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Published online 9 January 2007
Published in J Environ Qual 36:280-290 (2007)
DOI: 10.2134/jeq2006.0056
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Plant and Environment Interactions

Time-Dependent Distribution of Surface-Applied Radionuclides and their Recovery in Maize during the Growing Season

T. Centofantia,*, E. Frossarda and H. Flühlerb

a Plant Nutrition, Institute of Plant Sciences, ETH Zurich, Eschikon 33, CH-8315 Lindau (ZH) Switzerland
b Soil Physics, Institute of Terrestrial Ecology, ETH Zurich, Universitätstrasse 16, CH-8044 Zurich, Switzerland

* Corresponding author (tiziana.centofanti{at}gmail.com)

Received for publication February 10, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The spatial and temporal heterogeneity of field soils influences the fate and behavior of strongly sorbing pollutants and their entry into the food chain. We studied the redistribution of surface-applied 54Mn, 65Zn, 57Co, and 134Cs in the soil profile and their recovery in the aerial parts of maize grown on an untilled agricultural soil during the growing season. Radionuclides were more concentrated in the preferential flow paths (PFP) than in the soil matrix and their concentration decreased with time. The recovery of 54Mn in the aerial plant parts increased between pollen shed and maturity, while the recovery of 65Zn and 57Co did not show any significant difference, and the recovery of 134Cs decreased with time. The amount and distribution of rainfall, and the chemical, physical, and microbiological soil characteristics are the major factors influencing the variation of radionuclide recovery with time.

Abbreviations: PFP, preferential flow path • AY, acid yellow 7


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
RECENT studies, performed in forest and agricultural soils (Bundt et al., 2000; Centofanti et al., 2005) demonstrated that surface-applied radionuclides distribute heterogeneously through the soil profile. This behavior was explained by the structure-induced water flow paths and the sorption of radionuclides onto the surface of preferential flow paths (PFPs). Centofanti et al. (2005) have also suggested that the small fraction (12.5 ± 2.5%) of the roots located within and near the PFPs was responsible for the uptake and translocation of surface-applied 134Cs and 57Co.

Studies on the response of roots to the heterogeneous and patchy distribution of nutrients demonstrated that roots display morphological (flexibility in architectural patterns) and/or physiological plasticity (altered nutrient uptake capacity and increased ion affinity) (Hodge, 2004). Centofanti and Frossard (2006) have shown that a single root of maize has the capacity of taking almost half of the 134Cs taken up by the whole root system when grown in a medium with a high 134Cs concentration and a low K concentration. These findings suggest that the uptake of radionuclides can change during the plant growth cycle because roots grow and explore larger soil volumes. This increases the root accessibility to those areas that exhibit elevated radionuclide concentrations.

The aim of the present study was to assess the time-dependent variations of the distribution of four surface-applied radionuclides (54Mn, 57Co, 65Zn, and 134Cs) in the soil profile and their recovery in maize as affected by the root distribution and accessibility of radionuclides within the soil profile. We postulated that surface-applied radionuclides are: (i) heterogeneously distributed in the soil profile, (ii) are more concentrated in active PFPs (zones, in which a rapid transport of water and solutes is observed) than in the soil matrix (bypassed soil regions), and (iii) radionuclide recovery by plant increases during the plant growth cycle because the root system develops and explores an increasing fraction of the soil volume, reaching more frequently the PFPs.

Variation of radionuclide recovery during the growing season of maize (April through September, 2002) was studied in an untilled agricultural soil. Plants were harvested at pollen shed and maturity. At harvest we analyzed plant biomass production, radionuclide content, and root weight density. After harvest, radionuclide distribution in the soil profile and the interrelation between roots and flow paths distribution patterns were also analyzed.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Design of Field Experiment
Soil Characteristics
The field experiment was conducted on an untilled agricultural soil near Tänikon (47°28'53'' N; 8°54'22'' E), Switzerland, at the Swiss Federal Research Station for Agricultural Economics and Engineering (FAT) on the Grund plot. The soil is a sandy loam Gleyic Cambisol (FAO, 1999). Soil chemical and physical properties are given in Table 1. The slope of the field is approximately 2%. The experimental site was untilled for more than 10 yr and wheat was grown before our experiment. A full description of this experimental site can be found in Anken (2004). On the vertical cut of the soil profile we observed many macropores, primarily earthworm burrows which formed a vertically oriented continuous network of pores.


