in the erosion system. Most erosion process research with REETM method is on rill development (Song et al. 2003; Xue et al. 2004a, b; Lei et al. 2006; Li et al. 2006). Previous research means for soil erosion can not distinguish sheet erosion from rill erosion effectively. This would limit the cognition of erosion mechanism and also the determination of parameters in physical-models. Song et al. (2003) studied the processes of sheet erosion changing to rill erosion quantitatively under simulated rainfall (Table 1). They revealed that the transformation was slow until the appearance of rill. Xue et al. (2004a, b) studied the spatial-temporal processes of soil erosion on a slope in three-dimensional space (Table 1). The two tests were carried out based on the assumption that sheet erosion depth was known, and then some REEs can be used to track sheet erosion and others for rill erosion. Song et al. (2003) and Xue et al. (2004a, b) provided new idea and measurement for erosion dynamics research. Soil erosion rate is the mass of soil lost from an area per unit time divided by the areas, the above studies did not use this term. Shen et al. (2007) (Table 1) further spread the technique in a field plot under natural consecutive rainfalls. A number of researches have reported that REETM can be used not only to determine erosion intensity distribution quantitatively, but also to reveal the changeable processes of erosion under both simulated rainfall (Zhang et al. 2003; Polyakov and Nearing 2004; Tang et al. 2006) (Table 1) and scouring conditions (Ding et al. 2003; Lei et al. 2006; Li et al. 2006; Wang X et al. 2008) (Table 2). Soil erosion is related with slope, rill length and water inflow rate or rainfall intensity. The rill erosion development can be divided into three stages (Zhang et al. 2005; Li et al. 2006). Lei et al. (2006) considered that slope gradient had a greater effect on sediment concentration than inflow rate, while Li et al. (2006) believed that water inflow rate had a closer relationship with rill erosion than slope. This different conclusion might come from the different slope used in their experiments. An effective way to solve the problem is to do more crossover trials. Erosion rates estimated with REETM methods correlated well with those calculated by Zhang et al. (2003) and Polyakov and Nearing (2004) using laser-scanned DEMs methods. In individual natural rainfall event, Wu P T et al. (1997) (Table 1) pointed out that the spatial distribution characteristic of soil erosion in a rainfall is different with the rainfall process. So much attention should be paid to the process of soil erosion and sediment yield. Determination of sediment sources and sedimentation The unified understanding of sediment sources and sedimentation features is scarcity (Rose et al. 1983). The information on sediment sources is needed for controlling soil loss and its associated nutrient and pollutant transport, and for developing appropriate watershed management tools (Kimoto et al. 2006a). It is difficult to identify sediment sources from the eroded materials because soil structure on a slope or in a watershed is similar. However, the development of multi-tracer techniques resolves the difficulty. Shi et al. (1996) initially introduced the REETM into sediment source study in a small watershed. Their simulation tests indicated that the method could interpret the origins of sediment yield satisfactorily. Stevens and Quinton (2008) demonstrated the suitability of REETM for identifying sediment sources over longer periods but not for calculating erosion rates over multiple events.

1172 ZHU Ming-yong et al. Polyakov et al. (2004) reconstructed the spatial distribution of sediment and sedimentation patterns within a small watershed and their results had important implications for understanding the morphologic evolution of hillslopes. A lot of researches about channel or river sediment delivery ratio have been done through investigation or evaluation, but so far we know little about the sedimentation of eroded soil on slope land (Tian 1997). Tian (1997) opened up a new way for quantification research of sedimentation in the erosion processes (Table 1). He indicated that sedimentation occurred mainly along the flow direction, and the deposition rate declined sharply with the distance increased. He also pointed out the location of deposition occurred. Based on this recognition, Wu C L et al. (1997) (Table 1) evaluated the rill erosion ratio. Both Tian (1997) and Matisoff et al. (2001) studied the relationship between tracers (REEs) concentration in the deposition and the distance of eroded materials transported. They gained the consistent conclusion: Soil material transported over relatively short distances and sedimentation mainly occurred in the neighboring region. Zhang et al. (2003), Ma et al. (2003), Stevens and Quinton (2008), and Polyakov et al. (2004) confirmed that conclusion. Determination of parameters in soil erosion prediction models The quantification relationship between erosion, transportation and deposition is clear in physical-based erosion prediction models, but the physical meaning of practical data used for validating the models is uncertain (Tang et al. 2006). These models need to be validated by using spatially distributed soil erosion data. REETM has no peer in this aspect. Both the detachment rate (D r ) and critical shear stress (τ c ) are important parameters in the physical-based prediction models. Using one type of soil, Wang et al. (2004) (Table 2) reported that D r increased with flow shear stress (τ) increasing and their logarithm was linear. By analyzing the dynamic equilibrium of runoff characteristics and erosion intensity along a slope, Tang et al. (2004) obtained τ c of a loess (Table 1). D r was found to increase with the slope or inflow rate increasing and to decrease with the sediment concentration or rill length increasing. These relationships were consistent with the detachment function in the WEPP model (Zhang et al. 2008) (Table 1). Xue et al. (2004a, b) conducted a preliminary study on kinetic characteristics of erosion with REETM. Zhang et al. (2005) (Table 1) evaluated the sediment feedback relationships of the WEPP model, and they found that the WEPP-calculated and REE-measured D r agreed reasonably well. Thus, REETM is a valid and advantageous technique for estimating the spatial distribution of D r. The designs of above-mentioned experiments showed great potential for determination of parameters in soil erosion prediction models by using REETM methods. But up till now, the application in this aspect is very limited. Following work on determination of τ c and D r for different soil types besides loess could refer to these designs. LIMITATIONS OF REETM APPLICATION The mobility of REE may increase with an increasing in soil acidity. Leaching or redistribution of REEs could cause a problem for tracing soil erosion in soils with very low ph (Land et al. 1999). Meanwhile, error will be relatively big when REETM used in the light texture soils because of low aggregate-forming abilities (Zhang et al. 2001; Kimoto et al. 2006b). The method in its current form requires the mixing of the tracer into the soil profile and the soil used as vector tracer generally passed screening. Compared with the undisturbed soil, physical properties such as permeability and anti-erosion ability of the trace vector (soil) change greatly. Therefore, REE-derived data can not truly reflect the erosion information of original soil. These may limit its full applicability for natural areas or no-till fields (Polyakov et al. 2004). It is difficult for the researchers to arrange the tracking sites reasonably in a small watershed so that the tracking sites can reflect erosion information comprehensively. The method may not be suitable for natural complex topographies because of ignoring the within-field deposition (Yang et al. 2008). Also, this tracer method is expense and time consuming compared with other methods. So far REETM is limited in laboratory or field plot experiment. It is very difficult to collect runoff samples in field experiments (Zhou et al. 1997), especially in a large scale area.

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