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DIVISION S-5-PEDOLOGY

Metal Concentrations in Aggregate Interiors, Exteriors, Whole Aggregates, and Bulk of Costa Rican Soils

^a Institute of Soil Science and Soil Geography, Univ. of Bayreuth, D-95440 Bayreuth, Germany

wolfgang.wilcke{at}uni-bayreuth.de

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
In many temperate soils the preferential weathering and leaching of aggregate surfaces and the nonaggregated material between aggregates depletes geogenic metals. It also shifts metals from strongly to more weakly bound metal forms. Deposited metals are sorbed preferentially on aggregate surfaces and between aggregates. We examined whether preferential desilication under tropical climate causes an enrichment in the aggregate exteriors in oxidic forms of metals. We also studied where deposited metals are bound in these soils. Aggregates (2–20 mm) were selected manually from the A horizons of eight Oxisols, six Andisols, two Mollisols, and two Inceptisols in Costa Rica. All samples were fractionated into interior and exterior portions and treated with a seven-step sequence to extract Al, Cd, Cu, Fe, Mn, Pb, and Zn. Total concentrations of all metals except Zn were higher in the aggregate exteriors than in the interiors (average differences in % of the interior concentrations: Al, 9.0; Cd, 2.2; Cu, 8.7; Fe, 4.9; Mn, 4.2; Pb, 64.7; Zn, 0.0). The average Cd and Pb concentrations in easily extractable fractions were significantly higher in the aggregate exteriors. There were no significant differences in metal partitioning between interiors and exteriors except for Pb, which had higher proportions in extractable forms with NH₂OH · HCl > NH₄ - acetate, pH 6.0 > EDTA in the exteriors. There were few significant differences in metal concentrations and partitioning between bulk soil and whole aggregates. The results may be explained by (i) preferential desilication of the aggregate exteriors and (ii) preferential sorption of deposited heavy metals mainly in easily extractable forms.

Abbreviations: BS, base saturation • ECEC, effective cation-exchange capacity • EF, enrichment/depletion factor • EF_ex, qualitative differences between exteriors and interiors • EF_bu, qualitative differences between bulk soil and whole aggregates

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
HETEROGENEOUS METAL DISTRIBUTION in aggregated soil influences metal bioavailability and transport. Plant roots grow mainly on aggregate surfaces and between aggregates where nutrients and possibly toxic heavy metals are taken up (Whiteley and Dexter, 1983; Materechera et al., 1994). An important part of the soil water flows along preferential pathways (Jury and Flühler, 1992; Flury et al., 1994; Heuvelman and McInnes, 1997) contacting only aggregate surfaces and interaggregate soil where probably a considerable part of the sorption and desorption processes occur in the soil.

In temperate soils, concentrations of easily extractable metals are higher in the aggregate exteriors, while concentrations of more strongly bound metals are higher in the aggregate interiors (Wilcke and Kaupenjohann, 1997). Those authors explained their results with (i) preferential weathering of aggregate surfaces causing a shift from strongly bound (mainly within silicates) to other metal forms and (ii) preferential sorption of deposited metals on the aggregate surfaces resulting in a total enrichment of metals in the aggregate exteriors, particularly in easily extractable forms.

As the aggregation may not include all soil material, there are also differences between aggregated and nonaggregated material. First results indicate that the bulk soil has higher concentrations of heavy metals introduced to the soils via atmospheric deposition and lower concentrations of geogenic metals than aggregates (Wilcke et al., 1998b). The interaggregate soil material seems to act as preferred sorbent for deposited heavy metals, whereas geogenic metals are preferentially leached. However, part of the small-scale heterogeneity of metal concentrations and partitioning in soils may also be explained by the heterogeneous distribution of metal sorbents. In temperate soils, which do not show illuviation phenomena such as clay or humus coatings on aggregate surfaces, the aggregate exteriors contain less organic C, less Mn and Fe oxides, and have a lower cation exchange capacity than the interiors (Amelung and Zech, 1996; Wilcke and Kaupenjohann, 1997). The bulk soil contains more organic C, has a higher CEC and lower oxide concentrations than the aggregates (Wilcke et al., 1998b).

