Soil Science Society of America Journal 63:1284-1290 (1999)
© 1999 Soil Science Society of America
DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION
A Tracer Sphere Detectable by Neutron Activation for Soil Aggregation and Translocation Studies
A.F. Plantea,
M.J.M. Dukea and
W.B. McGilla
a SLOWPOKE Nuclear Reactor Facility, Univ. of Alberta, Edmonton, AB T6G 2N8 Canada
aplante{at}ualberta.ca
 |
ABSTRACT
|
|---|
Current radiological and particle tracer methods used in soil translocation studies have several limitations. Studies of soil aggregate dynamics also require an improved particle tracer approach. Our objective was to develop an inert tracer sphere applicable in soil aggregation and translocation studies. We selected ceramic prills in varying size fractions, labeled with 10 to 15% (w/w) Dy during manufacture, as inert tracers to simulate soil aggregates because of their similar surface properties. Prills added to soil (Typic Cryoboroll) were detected and quantified via the Dy content of a mixed tracer–soil sample using instrumental neutron activation analysis (INAA). Detection limit measurements demonstrated that the tracers were easily detected; approximately one 300-µm sphere is detectable in 5 g of soil containing a background level of 3.1 µg g-1 Dy. Coefficients of variation for tracer sphere properties within size fractions were determined: 5.3 to 16.6% for mass in composite samples, 12.2 to 22.6% for diameter, and 6.5 to 10.8% for Dy concentration. However, no difference between actual and calculated numbers of spheres was detected (P = 0.05), indicating that the variability is insufficient to affect tracer sphere detection. Tracer integrity tests showed no leaching losses of the Dy label, and tracer sphere abrasion resulted in losses of <1% of sphere Dy content. The tracers have proven sufficiently homogeneous and robust for practical use and are currently being used to study soil aggregate dynamics.
Abbreviations: INAA, instrumental neutron activation analysis • XRD, x-ray diffraction
|
INTRODUCTION
|
|---|
SOIL TRANSLOCATION is an important physical process altering the structural state of soils and their location in the landscape. The movement of soil particles and aggregates is observed at several scales: (i) large scales in wind and water erosion, (ii) medium scales in the displacement of soil during tillage or associated with soil faunal activity (bioturbation), and (iii) small scales in macroaggregation (diameters 
250 µm).
Various fallout or applied radionuclides, such as 137Cs, are commonly used in field studies for tracing soil translocation (see Ritchie and McHenry [1990] for extensive bibliography). While useful in large-scale studies such as translocation within a watershed or across a field (10–100 m), radiochemical tracers have several limitations that make them inadequate for use in medium- or small-scale field studies (<1–10 m) or laboratory studies. The use of fallout 137Cs is confounded by the spatial variability of the original deposition and subsequent soil translocation prior to starting the experiment. Furthermore, the application of radiotracers may involve radiological risks to both researcher and the environment.
Several particle tracer approaches have been developed to overcome these limitations. Sand-sized glass particles labeled with a fluorescing agent were used in studies monitoring the transport of soil particles entrained by rainfall erosion (Young and Holt, 1968; Young and Mutchler, 1969). "Soil movement detection units", consisting of 11-mm steel hexagonal nuts detected with a metal detector, were used to measure soil translocation due to tillage (Lindstrom et al., 1990; Lindstrom et al., 1992), while Govers et al. (1994) used 15-mm plastic spheres with metal cores. Riebe (1995) used Eu-enriched glass beads of various sizes to examine soil translocation due to bioturbation and erosion.
While not often thought of as a soil translocation process, the formation and destruction of soil macroaggregates (>250 µm in diameter) is an important physical process involving the small-scale movement of soil particles (sand, silt, and clay) and microaggregates (<250 µm). Soil particles and microaggregates are continuously rearranged within the soil matrix by wet–dry and freeze–thaw processes, bioturbation, tillage, and traffic to form and disrupt macroaggregates (Dexter, 1988). These macroaggregates are stabilized by microbial action, various biochemical constituents, and physicochemical forces. Although aggregate turnover is a dominant control in the decomposition of soil organic matter (Van Veen and Kuikman, 1990), there are few published measurements of the rates at which soil aggregates form and degrade. Furthermore, even fewer publications report on soil aggregation by observing the incorporation or release of tracers into, or out of, various aggregate size fractions. It is our contention that determining how soil aggregation controls organic matter decomposition requires determining the dynamics of the aggregates themselves.