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  		Table 1. Selected chemical and physical properties of the soil at the field site.

 
Experimental Design
The experimental area was 4.9 m long and 4.2 m wide, subdivided into three plots (Fig. 1). The plants were distributed in rows with a distance of 0.7 m between the rows and a distance of 0.15 m between plants of the same row. Each plot contained seven rows of eight plants, totaling 56 plants per plot. The radionuclide and dye tracer solutions were applied onto three inner areas (0.6 x 0.7 m) of each plot encompassing one row and four plants per row (Fig. 1). Plants were sampled in these inner areas only. The plant rows outside the inner area were used as buffer. On 21 Apr. 2002, four time domain reflectometry (TDR) probes and four tensiometers were installed at two vegetation-free locations near the plant rows, at depths of 10, 20, 45, and 60 cm, and of 15, 30, 45, and 60 cm, respectively. One trench was excavated directly adjacent to a plant row, and the other was placed at 50 cm from the nearest maize's stalk (Fig. 1).


Figure 1
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  		Fig. 1. Plot design. (A) Overview of the layout in the field experiment. x = plants; gray rectangles = inner areas, traced with radionuclides and dyes; white areas = buffer area to minimize edge effects; black rectangles = instrumented trench close to the maize row (maize soil) and 50 cm from the experimental plots (vegetation-free soil); (B) sampling scheme in the inner areas.

 
Application of Radionuclide Solution
On 10 Apr. 2002, a solution of 1% of glyphosate (Roundup) was applied at a rate of 0.42 mL m–2. On 28 Apr. 2002, a solution containing 54Mn, 65Zn, 57Co, and 134Cs, all in chloride form diluted in an acidic solution, was applied onto the soil surface of each inner area (Table 2). Radionuclides were purchased at Amersham (Germany). We have chosen these man-made {gamma}-emitter radionuclides because of their absence in the natural system. They can also be readily measured by {gamma}-spectroscopy at very low activities, allowing field application below the Swiss safety standards (Bewilligungsgrenze der Strahlenschutzverordnung). Furthermore, their different chemical and physiological properties (Centofanti, 2005) allowed us to make a more consistent and more widely applicable evaluation of the influence of the physical and chemical soil heterogeneities on the soil to plant transfer of radionuclides.


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  		Table 2. Characteristics and quantities of the four radionuclides (54Mn, 65Zn, 57Co, and 134Cs) applied onto the soil surface of an untilled agricultural soil.

 
The solution was applied manually, using a watering can with a sprinkling bar fixed at the end of the spout. To avoid surface ponding the solution was applied in six slops of equal amounts of about 6.6 L within a period of 6 h. Forty liters of solution containing 18.7, 91.3, 18.8, and 29.2 MBq l–1 of 54Mn, 57Co, 65Zn, and 134Cs, respectively, were applied on each inner area. This amount of water corresponded to a heavy thunderstorm shower. The radionuclides were dissolved in Osmosis I water of an electrical conductivity of 17 µS cm–1 at 25°C. To avoid lateral infiltration toward the rows, a similar amount of Osmosis I water was applied at the same rate on the buffer area outside the central plots.

