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Department of Soil Science, Institute of Ecology, Berlin University of Technology, Salzufer 11-12, D-10587 Berlin, Germany
* Corresponding author (katrin.ilg{at}tu-berlin.de)
Received for publication March 8, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DPS, degree of phosphorus saturation • Pox, Feox, Alox, oxalate-extractable phosphorus, iron, and aluminum, respectively
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In sandy soils, the DPS is a central factor controlling the concentration of dissolved P in drainage water and therefore subsurface P leaching (Behrendt and Boekhold, 1993; Breeuwsma and Silva, 1992). The ratio of oxalate-extractable P to (Fe + Al) is a good measure of the DPS (van der Zee and van Riemsdijk, 1988; van der Zee and de Haan, 1994). Concentrations of dissolved P increase sharply if the DPS exceeds a certain critical value that has been termed the "change point" (Maguire and Sims, 2002; McDowell and Sharpley, 2001a). This change point can be related to the nonlinear sorption characteristics of orthophosphate 
to soil (Ryden and Syers, 1977; Barrow, 1983; Koopmans et al., 2002). Several authors have quantified distinct change points for various soils (Celardin, 2003; Nair et al., 2004). However, as Koopmans et al. (2002) pointed out, these change points depend both on experimental conditions, such as the soil to solution ratio, and on soil characteristics. Highly fertilized sandy soils that are poor in the main P sorbents Fe and Al oxides and rich in P often exceed a critical level of DPS and are therefore vulnerable to P leaching. Such soils are found in areas with high livestock densities, for example, in the Netherlands, Belgium, and the northwestern part of Germany (Breeuwsma and Silva, 1992; De Smet et al., 1996; Leinweber et al., 1997). In these areas, subsurface leaching of P is often equally important as surface erosion for P inputs into surface waters (Driescher and Gelbrecht, 1993).
In addition to dissolved P, P bound to suspended particles and colloids contributes to P leaching from agricultural soils (Jensen et al., 2000; Hesketh et al., 2001; Hens and Merckx, 2001; Motoshita et al., 2003). Soil colloids are defined as particles ranging from >1 nm to <1 µm, which remain suspended in water and are therefore mobile (Kretzschmar et al., 1999). Phosphorus may be bound to mineral colloids, such as Fe and Al oxides, or to organic or organo–mineral colloids (Celi et al., 2001; Hens and Merckx, 2002). Colloidal P in soil water samples may account for 13 to 95% of total P, but its relevance for P leaching and the processes governing its release from soils are not fully understood (Haygarth et al., 1997; Hens and Merckx, 2001; Shand et al., 2000).
Zhang et al. (2003) reported that application of P to sandy soils packed into columns induced the mobilization of colloidal P and Fe. In accordance with this finding, Siemens et al. (2004) showed that sorption of P caused the release of colloidal P from sandy soils in batch experiments. Referring to Celi et al. (1999), Puls and Powell (1992), and Stumm and Sigg (1979), who found that P sorption decreases the surface potential of iron oxides, they hypothesized that a certain P saturation of the sorbent may mark a change point for the release of colloidal P, similar to the change point for the mobilization of dissolved P.
Experience with soil test methods as tools for assessing the risk of dissolved P export from soil is rapidly growing; however, no data are currently available that relate soil P parameters to the risk of subsurface transport of colloidal P (Schouwmans and Chardon, 2003).