The use of particle tracers in aggregation studies poses a particular problem, as the physical process is less of a translocation, where tracers travel with soil particles and aggregates, than a rearrangement of soil where the tracers may be incorporated into aggregates of various sizes. Radionuclides, glass beads, and paint have been used to label constituents of aggregates for use as tracers. Toth and Alderfer (1960a) tagged water-stable aggregates with radiocobalt by soaking the aggregates in a radioactive solution of 60Co and leaching excess Co. The method was used in incubation and greenhouse experiments to study changes in soil aggregation (Toth and Alderfer, 1960b). Terpstra (1989) used glass beads as surrogates for soil particles and aggregates, and Staricka et al. (1992) used ceramic prills coated with fluorescent paint to examine the longevity of soil aggregates under field conditions and varying tillage methods.
However, the primary deficiency of many previous studies using particle tracers is the lack of similarity between the selected tracer and soil. The surface properties of plastic or glass beads are unlikely to simulate those of soil aggregates, which are generally anionic, amphiphilic, and porous. Tracer-soil similarity is important in ensuring realistic physicochemical interactions between tracer and soil. In addition, in the majority of reports, tracers are larger than soil micro- or mesoaggregates (>1 mm in diameter). Such tracers may be avoided by soil fauna and can only be incorporated into large macroaggregates or soil clods.
Studies focusing on soil organic matter and aggregation interactions have primarily been designed to examine aggregates <6 to 8 mm (Christensen, 1998), as these aggregates are the most abundant and contain the highest concentration of sequestered organic matter according to the current paradigm of hierarchical soil aggregation (Tisdall and Oades, 1982; Oades and Waters, 1991). Examining the interactions between soil organic matter and aggregation would therefore require smaller tracers than those used in examples cited above. While stable isotopes of C in various organic materials have been used as tracers for aggregate dynamics to alleviate problems associated with tracer particle sizes (Puget et al., 1995; Jastrow et al., 1996), there is a conceptual problem in using a dynamic tracer substance (e.g., decomposing carbonaceous materials) to study a second dynamic system (e.g., soil macroaggregates).
The application of INAA in soil science has become increasingly widespread. Its principal use has been in determining the elemental composition of soil samples in analytical and contamination contexts (e.g., Essington and Mattigod, 1990). While INAA and inert tracers have often been used in medical and pharmaceutical tracer studies, their application to tracer studies in soils has not been widespread. In experiments monitoring the movement of soil particles due to erosion and bioturbation, Riebe (1995) used neutron activation analysis to detect the glass beads, which had been enriched in Eu.
We developed an inert tracer sphere for use in soil particle translocation studies. The development process includes the manufacture of this tracer sphere as a representative surrogate for soil macro- and microaggregates, the detection and quantification of the spheres by INAA, and a preliminary approach to develop the tracers for observing soil aggregate formation and disruption directly.
|
Materials and methods
|
|---|
Tracer Sphere Development: Concepts
Tracer Selection
A ceramic prill manufactured by Kinetico Inc. (Nashwauk, MN), under the tradename Macrolite was selected as a candidate tracer. In field studies, the Macrolite prills were effective surrogates for soil macroaggregates, as well as for weed seeds and crop residues (Allmaras et al., 1996; Staricka et al., 1990, 1991, 1992). Macrolite is made primarily of nepheline (a feldspathoid mineral) syenite and is prilled into spheres of various sizes with a rough, sealed outer surface and porous interior (Fig. 1) . According to product literature, the prills have slightly alkaline, anionic, and hydrophilic surface properties, which are a suitable analog for soil particles and aggregates.