Plants
On 30 Apr. 2002, furrows were manually prepared and maize (Zea mays L. cv. Corso) was sown. After sowing, the soil was fertilized with 110 kg N ha–1 as NH4NO3 in solid form as recommended for maize production in Switzerland (Ryser et al., 2001) and was mechanically spread onto the whole area. The amount of available P and K in this soil was sufficient to allow a proper maize growth (Anken, 2004). The plants were not irrigated to reduce irrigation-induced leaching of the radionuclides. During the growing period shoot height was regularly measured. A weather station placed 60 m from the site recorded air and soil temperatures, rainfall, and solar radiation during the entire experiment (Table 3). Plants were harvested at two development stages: (a) at the pollen shed, 29–31 July 2002, and (b) at maturity, 28–30 Sept. 2002.


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  		Table 3. Monthly meteorological data recorded during the growing period of the maize plants in the field experiment in 2002.

 
On 6 June 2002, hail damaged about one to two plants per inner area. The hail destroyed part of the leaf and in some cases half of the shoot was cut off. These shoots were collected and used for analysis. Their dry weight was 0.4 ± 0.08 g and their radionuclide recovery, expressed as Bq Bq–1 x 103 (shoot activity/total of applied activity per plant), was: 0.006 ± 0.002, 0.02 ± 0.003, 4.7 x 10–6 ± 1.3 x 10–6, and not detectable for 54Mn, 65Zn, 57Co, and 134Cs, respectively. The damaged plants were replaced by planting 10-d-old seedlings to allow their harvest at pollen shed and at maturity. One row per plot was cut at harvest for each development stage (Fig. 1), having 12 replicates per development stage. The shoots were harvested by cutting the stem 1 cm above the soil level, and then washed with tap water, chopped, and oven-dried at 105°C for 24 h. The whole shoot (stem, leaves, and inflorescence of male flowers) was used for plant analyses, and at maturity grains were separated from the rest of the plant. The dry weight was determined and the dry material milled to powder. The ground mass of shoots and grains was weighed and transferred into calibrated {gamma}-spectrometry containers. Part of the ground samples (0.25 g) of shoots harvested at pollen shed and maturity were digested in a microwave oven (Centofanti and Frossard, 2006) and analyzed for total Mn, Zn, Co, Cs, P, and K concentration by ICP–MS. Nitrogen content was determined with a CN analyzer.

Dye Tracers Application
One week before harvest a fluorescent tracer, acid yellow 7 (AY; Fluka Chemie AG, Buchs, Switzerland) was applied onto the soil surface of the inner area (Fig. 1). The area was irrigated for 8 h per day over a period of 5 d with AY solution (8 g l–1 of Osmosis I water). The cumulated amount of irrigation was 48 L of solution per inner area (23 mm d–1). The solution was applied with a spraying can which was manually operated. The surrounding area was protected with a tarp to prevent lateral spreading of the solution to adjacent plots. The fluorescence intensity on the excavated profile surfaces does not change when exposed to daylight for a few hours and it is pH independent in the range from 4 to 9. The background fluorescence of the soil itself does not interfere significantly with the dye emission (Aeby et al., 2001). After cutting the plants at harvest, 16 L of Osmosis I water solution containing the food stuff dye brilliant blue FCF (C.I. 42090) (5 g l–1), was manually applied onto the soil surface of each inner area over a period of 6 h (6.4 mm h–1). The dye tracer brilliant blue is well suited to visualize the cumulative flow pattern of the infiltrating water because of its low toxicity, high visibility, and high mobility (Flury and Flühler, 1994). The dye tracer brilliant blue is slightly more mobile than the tracer AY, but both can be used under field conditions using them as observable surrogate compounds that behave similarly to radionuclides (Aeby et al., 2001). Therefore we applied the dye tracer AY to obtain detailed imaging of the PFPs areas in the soil profile excavated after harvest, whereas we used brilliant blue to sample the PFP areas (stained) and the soil matrix (unstained).