Compared with temperate soils, soils of the humid tropics are characterized by higher leaching rates and faster weathering because of elevated soil temperatures and high precipitation. This results in desilication i.e. the leaching of Si from silicates and the enrichment of primary quartz, secondary Al and Fe oxides, and two-layer clay minerals (mainly kaolinite) relative to all other soil constituents (Duchaufour, 1977). Preferential desilication under tropical climates may cause an enrichment of metals strongly bound in oxidic forms in the aggregate exteriors compared with the interiors which would be in contrast to many non-illuvial soil horizons in temperate regions.

The objective of this study is to compare Al and heavy metal concentrations and partitioning between aggregate exteriors and interiors and between bulk and whole aggregates of tropical soils from Costa Rica. The study soils have received heavy metal inputs via fungicides, fertilizers, and deposition from the atmosphere resulting in surface accumulation of Cd, Pb, and in part Cu (Kretzschmar et al., 1998).

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Study Sites and Soil Sampling
We sampled A horizons of 18 soils in the coffee (Coffea arabica L.) cultivation areas of Costa Rica (locations and soil properties are shown in Wilcke et al., 1998a). Mean annual precipitation ranges between 1500 mm in the west and 3500 mm in the east. Mean annual temperature ranges between 20 and 26°C. Separate samples were taken for aggregates and for bulk soil from 16 coffee plantations and two natural forests. Five subsamples from homogenous 10 x 10 m plots were taken and mixed. The bulk soil was air dried, crushed and sieved to <2 mm. As there were no significant differences in metal concentrations and partitioning of the bulk soil between the coffee grown soils and the forest soils (Wilcke et al., 1998a) we pooled all data for this paper.

Aggregate Fractionation
We separated soil aggregates mechanically into exteriors and interiors fractions using the method described by Kayser et al. (1994) as modified by Bundt et al. (1997) for field conditions. Macroaggregates (2-20 mm) were selected manually from the field moist sample. To prevent solute movement and to stabilize the aggregates, they were frozen in liquid N₂ immediately after sampling. The 300 g of frozen aggregates were placed on a 2-mm-sieve above a polyethylene bowl with distilled water. We shook the bowl manually with approximately 70 to 90 rounds per minute horizontally, so that water washed over the frozen aggregates and thawed them slowly from the outside. The exteriors dispersed, fell through the sieve and were collected in the bowl. The interiors remained on the sieve and were removed to dry. We used shaking times between 0.5 and 20 min depending on the aggregate stability, which resulted in peeling off of between 3 and 39 mass % of the aggregates as the exterior fraction. The procedure was repeated without changing the water until 1000 g of aggregates were separated. The water used for fractionation was analyzed for heavy metals and Al, which were assigned to the aggregate exteriors. After air drying the aggregate interiors were crushed and sieved to <2 mm. The mass proportions were calculated on a gravel-free basis. All samples contained <2% gravel. This method provides a crude field-applicable separation of aggregate interior and exterior material, which allows only a qualitative comparison.

Extractions and Analyses
Both aggregate fractions and the bulk soil were sequentially extracted by the method of Zeien and Brümmer (1989). Approximate assignments to chemical forms (in brackets) have been examined by Zeien (1995) for soils of the temperate regions and by Wilcke et al. (1998a) for Costa Rican soils:

1. unbuffered 1 M NH₄NO₃, shaken end-over-end for 24 h (readily soluble and exchangeable),
   
2. 1 M NH₄-acetate, pH 6.0, shaken for 24 h (specifically adsorbed and other weakly bound species),
   
3. 0.1 M NH₂ OH·HCl + 1 M N₄-acetate, pH 6.0, shaken for 30 min (bound to Mn oxides),
   
4. 0.025 M NH₄EDTA, pH 4.6, shaken for 90 min (bound to organic matter in stable complexes)
   
5. 0.2 M NH₄-oxalate, pH 3.25, shaken for 4 h (bound to amorphous and poorly crystalline Fe oxides, in Andisols additionally bound to allophanes),
   
6. 0.1 M ascorbic acid in 0.2 M NH₄-oxalate, pH 3.25, 30 min. in boiling water (bound to crystalline Fe oxides), and
   
7. 3 parts concentrated HNO₃ and 1 part concentrated HClO₄ (residuum, mainly bound within silicates).
   