View larger version (100K):
[in this window]
[in a new window]
|
Fig. 1 Scanning electron micrographs of Macrolite tracer spheres (a) illustrating sealed outer surface and (b) fractured to reveal porous interior
|
|
Tracer Detection
Many soil aggregate studies have used relatively large spheres as analogs. However, to achieve our objective of studying smaller soil aggregates, smaller tracers were required. Examinations at a much smaller spatial scale than that used by Staricka et al. (1992) would require a better method of detection, recovery, and quantification than visual inspection. Because of its extreme sensitivity and nondestructive nature, INAA (DeSoete et al., 1972; Erdtmann and Petri, 1986; Parry, 1991) was investigated. Neutron activation analysis involves bombarding a sample with neutrons, some of which are absorbed by the nuclei of stable isotopes, to create radioactive nuclides. The emission of characteristic gamma rays often accompanies the decay of these radionuclides. Detection and quantification of these gamma rays affords a means of elemental analysis.
Three approaches with INAA were considered to study soil aggregate formation and destruction: (i) the addition of nonlabeled ceramic spheres to soil and their later determination by INAA, (ii) radiolabeling of the spheres by neutron irradiation before their use as a radiotracer, and (iii) labeling the spheres with an inert but readily activated element that was absent (or in low concentration) in the soil matrix, with subsequent determination in the soil–sphere mixture by INAA.
Initial INAA of spheres, as supplied by the manufacturer, and soil samples indicated no element or group of elements that would readily distinguish spheres from the soil matrix. Moreover, the spheres did not contain sufficient concentrations of those elements capable of producing the necessary quantities of long-lived radioactivity for use of the spheres as radiotracers. In addition, the use of radiotracers in field-scale experiments has many restrictions. Consequently the third option, that of labeling the spheres with a readily activated element was pursued.
Tracer Labeling
Several criteria guide the selection of an element with which to label the tracer spheres. The element must: (i) be absent, or present in very low quantities in the background matrix, (ii) have a high sensitivity by INAA, that is, activate readily due to a high neutron cross section, (iii) be nonvolatile at the high temperatures experienced during kiln firing of the prill, and (iv) be relatively inexpensive and nontoxic. Various metals (e.g., Co, Ag, In, Ir, Au) and most of the rare earth elements (e.g., Sm, Eu, Tb, Dy, Ho) make suitable inert labels readily determined by neutron activation analysis. While Riebe (1995) selected Eu (as Eu2O3) to label glass beads in his study, dysprosium oxide (Dy2O3) was selected for the purposes of this study. Dysprosium activates more readily than Eu (cross sections of 2000 x 10-24 vs. 320 x 10-24 cm2, respectively), and was significantly less costly. An additional benefit of using Dy as the labeling element is the presence of both long- and short-lived radioisotopes after neutron activation. This property was used to our advantage when performing preliminary characterization experiments.
To achieve a reasonably low detection limit in soils, a tracer sphere Dy content of 15% (w/w) Dy2O3 (99% pure; Stanford Materials, Stanford, CA) by weight was prepared. A pilot batch of Dy-labeled spheres was manufactured for use in trial experiments to characterize the prill and its behavior in soil, and to determine the detection limit for spheres in soil samples using INAA. The pilot batch, as received from the manufacturer, consisted of 
4.5 kg of prills in three size fractions. The sphere size fractions examined in this study are referred to simply by their relative size: small, medium, and large. While the medium and large size fractions are expected to be useful in tillage translocation and erosion studies, only the small fraction is expected to be of interest in aggregation studies because they are small enough to simulate microaggregates. Research is in progress to examine the use of prills smaller than 200 µm.
Tracer Sphere Development: Characterization
Surface Characteristics and Mineralogy
The manufacturing process of Macrolite includes a kiln firing of the prills at 1500°C. This caused some concern about the integrity of the mineral and surface characteristics of the prill. Consequently, x-ray diffraction (XRD) analyses were performed on both unfired and fired samples of prills to determine if the prills maintained their mineral structure after kiln firing. Samples were analyzed at 0% relative humidity using a Philips diffractometer (Model 1710, Philips Electronics Ltd., Scarborough, ON, Canada) equipped with a LiF curved crystal monochromator using CoK
radiation generated at 50 kV and 25 mA. Step sizes of 0.05° 2
and an accumulation time of 2 s step-1 were used for data acquisition.
Mass and Size Variability
Before using the pilot batch, the mass and size variability within each of the size fractions was characterized. The mean sphere mass in each size range was determined by weighing and counting spheres collected on a piece of previously weighed adhesive tape. Sphere diameters were determined using image analysis after samples of spheres from each sieve size were mounted on microscope slides and digitally photographed. Cross-sectional area, and maximum X and Y length data were collected using image analysis software. From the cross-sectional area data, mean effective sphere diameters and their distributions were calculated.