Mapping Root and Flow Path Distribution
On the inner area six horizontal profiles were prepared at consecutive depths of 0.13, 0.18, 0.2, 0.35, 0.38, and 0.4 m (Fig. 1). They had an area of 0.6 m parallel to the row and 0.5 m perpendicular to the row. On the horizontal planes the occurrence of roots intersecting the plane of observation was mapped onto polythene sheets using a felt-tip pen. A grid of 5 cm x 5 cm was drawn on the sheet to systematically locate and document root occurrence (Tardieu and Manichon, 1986). Root maps were scanned and digitized using Scion Image (version beta 4.02) for recording the x, y coordinates of the individual roots. Images of the distribution of the surface-applied fluorescent AY were taken on the same area used for the root mapping by using an imaging device consisting of a high-power xenon lamp and a sensitive charge coupled device (CCD) camera. The fluorescence images (1242 by 1152 pixels) were corrected for nonuniform lighting, changing surface roughness, and varying optical properties of the soil profile. A detailed description of the device used and of the image processing procedure is given in Aeby et al. (2001). The corners of the grid borders were used as reference points to superimpose the fluorescence images and the digitized root maps. The area stained by AY (PFPs) and the number of roots occurring within the PFPs were determined using IDL (version 4.01 of Interactive Data Language, Research Systems).

Analysis of Radionuclide Distribution on Soil Horizontal Profiles
Soil samples were taken from the regions stained by the brilliant blue, interpreted as the areas of PFPs, from the unstained soil matrix, and from 2-cm fringe areas surrounding the flow paths. The samples were cut with a sharp knife at 1-cm thickness. Samples from the stained regions and from the soil matrix were taken in equal number for each horizontal plane analyzed. All the samples were oven-dried at 60°C for 48 h until they reached a stable weight. Then they were ground and put in plastic containers for {gamma} spectrometry measurement. The Brilliant Blue concentration of the soil samples was quantified by centrifuging 3 g (dry matter basis) of soil with 30 mL of nanopure water repeatedly until the water solution appeared clear (no more blue). The sequential water extracts from each sample were pooled together and the concentration of dye was measured with a UV/visible light spectrophotometer at a wavelength of 630 nm. The soil matrix was defined as soil containing <0.03 mg g–1 of brilliant blue.

Root Weight Density
In each inner area six cylindrical cores (20 cm long and 5 cm in diameter) were sampled at two depths (0 to 15 cm and 15 to 30 cm) using a hand-held auger. To obtain a realistic representation of the root distribution in relation to the plant position, samples were taken at 5 cm, 15 cm, and 30 cm from the plant row, on both sides of the plant rows (Fig. 1). Soil was rinsed off from the root samples using the hydropneumatic elutriation system developed by Smucker et al. (1982). Roots were freeze-dried for 48 h. Root dry weight was determined and the root weight density (g cm–3) calculated. Measurements of the radionuclide activities in root samples obtained from the field experiment were erratic and unreliable due to the fine soil particles adhering on the root epidermis.

{gamma} Spectrometry
All samples were analyzed for 65Zn (half-life t1/2 = 243.9 d, 1115.55 keV), 57Co (t1/2 = 271.7 d, 122.06 keV), 54Mn (t1/2 = 312.2 d, 834.84 keV), and 134Cs (t1/2 = 2.07 yr, 475.34 keV) at the {gamma} spectrometry laboratory of EAWAG (Swiss Federal Institute for Environmental Science and Technology, Dübendorf, Switzerland) using high purity Ge detectors. Plant, roots, and soil samples were measured with flat crystals. Radionuclide activities were determined in Bq g–1 (dry weight); decay corrected to the common date of 3 July 2002 at 12:00 a.m. The measurement errors were 5 to 10%. Geometry correction and calibration are based on standard solutions. Samples were measured for different periods of time with varying soil masses, which resulted in different detection limits. On average, samples were measured for 20 h and the detection limits were in the range of 10–2 to 10–3 Bq g–1 (dry weight).

Statistical Analysis
Mean values for plant dry matter production, recovery of radionuclides, concentrations of stable elements and root weight density between plant at pollen shed and maturity were tested by Duncan's multiple range test after ANOVA. All tests were conducted at the 5% significance level.