Soil:solution ratios of Steps 1 through 6 were 1:25, of Step 7, 1:10. After each of the extraction Steps from 2 to 6, the samples were washed with the preceding extractant (2, 3, and 6), the same extractant (5) or NH₄OAc, pH 4.6 (4) (soil:solution ratio 1:25, shaken for 10 min., Steps 2, 4, 5, and 6 one time, Step 3 two times). The washes were combined with the preceding extracts. The readers should be aware of the fact that it is not possible to exactly extract a specific metal form from soil and that the assignments are approximate.

Using conventional procedures the following soil properties were determined in aggregate fractions and bulk soil: effective cation-exchange capacity (ECEC) with 1 M NH₄-acetate, pH 7 (Page et al., 1982), base saturation (BS) as the percentage of Ca + Mg + K + Na of the ECEC, and total C with a CHNS-Analyzer (Elementar vario EL, Elementar Analysensysteme GmbH, Hanau, Germany). Amorphous and poorly crystalline Fe oxides were extracted according to Schwertmann (1964)(oxalate buffer method = Fe_o), crystalline Fe oxides according to Mehra and Jackson (1960)(dithionite–citrate buffer method = Fe_d). Texture was determined after destruction of organic matter with H₂O₂ and removal of Fe oxides with dithionite–citrate and chemical dispersion with Na-pyrophosphate (Schlichting et al., 1995). Aluminum, Cd, Ca, Cu, Fe, K, Na, Mg, Mn, Pb, and Zn were measured by atomic absorption spectrometry using graphite tube or flame techniques (Varian AA 400 Z, AA 10, or AA 400, Varian GmbH, Darmstadt, Germany).

Calculations and Statistical Evaluation
Total metal concentrations of the A horizons were taken as the sum of the concentrations extracted with the seven fractions.

Whole aggregate concentrations were calculated from the interiors' and exteriors' concentrations after weighting them according to their mass percentages of the whole aggregate (Eq. [1]).

(1)

where c is the concentration of a metal in a single fraction, i a metal, j a metal fraction (or ij a metal sorbent), ag the whole aggregate mean, m the mass fraction of the interior or exterior portion of the whole aggregate (0 < m < 1), in the aggregate interiors, and ex the aggregate exteriors.

The enrichment/depletion factor EF was defined as:

(2)

where t is the total concentration of a metal and bu the bulk soil (the other suffixes are the same as in Eq. [1]).

Data sets were tested for normality with the Kolmogorov-Smirnov test (Lillifors probabilities). Differences in average concentrations between aggregate interiors and exteriors and between bulk soil and whole aggregates were compared with a t test ("paired differences"). Mean enrichment/depletion factors were tested for significant differences from 1 with the "Wilcoxon matched pairs" test. Statistical analyses were performed with the software package STATISTICA for Windows 5.1 (Loll and Nielsen, Hamburg, Germany). The significance level was set at P < 0.05.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Metal Sorbents
In Oxisols, Mollisols, and Alfisols average organic C concentrations in the aggregate exteriors (39.0 ± SD: 11.9 g kg^-1) are, like in many temperate soil horizons without illuvial enrichment of organic matter, significantly lower than in the interiors (41.3 ± 13.9). In temperate soils, this has been attributed to favorable conditions for microorganisms and thus enhanced degradation of soil organic matter (Amelung and Zech, 1996; Wilcke and Kaupenjohann, 1997). The studied Andisols contain a much higher organic C concentration than the other soil orders. Organic C concentrations are, on the average, also lower in the exteriors (84.5 ± 1.71) than in the interiors (88.6 ± 13.9); however, the difference is not significant. Andisols show, on the average, significantly higher oxalate-extractable Fe concentrations in the aggregate exteriors (9.4 ± 1.9 g Fe kg^-1) than in the interiors (8.5 ± 1.5 g Fe kg^-1). On the average of all soils and when the soils are grouped according to their orders there are no significant differences in organic C, oxalate- and ascorbic acid-extractable Fe concentrations, and CEC between bulk soil and whole aggregates.