Dysprosium Concentration
Although the manufacturing process of the pilot-scale batch of tracer spheres called for 15% Dy2O3 content by weight (equivalent to 13.07% Dy), measurements were made to determine the accuracy and variability of this value. Replicated single sphere samples from the three size fractions were prepared for analysis and their Dy content determined by INAA. All irradiation and counting operations were performed at the University of Alberta SLOWPOKE II Nuclear Reactor Facility. Individual spheres were sealed in polyethylene microcentrifuge tubes, which were subsequently placed in 7-mL rabbit vials ready for irradiation. Samples were irradiated for 30 s at a nominal neutron flux of 1 x 1011 n cm-2 s-1. Following a decay time of 30 s (during which time the microcentrifuge tube was removed from the 7-mL irradiation rabbit) each sample was counted for 30 s, in live time. The Dy content of the spheres was quantified by measuring the 108 keV gamma-ray emission of the short-lived radionuclide 165mDy
with a hyperpure Ge detector. Signals from the Ge detector were amplified using an Aptec 6300 spectroscopy amplifier before being processed by an Aptec V4.3 PC-based MCA card (Aptec Instruments Ltd., Concord, ON, Canada). Dysprosium was determined via the semiabsolute method of neutron activiation analysis (Bergerioux et al., 1979).
Detection Limit
Estimates of the detection limit number of spheres were calculated on the basis of an estimated soil background Dy content of 4 µg g-1, an assumed 13.7% Dy content of the spheres, and the measured mean sphere mass. These values were used as a basis for the addition of known numbers of spheres to 5-g soil samples (Typic Cryoboroll, sampled from Ellerslie Research Station, Edmonton, AB, Canada) and subsequent INAA for Dy content, and hence sphere content. The number of spheres added to each soil sample represented approximately 0.5, 1, and 2 times the estimated detection limit. Samples of each number of added spheres were replicated eight times in the large fraction, and six times in the other fractions. The 5-g soil–sphere mixtures were irradiated in the SLOWPOKE reactor for 240 s at a nominal neutron flux of 0.2 x 1011 n cm-2 s-1. Activation of the soil matrix released an initially high level of short-lived radioactivity (e.g.,
, and
). Consequently, Dy in the spheres contained in the soil sample was detected following a decay period of 30 min to reduce the interfering signal from high levels of 28Al activity. The samples were counted for 300 s in live time using a vertically facing 19% hyperpure Ge detector attached to an Aptec PC-based MCA card (Aptec Instruments Ltd.) and automatic sample changer. The Dy content of each sample was determined by measuring the 94 keV gamma-ray emission of the longer-lived Dy radionuclide,
. Dysprosium was again determined via the semiabsolute method of NAA (Bergerioux et al., 1979).
Tracer Integrity
Tracer sphere and label element integrity were tested by leaching from spheres and by sphere disintegration experiments. In leaching experiments, 200 mg of spheres were placed in a 125-mL Nalgene bottle with 100 mL of deionized water and allowed to stand for 48 h. The disintegration experiments were similar with the addition of three 6-mm diameter glass beads and reciprocal shaking for 48 h. Dy label released due to leaching or sphere disintegration was determined by INAA, by analyzing 2-mL aliquots of the respective solutions taken after a short shaking to resuspend any fine particles. Samples were irradiated for 30 s at a nominal neutron flux of 5 x 1011 n cm-2 s-1 and following a decay period of 45 s were counted for 30 s in live time. The 22% Ge detector and spectroscopy system outlined above were used for data collection. The Dy content of the samples was quantified using the short-lived isotope, 165mDy. Aluminum, as indicated by 28Al, was also measured to assess tracer sphere disintegration.
|
Results
|
|---|
Surface Characteristics
The results of the XRD analyses of the kiln-fired and unfired sphere samples indicate that the mineral structure of the spheres is maintained during firing. Strong reflections associated with feldspathoid minerals (d-spacing of 3.30–3.18 
) were generated by both the fired and unfired samples, although the fired sample showed decreased intensity. The decreased intensity indicates a minor loss of crystalinity during firing of the prills.