The linear relation between the concentration of radionuclides and of brilliant blue is expressed by the Pearson product-moment correlation coefficient, denoted r.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant Growth and Recovery of Radionuclides
The shoot dry matter production is shown in Table 4. These results are consistent with the dry matter production of maize grown on an untilled Gleyic Cambisol located in Switzerland (Centofanti et al., 2005).


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  		Table 4. Plant height and dry matter measured at two development stages of maize grown in an untilled agricultural soil. Means and standard error between twelve replicates are given.

 
Value of the root weight density (Table 5) showed a slight increase with time, although not statistically significant, and a significant decrease with increasing soil depth. These results agree with those of Chassot et al. (2001) who found that root length density of maize grown in an untilled soil decreased strongly with increased depth and horizontal distance from the plant row.


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  		Table 5. Root weight density measured at three distances from the plant row (5, 15, and 30 cm) and at two soil depths (0 to 15 cm and 15 to 30 cm). Means and standard errors between six replicates are given.

 
The recovery of 54Mn in the aerial part of maize increased with time showing significantly higher values at later harvest times (Table 6). Contrarily, the recovery of 65Zn in the aerial part of maize (shoots and grains) at maturity was lower than at pollen shed, although these differences were not statistically significant. The recovery of 57Co in the aerial part of maize was similar in shoots harvested at pollen shed and maturity (Table 6). 134Cs was not detectable at maturity. Only 54Mn and 65Zn were detectable in the grains.


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  		Table 6. Recovery of surface-applied radionuclides in plants harvested at pollen shed and maturity. Means and standard errors of twelve replicates are given.

 
The total concentration of Mn, Zn, and Co remained constant between pollen shed and maturity while that of Cs increased (Table 7). The higher concentration of P at pollen shed than at maturity was not statistically significant, whereas the concentration of N and K was significantly lower at maturity (Table 7). At pollen shed plants showed an adequate P and K nutritional status as 2.5 to 3.5 mg P g–1 and 17 to 23 mg K g–1 are needed in the aerial part of maize for optimum growth (Bergmann, 1992). The use of the model proposed by Plénet and Lemaire (1999) to assess critical concentration of N in maize shoot indicates that plants at pollen shed were N sufficient. The specific activity of 54Mn, 65Zn, and 57Co was significantly lower at maturity; the specific activity of 134Cs at pollen shed shows a high value in comparison to the other radionuclides (Table 8).


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  		Table 7. Concentration of Mn, Zn, Co, Cs, P, N, and K in the shoots of maize harvested at pollen shed and maturity. Means and standard errors of twelve replicates are given.

 

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  		Table 8. Specific activity of Mn, Zn, Co, and Cs calculated for the shoot of maize harvested at pollen shed and maturity.

 
Precipitation and Soil Water Content
Several short-time (1 to 2 d) rainfall events of relatively high intensity occurred during the 6 mo of the experiment (Fig. 2) and caused fluctuation in the soil water content (Fig. 3). Thus, starting from beginning of April until the end of the experiment, periods of 10 to 15 d showing volumetric water contents of 0.32 m3 m–3 alternated with periods of 20 to 30 d of water contents close to saturation (0.40 m3 m–3). Except shortly after harvest (maturity), the matric potential remained in the tensiometric range of h > –850 mbar, indicating suf ficient water availability.


Figure 2
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  		Fig. 2. Diagram of the daily rainfall intensity (mm d–1) measured during the plant growing season.

 

Figure 3
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  		Fig. 3. Volumetric water contents and matric potentials measured at different soil depths and at 5 and 40 cm from the maize row (as indicated in Fig. 1). The measurements were performed during the entire growing season.