Total Aluminum and Heavy Metal Concentrations
On the average of all studied soils, total concentrations of all metals except for Zn are higher in the aggregate exteriors than in the interiors (Fig. 1) . The differences are significant for Al, Cu, and Pb. Significantly higher concentrations of the mainly geogenic Al (Kretzschmar et al., 1998) in the aggregate exteriors than in the interiors are in contrast to temperate soil horizons without illuviation where the exteriors generally have lower concentrations of geogenic metals (Wilcke and Kaupenjohann, 1997). The findings may be explained by preferential desilication of the aggregate surfaces or by a surface accumulation of Al-rich material, such as clay coatings. However, no coatings were visible macroscopically in the studied A horizons. To be certain, we also determined exemplarily the particle-size composition in aggregate exteriors and interiors of six soils where enough material was available (one Mollisol, one Andisol, four Oxisols). The aggregate exteriors have, on average, 4.5 ± 12.1% higher clay concentrations than the interiors. However, the differences are not consistent and not significant. Only in one individual soil (Andisol) did we find a 28% higher clay concentration in the exteriors than in the interiors. As higher Al concentrations in the exteriors than in the interiors were found in all studied soils, preferential desilication seems to be a more probable explanation than clay coatings. However, the latter may contribute to higher Al concentrations in the exteriors, particularly in Andisols.

View larger version (19K): [in this window] [in a new window]   Fig. 1 Average differences in Al, Fe, Mn, Cd, Cu, Pb, and Zn concentrations and standard deviations between tropical soil aggregate exteriors and interiors in percentage of the concentration in the interiors (n = 18, positive values indicate higher concentrations in the aggregate exteriors than in the interiors, negative values lower ones, an asterisk illustrates that the average differences between exteriors and interiors are significant at P < 0.05)

When Andisols and Oxisols are treated separately (Fig. 2) similar results are observed as shown in Fig. 1. However, differences in Al concentrations between aggregate exteriors and interiors are smaller in Oxisols than in Andisols and not significant. This may indicate that in Andisols aggregates have a longer residence time because of the stabilizing effect of high organic C concentrations (Oades, 1984). The consequence would be a longer time to establish differences between exteriors and interiors. Another reason may be the fact that Andisols contain more weatherable minerals than the highly weathered Oxisols, which allow a more pronounced differentiation between aggregate interiors and exteriors by preferential weathering.

View larger version (22K): [in this window] [in a new window]   Fig. 2 Average differences in Al, Fe, Mn, Cd, Cu, Pb, and Zn concentrations and standard deviations between tropical soil aggregate exteriors and interiors in percentage of the concentration in the interiors (Oxisols: n = 8; Andisols: n = 6; positive values indicate higher concentrations in the aggregate exteriors than in the interiors, negative values lower ones; an asterisk illustrates that the average differences between exteriors and interiors are significant at P < 0.05)

View larger version (16K): [in this window] [in a new window]   Fig. 3 Average differences in Al, Fe, Mn, Cd, Cu, Pb, and Zn concentrations and standard deviations between manually selected whole aggregates and bulk tropical soil in percentage of the concentration in the aggregate (n = 18, positive values indicate higher concentrations in the aggregate exteriors than in the interiors, negative values lower ones, concentrations of the whole aggregates are calculated from those of the exteriors and interiors and weighted according to their mass percentages of the whole aggregate, an asterisk illustrates that the average differences between the whole aggregate and the bulk soil are significant at P < 0.05)

Quantitative Differences
Lead concentrations are significantly higher in the aggregate exteriors than in the interiors in all metal fractions except for Fraction 1. There are no significant differences between bulk soil and whole aggregates except for Fraction 6 where Pb concentrations are significantly lower in bulk soil than in whole aggregates (Table 1) . This indicates preferential sorption of probably atmospheric Pb (Kretzschmar et al., 1998) to all sorbents characterized with the extractions at the aggregate surfaces, but not or to a far lower extent to the interaggregate soil.