Mass and Size Variability
Variability of sphere masses in the samples mounted on adhesive tape was a function of sphere size, wherein variability decreased with decreasing sphere size (Table 1)
. When compared with the much smaller data set of the single sphere masses collected for INAA (Table 2)
, sample size shows a significant effect on the variability in mass data.
Particle-size distribution analysis by image analysis are reported as calculated mean effective sphere diameters (Table 3)
and histograms of the distributions of these diameters within size fractions (Fig. 2)
. Variability in the size of spheres in each of the size fractions decreased with decreasing sphere size, which is consistent with mass variability.
Dysprosium Concentration
Mean single sphere Dy concentrations by mass were 11.0 ± 1.2% for the small, 11.8 ± 1.1% for the medium, and 11.4 ± 0.7% for the large size fractions, respectively. Within sphere size fractions, the respective coefficients of variation were 10.8, 9.0, and 6.5%. The mean Dy content for each fraction does not appear to be related to particle size (Fig. 3)
. Differentiation of tracer spheres from differing size fractions would require the use of additional labeling elements. However, our experiments are designed to use only one size fraction.
Detection Limit
Analysis of sphere–soil mixtures revealed that the detection limit of the method was more sensitive than anticipated from preliminary calculations (Table 4)
. Instrumental neutron activation analysis of soil samples containing no spheres showed a background level of 3.1 µg g-1 soil of Dy. In addition to determining the threshold numbers of spheres, the INAA data were used to determine a calculated number of spheres present based on mean sphere mass and measured Dy content. The calculated number was then compared with the actual number by a paired t-test; actual and calculated numbers of spheres did not differ significantly at P = 0.05 (Table 4).
Tracer Integrity
Dysprosium was not detected in the liquid aliquots taken from samples after 48 h of leaching. However, some Dy and Al were measured in aliquots taken from samples that had been shaken with glass beads for 48 h. These results indicated the Dy does not readily leach from the spheres, but that the spheres are subject to some minor disintegration when abrading forces are applied. However, the amount of Al detected by INAA in the aliquots was not significantly higher than in blank water samples. One would expect that if the spheres are disintegrating and releasing Dy, they would also be releasing Al from the nepheline syenite. It is suspected that the Dy released during shaking was originally in particulate Dy2O3 form and located on the surface of the tracer particles. In any case, the amount of Dy measured after 48 h of shaking represented <1% of the Dy content in the particles. Tracer and label integrity are sufficient for practical experimental use.
|
Discussion
|
|---|
For a tracer particle to be used as a surrogate for soil aggregates and particles, it must have similar physical and chemical properties to soil particles. Unlike plastic or glass beads, the ceramic prills have surface properties similar to soils, which allows for normal interactions between tracer and soil. The XRD results indicate that the kiln firing of the prills does not fuse the material to create a physicochemically amorphous surface. This is an important result as it supports the decision to use the ceramic prills as an appropriate surrogate for soil particles and aggregates.
An important property of particle tracers is their density. The density of particle tracers reported in the literature range from 0.35 g cm-3 (Staricka et al., 1992) to upwards of 3.5 to 4 g cm-3 (estimated from Lindstrom et al., 1990), most lying near the particle density of soil (2.65 g cm-3). The densities of the tracers developed in this work, as calculated from mean mass and diameter data, were 3.04, 3.11, and 1.63 g cm-3 for the large, medium, and small size fractions, respectively. Tracer density is an important property that determines how a tracer may behave within the soil matrix or on the surface of a hillslope. An advantage of using Macrolite as a tracer material is that the density of the spheres may be customized to some degree by altering the kiln firing process to produce heavier or lighter spheres. Lighter spheres such as those used by Staricka et al. (1992) can be useful, as they are easily extracted by floatation when soil samples are washed. However, the use of INAA precludes the need for tracer sphere extraction, and hence tracers can more closely simulate soil particle or aggregate densities.