 
Displacement of Surface-Applied Radionuclides and Relation between Roots and Preferential Flow Path Distribution Patterns
Surface-applied radionuclides were distributed heterogeneously in the soil profile. All applied radionuclides showed significantly higher concentrations in the PFPs compared to the soil matrix. The samples from the 2-cm fringe outside the stained PFPs (Fig. 4 and 5) showed concentrations comparable to those of the soil matrix. A higher concentration of 57Co was observed in the soil matrix at 18- to 20-cm depth. At pollen shed and at maturity the amount of radionuclides found in the PFPs (from 0- to 40-cm depth) was about 14 and 4% of that applied, respectively. The concentrations of the radionuclides in the first 5 to 10 cm of the soil profile measured at pollen shed were in the range of: 0.07 to 0.004 Bq g–1 for 134Cs, 0.11 to 0.008 Bq g–1 for 65Zn, 0.64 to 0.008 Bq g–1 for 54Mn, and 1.07 to 0.01 Bq g–1 for 57Co (R. Penfield, personal communication, 2004). No data are available for this horizon at maturity. All four radionuclides were measurable down to a depth of 40 cm, though their concentration in the PFPs decreased with time.


Figure 4
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  		Fig. 4. Growth stage: pollen shed. Activities of 54Mn, 65Zn, 57Co, and 134Cs measured in the unstained soil matrix, in the stained preferential flow paths, and in the 2-cm fringe just outside the stained flow paths. Samples were taken at harvest time when plants reached pollen shed. Means and standard errors are given. The symbols *, **, and *** indicate significance at the 0.05, 0.01, and 0.001 probability levels, respectively; NS is not significant at the 0.05 probability level and ND is not detectable.

 

Figure 5
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  		Fig. 5. Growth stage: maturity. Activities of 54Mn, 65Zn, 57Co, and 134Cs measured in the unstained soil matrix, in the stained preferential flow paths, and in the 2-cm fringe just outside the stained flow paths. Samples were taken at harvest time when plants reached maturity. Means and standard errors are given. The symbols *, **, and *** indicate significance at the 0.05, 0.01, and 0.001 probability levels, respectively; while NS is not significant at the 0.05 probability level and ND is not detectable.

 
The soil samples taken from the matrix and the 2-cm fringe outside the PFPs exhibit brilliant blue concentrations within the conservative threshold of 0.03 mg of dye per g of soil (Fig. 6 and 7). Linear relations were observed between 54Mn and 57Co activity and brilliant blue concentration at pollen shed. At maturity, the data points were more scattered. No linear relation could be observed between 65Zn and 134Cs concentrations and brilliant blue, since much lower activity was measured in the PFPs (Fig. 6 and 7).


Figure 6
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  		Fig. 6. Growth stage: pollen shed. Relation between brilliant blue and the concentration of radionuclides in the three soil compartments: preferential flow paths; 2-cm fringe just outside the stained flow path areas; soil matrix; each point represents the average of the soil samples taken on horizontal profiles at 15-, 18-, 20-, 35-, 38-, and 40-cm depth. Samples were taken at harvest time when plants reached pollen shed. r denotes the Pearson product-moment correlation coefficient.

 

Figure 7
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  		Fig. 7. Growth stage: maturity. Relation between brilliant blue FCF and the concentration of radionuclides in the three soil compartments analyzed: preferential flow paths; 2-cm fringe just outside the stained flow path areas; soil matrix; each point represents the average of the soil samples taken on horizontal profiles at 15-, 18-, 20-, 35-, 38-, and 40-cm depth. Samples were taken at harvest time when plant reached maturity. r denotes the Pearson product-moment correlation coefficient.

 
The analysis of the overlaid root maps and fluorescent tracer images (Fig. 8 and 9) showed that the area covered by the dye tracer increased between pollen shed and maturity. In the upper 0 to 30 cm of soil depth the dye coverage area was larger and the number of roots occurring in the PFPs was higher than at deeper soil depths (Table 9). The ratio between percentage of roots occurring in the stained areas and percentage of stained areas was similar at both harvest times.