There are no significant differences between the aggregate fractions in the metal concentrations extracted with Fraction 3 except for Pb and none between bulk soil and whole aggregates. Average Cu concentrations in Fractions 4 and 5 are significantly higher in the aggregate exteriors than in the interiors and the average Cu concentration in Fraction 4 is significantly higher in bulk soil than in aggregates. This indicates that Cu introduced to the soils via fungicides mainly is sorbed to organic matter at aggregate surfaces and between aggregates and to poorly crystalline Fe oxides at aggregate surfaces. It is known that these sorbents are of particular importance for Cu (McLaren and Crawford, 1973; König et al., 1986). Al concentrations in Fractions 6 and 7, and Cu concentrations in Fraction 7 are significantly higher in the aggregate exteriors than in the interiors. This is in contrast to results from many temperate soil horizons without illuviation where concentrations of mainly geogenic metals only extractable with strong acids generally are lower in the aggregate exteriors than in the interiors.

Qualitative Differences
We used the enrichment/depletion factors (EF) to describe qualitative differences in metal partitioning between aggregate interiors and exteriors (EF_ex) and between bulk soil and whole aggregates (EF_bu). It must be kept in mind that the EF only is a qualitative measure because it depends on the exteriors/interiors ratio. The EF_ex of the easily extractable Fractions 1 + 2 are significantly >1 for Cd and Pb (Table 2) . There are no significant differences from 1 for all metals in all other fractions except for Pb whose EF_ex are significantly different from 1 for all fractions. To illustrate the differences we contrast the EF_ext of Pb to those of Al in Fig. 4 . The figure shows that there are almost no qualitative differences between aggregate exteriors and interiors in Al. partitioning whereas Pb is mainly enriched in the less strongly bound Fractions 1 to 4. As these fractions are known to be plant available (Symeonides and McRae, 1977; Köster and Merkel, 1982) and as plants mainly grow on aggregate surfaces (Materechera et al., 1994) this may cause an increased Pb uptake by plants. The findings for the Costa Rican soils, except for Pb, are substantially different from those for temperate soils (both with and without illuviation, Wilcke et al., 1996, Wilcke et al., 1998a) where all metals show higher percentages of the total concentrations in the aggregate exteriors than in the interiors whereas the strongly bound Fractions 5 to 7 have lower percentages. Thus, in the studied tropical soils metals released from minerals through weathering seem to be leached rather than adsorbed. Only when the inputs are high compared with the soil concentrations a visible enrichment in the aggregate exteriors compared with the interiors occurs mainly in the easily extractable Fractions 1 through 4.

View larger version (15K): [in this window] [in a new window]   Fig. 4 Average enrichment factors for tropical soils of Al and Pb in the seven fractions of the extraction sequence (= qualitative differences between exteriors and interiors) and standard deviations (n = 18)

	Conclusion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusion REFERENCES

 
Our results show that in tropical soils, like in many temperate soils, there are small but consistent differences between aggregate exteriors and interiors and between bulk soil and whole aggregates. Similar to many temperate soils, deposited heavy metals are sorbed preferentially in easily extractable forms onto the aggregate surface and onto interaggregate soil. However, in contrast to temperate soil horizons, except for those with illuviation of clay, humus or oxides, the studied tropical A horizons show higher concentrations of geogenic metals in the aggregate exteriors than in the interiors mainly in strongly bound forms. This probably is caused by preferential desilication of aggregate surfaces. In some soils an enrichment of clay at aggregate surfaces may also contribute to explain the observed metal distribution in soil aggregates.

	ACKNOWLEDGMENTS

 
We thank Dr. Luís Alpízar, Guillermo Ramirez, and Juan Bautista of the Costa Rican Ministery of Agronomy (MAG), as well as Carlos Chacón and Gerardo Quesada and his family of the Cooperativa Pilangosta in Hojancha, Costa Rica, for their important support during the field work. We also thank Prof. Dr. Hans-Werner Faßbender, Prof. Dr. Martin Kaupenjohann, and Prof. Dr. Heiner Goldbach for their help during project planning and their scientific assistance. We are highly indebted to the German Research Foundation (DFG) which funded this study (Ze 154/28-1).

Received for publication May 30, 1998.

	REFERENCES

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Wilcke, W. Articles by Zech, W. Search for Related Content PubMed Articles by Wilcke, W. Articles by Zech, W. GeoRef GeoRef Citation Agricola Articles by Wilcke, W. Articles by Zech, W.