Variability in the size, mass, and Dy concentration of tracer spheres are significant concerns because the detection of the tracers is based on the measured Dy content of a mixed sphere and soil sample. The variability affects the ability of INAA to detect and determine a given number of spheres in a soil sample. However, results of experiments indicate that this source of error may not be as significant as originally suspected. The t-tests performed to compare the actual and calculated sphere numbers showed no significant differences within sphere size fractions. This is an important result as it relieves some of the concern caused by variability in tracer sphere size, mass, and Dy concentration. The overall variability appears to be insufficient to affect the counting of spheres even at a level where few spheres are present. In addition, it should be noted that significant differences in sphere properties between size fraction is not a concern because a given experiment will use only one size fraction.
Subsequent to the development of the tracer spheres as reported here, their application in studies examining soil translocation due to bioturbation, tillage, or water and wind erosion is similar to previously reported studies, save that the method of detection in this case is by INAA. Soil translocation at field scales can be examined by uniformly applying the Dy-labeled tracer spheres and subsequently sampling the soil spatially and quantifying the distribution of tracers by INAA. Knowing two or more states of tracer distribution, the translocation of the tracers, and hence soil, can be calculated similarly to Lindstrom et al. (1990, 1992).
Field- and laboratory-scale experiments are planned which utilize the small size fraction of the developed tracer spheres in studies examining soil aggregation dynamics. These are smaller than those in previous studies of Staricka et al. (1992). Aggregate dynamics are studied by observing the incorporation and release of tracer particles into and out of soil aggregate size fractions with time and under various conditions. The experimental protocol involves adding the tracer spheres to soil, allowing the treatments of concern to express themselves, sampling the soil, separating the aggregate fractions of concern by appropriate means (e.g., dry or wet sieving), and applying INAA to each aggregate fraction to determine Dy content, and hence sphere content. One such experiment that is currently underway involves the field application of tracers to plots where the organic C dynamics are well known to link the C dynamics with the aggregate dynamics. The expectation is that soil with a rapid turnover of organic C will also display a more rapid aggregate turnover, when compared with soil with slower organic C dynamics.
Use of INAA provides a detection method that is not only highly sensitive, but also independent of many soil and environmental variables. Detection of tracer spheres depends only on the Dy-label concentration of the spheres, sample mass, and the soil background concentration. This allows its application in varying water contents, bulk densities, and soil and vegetation types. It should be noted that while specialized, neutron irradiation facilities are not uncommon (e.g., six SLOWPOKE reactors in Canada, >40 TRIGA and other research reactors in the U.S., and other reactor facilities around the world) and the cost of analyses is moderate ($6.00–10.00 per sample).
|
Conclusions
|
|---|
Ceramic prills of various sizes, enriched with a Dy label (10–15% Dy), and detected by instrumental neutron activation analysis can provide a sensitive tool for use in various studies examining soil particle and aggregate interactions, such as translocation (e.g., erosion) and rearrangement (e.g., aggregate dynamics). The use of the Macrolite prills is preferred over other potential tracers since the tracer closely simulates soil aggregate properties. The addition of Dy as a labeling element allows for the use of a rapid, routine, and nondestructive INAA detection technique, eliminating the need for visual inspection, tracer extraction, or radioactive tracers. While developed for use in studies of soil aggregate dynamics, the tracer spheres may be ideally suited to soil erosion, bioturbation, or other soil translocation studies.
|
ACKNOWLEDGMENTS
|
|---|
Mr. J.C. Stensrud of Kinetico Inc. for the kind contribution of Macrolite material and addition of Dy label during manufacture, Dr. M.J. Dudas for XRD analyses, Dr. D.A. Craig for particle-size distribution analyses. This work has been supported by NSERC operating funds (to W.B. McGill) and an Agriculture & Agri-Food Canada scholarship (to A.F. Plante).
Received for publication September 22, 1998.
|
REFERENCES
|
|---|
- Allmaras R.R., Copeland S.M., Copeland P.J., Oussible M. Spatial relations between oat residue and ceramic spheres when incorporated sequentially by tillage. Soil Sci. Soc. Am. J. 1996;60:1209-1216.[Abstract/Free Full Text]
- Bergerioux C., Kennedy G., Zikovsky L. Use of the semi-absolute method of neutron activation analysis. J. Radioanal. Chem. 1979;50:229-234.
- Christensen B.T. Carbon in primary and secondary organo-mineral complexes. In: Carter M.R., Stewart B.A., eds. Structure and organic matter storage in agricultural soils. Boca Raton, FL: CRC Press, 1998:97-165.