Figure 8
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  		Fig. 8. Growth stage: pollen shed. Superimposed maps of root occurrence (black open circles) and fluorescence images of the distribution and concentration of the surface-applied fluorescent tracer (acid yellow 7). Horizontal profiles were cut at soil depths of 15, 18, 20, 35, 38, and 40 cm. Plants were at the pollen shed.

 

Figure 9
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  		Fig. 9. Growth stage: maturity. Superimposed maps of root occurrence (black open circles) and fluorescence images of the distribution and concentration of the surface-applied fluorescent tracer (acid yellow 7). Horizontal profiles were cut at soil depths of 15, 18, 20, 35, 38, and 40 cm. Plants were at maturity.

 

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  		Table 9. Analysis of spatial relation between root and fluorescent tracer distribution based on Fig. 8 and 9. For each horizontal profile the percentage of roots present within the stained flow path areas and the percentage of the area occupied by the stained flow paths expressed relative to the mapped area are reported.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The hypotheses that surface-applied radionuclides distribute heterogeneously in the soil profile and are more concentrated in the PFPs are confirmed, whereas the hypothesis that the recovery of radionuclides increases with time has been supported only for 54Mn, while the recovery of 57Co and 65Zn remained constant and that of 134Cs decreased.

The surface-applied radionuclides have been heterogeneously displaced into and through the soil profile due to the structure-induced non-uniform water flow and were mainly concentrated in the PFPs compared to the soil matrix. These results support those reported by Centofanti et al. (2005) who found a higher concentration of the same surface-applied radionuclides in the PFP relative to that in the soil matrix in an untilled agricultural soil. The concentration of radionuclides in the PFP decreased with time, although the surface areas of PFPs increased with time and in the upper 0 to 25 cm of soil profile. These variations could be due to the amount and distribution of rainfall during the experiment. The rainfall fluctuations and a sequence of periods of water content close to saturation (0.40 m3 m–3) alternating with periods of low water content (0.32 m3 m–3) strongly suggest that rapid drainage most probably has occurred through the macroporous PFPs. Thus, since radionuclides were sorbed on the solid surfaces in these pathways and none or very small amounts were found in the surrounding soil, they might have been transported further down the soil profile. It is very unlikely that the fluctuations in soil water content at these soil depths were caused only by root water uptake, since a similar pattern of water content fluctuation was observed in the nearby bare soil (Fig. 3).

An additional factor that may have caused the migration of fractions of radionuclides is the presence of numerous earthworm burrows that potentially act as important pathways for rapid infiltration (Edwards et al., 1990; Anken et al., 2004). Such macrostructures facilitate the infiltration and transport of soil particles acting as suspended carriers of sorbed radionuclides.

The transport of such fine-dispersed suspended particles is controlled by the hydraulic properties of the soil (Kuznetsov et al., 2001).

The amount of water applied with the radionuclides and the dye tracers did not significantly influence the downward transport of radionuclides because the solutions were applied during relatively dry periods with a low soil water content in the upper soil layers (0 to 30 cm) and low rainfall intensity.

This varying spatial distribution of the surface-applied radionuclides in the course of the growing cycle has probably modified the temporal and spatial patterns of radionuclide uptake by roots.