- DeSoete D., Gibjels G., Hoste J. Neutron activation analysis. London: Wiley, 1972.
- Dexter A.R. Advances in characterization of soil structure. Soil Tillage Res. 1988;11:199-238.
- Erdtmann G., Petri H. Nuclear activation analysis: Fundamentals and techniques. In: Elving P.J., ed. Treatise on analytical chemistry. Pt. 1. Vol. 14. New York: Wiley, 1986:419-643.
- Essington M.E., Mattigod S.V. Element partitioning in size- and density-fractionated sewage sludge and sludge amended soil. Soil Sci. Soc. Am. J. 1990;54:385-394.[Abstract/Free Full Text]
- Govers G., Vandaele K., Desmet P.J.J., Poeson J., Bunte K. The role of tillage in soil redistribution on hillslopes. Eur. J. Soil Sci. 1994;45:469-478.
- Jastrow J.D., Bouton T.W., Miller R.M. Carbon dynamics of aggregate-associated organic matter estimated by carbon-13 natural abundance. Soil Sci. Soc. Am. J. 1996;60:801-807.[Abstract/Free Full Text]
- Lindstrom M.J., Nelson W.W., Schumacher T.E. Quantifying tillage erosion rates due to moldboard plowing. Soil Tillage Res. 1992;24:243-255.
- Lindstrom M.J., Nelson W.W., Schumacher T.E., Lemme G.D. Soil movement by tillage as affected by slope. Soil Tillage Res. 1990;17:255-264.
- Oades J.M., Waters A.G. Aggregate hierarchy in soils. Aust. J. Soil Res. 1991;29:815-828.
- Parry S.J. Activation spectrometry in chemical analysis. New York: Wiley, 1991.
- Puget P., Chenu C., Balesdent J. Total and young organic matter distributions in aggregates of silty cultivated soils. Eur. J. Soil Sci. 1995;46:449-459.
- Riebe B. Monitoring the translocation of soil particles using a neutron activated tracer. In: Hartge K.H., Stewart B.A., eds. Soil structure: Its development and function. Boca Raton, FL: CRC Press, 1995:277-297.
- Ritchie J.C., McHenry J.R. Application of radioactive fallout Cesium-137 for measuring soil erosion and sediment accumulation rates and patterns: A review. J. Environ. Qual. 1990;19:215-233.[Abstract/Free Full Text]
- Staricka J.A., Allmaras R.R., Nelson W.W. Spatial variation of crop residue incorporated by tillage. Soil Sci. Soc. Am. J. 1991;55:1668-1674.[Abstract/Free Full Text]
- Staricka J.A., Allmaras R.R., Nelson W.W., Larson W.E. Soil aggregate longevity as determined by the incorporation of ceramic spheres. Soil Sci. Soc. Am. J. 1992;56:1591-1597.[Abstract/Free Full Text]
- Staricka J.A., Burford P.M., Allmaras R.R., Nelson W.W. Tracing the vertical distribution of simulated shattered seeds as related to tillage. Agron. J. 1990;82:1131-1134.[Abstract/Free Full Text]
- Terpstra R. Formation of new aggregate under laboratory-simulated field conditions. Soil Tillage Res. 1989;13:13-21.
- Tisdall J.M., Oades J.M. Organic matter and water-stable aggregates in soils. J. Soil Sci. 1982;33:141-163.
- Toth S.J., Alderfer R.B. A procedure for tagging water-stable soil aggregates with Co60. Soil Sci. 1960;89:36-37 a.
- Toth S.J., Alderfer R.B. Formation and breakdown of Co60-tagged water-stable soil aggregates in a Norton silt loam soil. Soil Sci. 1960;90:232-238 b.
- Van Veen J.A., Kuikman P.J. Soil structural aspects of decomposition of organic matter by micro-organisms. Biogeochemistry 1990;11:213-233.[ISI]
- Young R.A., Holt R.F. Tracing soil movement with fluorescent glass particles. Soil. Sci. Soc. Am. Proc. 1968;32:600-602.
- Young R.A., Mutchler C.K. Soil movement on irregular slopes. Water Resour. Res. 1969;5:1084-1089.
TOP
ABSTRACT
INTRODUCTION