Despite the decrease of 54Mn, 65Zn, and 57Co concentration in the PFP with time, their recovery in maize increased or was constant with time. In our experiment the occurrence of periods of soil saturation might have caused temporarily reducing conditions which increased the 54Mn, 57Co, and 65Zn availability to the roots growing in the PFPs (McBride, 1989). The fluctuations in soil water content were most dramatic in the well-rooted top soil (0 to 30 cm) where a larger proportion of roots were located within the PFP areas. This suggests that roots growing in these layers and in the PFP areas were primarily responsible for radionuclide uptake, and also because zero or low activities were found in the adjacent soil matrix. In addition, many members of the bacterial and fungal genera Bacillus, Pseudomonas, Arithrobacter, Streptomycetes, and Aspergillus can mediate Mn reduction via abiotic and/or enzymatic reactions using Mn4+ as a terminal electron acceptor (Posta et al., 1994). Fungal hyphae of arbuscular mycorrhizal fungi having a diameter of a few µm can penetrate very fine pores and can transport Zn over distances up to 10 cm from the roots (Jansa et al., 2003). The decrease of the specific activity between pollen shed and maturity for Mn, Zn, and Co is due to the continuous uptake of the stable elements which are necessary for plant growth. The faster uptake of stable elements suggests: (i) that radionuclides have been isotopically diluted in the pools of stable elements in the soil and (ii) that these elements are taken up by the entire root system, both from the PFP and the soil matrix through the above described processes.

At pollen shed the high specific activity of Cs indicates that 134Cs was preferably taken up in relation to Cs. However, the uptake of Cs significantly increased with time. Although there is no known role of Cs in plant nutrition (Marschner, 1995), Cs can be transferred into plants due to its physiological similarity with K (Shaw and Bell, 1991). As Cs uptake depends on the K concentration in solution and on K uptake by the plant (Zhu and Smolders, 2000), the increase of Cs uptake may be a consequence of the plant K demand for grain maturation.

The decrease in the recovery of 134Cs with time can be attributed to the migration of 134Cs below the rooting zone. It is known that Cs is strongly sorbed on micaceous clay minerals (illite), and thus Cs migration rates are generally <2 cm yr–1 (Delvaux et al., 2001). Such migration rates very likely only apply to cases where matrix flow dominates, that is in absence of preferential flow. Furthermore, the high soil K content in the upper 0 to 20 cm of the soil profile might have caused a lower Cs sorption to the clay particles thus increasing the Cs mobility within the soil profile. In agreement with our results various authors (Ehlken and Kirchner, 1996; Krouglov et al., 1997; Rigol et al., 2002) have shown that the transfer of radiocaesium from soil to plant appears to decrease with increasing time between soil contamination and harvest of crops. They attributed it to a variety of processes including sorption on soil minerals, incorporation by soil microorganisms, and climatic conditions (Ehlken and Kirchner, 1996; Krouglov et al., 1997).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our experiment has shown that the displacement and transport of surface-applied radionuclides through the rooting zone can play an important role on time-dependent variations of the soil to plant transfer of some radionuclides. Indeed, this effect can vary depending on the type of radionuclide, the amount and distribution of rainfall, and the chemical, physical, and microbiological soil characteristics.

Macroporous flow is the most extreme form of preferential flow and most affecting for leaching strongly sorbing compounds. It occurs when fissures, cracks, furrows, and probably also roots form a network of continuous macropores. Such pathways are activated when high intensity and amount of rainfall take place (Weiler and Naef, 2003). However, flow is also heterogeneous under normal infiltration regimes and leads to an unexpected mobility of reactive molecules such as pesticides (Flury et al., 1995).


   ACKNOWLEDGMENTS
 
We thank Dr. T. Anken (FAL, Tänikon, Switzerland) for allowing experimentation on the field site Grund, for rendering available water and energy supplies of the FAL, and for providing soil physical, chemical, and meteorological data. A special thanks to R. Penfield (ITOe, ETH Zurich) for the results presented in section 3.3 and to H. Wydler, J. Leuenberger (ITOe, ETH Zurich) for their help in establishing, controlling and carrying out the field experiment, setting up and operate instrumentation devices, and helping in processing and analyzing image data. We also thank E. Grieder and J. Beer (EAWAG, Dübendorf) for the {gamma}-spectrometry measurements and Dr. A. Albrecht for preparing the radioactive solutions. T. Centofanti acknowledges the financial support of the Swiss National Foundation for Scientific Research (Project n. 3152-064135.00) and the Swiss Federal Nuclear Safety Inspectorate (HSK, Villigen-HSK, Switzerland).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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