The objective of this study was to evaluate the impact of P fertilization and the initial degree of P saturation on concentrations of dissolved and colloidal P in sandy soils. We hypothesize that (i) increasing DPS not only increases dissolved P concentrations, but also enhances the release of colloidal P from soils and (ii) a critical level of DPS exists above which concentrations of dissolved and colloidal P increase sharply. To test these hypotheses, we sampled four long-term fertilization experiments on sandy soils to ensure a wide range of P contents and DPS as a result of different P additions.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Degree of Phosphorus Saturation
Oxalate-extractable phosphorus, iron, and aluminum concentrations (Pox, Feox, Alox) were determined by extracting 2 g of soil with 100 mL ammonium oxalate (0.2 M, pH 3.25) for 1 h in the dark (Schlichting et al., 1995). Iron and Al concentrations were measured using AAS. The detection limits were 0.1 mg L–1 for Fe and 1 mg L–1 for Al. We measured P concentrations by the method of Murphy and Riley (1962) using a continuous flow analyzer (CFA) (Skalar, Erkelenz, Germany). The detection limit was 0.01 mg P L–1. All vessels were rinsed with 0.01 M HNO3 before P analyses. The DPS was calculated according to Breeuwsma and Silva (1992) and van der Zee and de Haan (1994):
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Dissolved and Colloidal Phosphorus
We used the concentration of water-dispersible P as a measure for potentially mobile, colloidal P (Kaplan et al., 1997). Ten grams of soil were shaken end-over-end with 80 mL of deionized H2O for 24 h. The extracts were centrifuged at 3000 x g for 10 min and filtered (no. 512 1/2; Schleicher and Schuell, Dassel, Germany) to remove coarse particles. Thereafter, the extracts were filtered through 1.2-µm cellulose acetate filters (Sartorius, Göttingen, Germany) to capture only particles < 1.2 µm, which are defined as colloids (Kretzschmar et al., 1999). The first 5 mL of both filtrates were discarded. An aliquot of the filtrate was ultracentrifuged at 300000 x g for 1 h to remove colloids (Optima TL; Beckman, Unterschleissheim, Germany). Colloidal P was calculated as the difference between the concentration of total P in non-ultracentrifuged and ultracentrifuged samples. We measured total P concentrations after oxidation and acid hydrolysis of organically bound P using the method of Murphy and Riley (1962). To achieve a complete oxidation, 3.2 M H2SO4 and 37 mM K2O8S2 were added, and the samples were heated to 95°C and irradiated with UV radiation for 6 min (CFA, see determination of DPS). The detection limit was 0.01 mg P L–1.
Characterization of Colloids
To characterize the released colloids we extracted a randomly chosen set of 16 soil samples per depth. In addition to the parameters mentioned above, we determined the average particle size (High Performance Particle Sizer HPP 5001; Malvern Instruments, Malvern, UK) and the zeta potential (Zeta Sizer DTS 5200, S/N 34132/35; Malvern Instruments). The optical density of the filtrate at a wavelength of 525 nm was quantified as a measure of the concentration of colloids (Specord photometer; Analytik Jena AG, Jena, Germany). Concentrations of Fe and Al were measured in ultracentrifuged and non-ultracentrifuged samples to quantify colloidal Fe and Al. Colloidal C was determined on a TOC analyzer (TOC-Analyzer 5050A; Shimadzu, Kyoto, Japan).
The effect of ionic strength and electrolyte composition on the release and properties of colloids was tested by extracting a set of five samples, randomly chosen from each depth, with deionized H2O and 0.01 M KCl. Potassium chloride was chosen as background electrolyte instead of CaCl2 to avoid the precipitation of apatite. The same parameters as mentioned above were determined.
Precision
To check the quality of laboratory methods, 12 randomly chosen samples were extracted in duplicate. The analytical reproducibility of dissolved and colloidal P was 91 and 84%, respectively. The reproducibility of field replicates was 61 and 50%, respectively. The three sets of 72, 16, and 5 samples from each sampling depth were analyzed in separate analytical runs. The coefficients of variation of dissolved and colloidal P concentrations between these runs were large (52 and 38%, respectively), because concentrations of water-extractable P were small especially for soil samples in the 30- to 60- and 60- to 90-cm depths (samples below detection limit: 16 and 25%, respectively). The concentration of colloidal P is a calculated parameter, therefore error propagation reduces the reproducibility. To account for the limited reproducibility between different analytical runs, we compared only values that were determined within the same run.
Calculations and Statistical Evaluations
Arithmetic means, standard deviations, and coefficients of variation were calculated for all variables. Values below detection limit and concentrations of colloidal P, Fe, and Al < 0 were set to zero. In case of large variations of the input parameters total and dissolved P concentration, error propagation leads to varying quantification thresholds. Setting negative concentrations of composite parameters as colloidal P concentrations to one half of the respective quantification threshold introduces "artificial" differences into the dataset, which may influence the statistical analysis (Siemens and Kaupenjohann, 2002). For this reason, we preferred setting negative colloidal P concentrations to zero. We think that this approach is straightforward as it provides a clearly conservative estimate of colloidal P concentrations. The significance of fertilization effects on concentrations of Pox, colloidal P, dissolved P, and DPS were tested using the nonparametric Kruskal–Wallis test. We applied the nonparametric Nemenyi test for post-hoc comparisons (Köhler et al., 1996). Parameters were analyzed for correlations by the nonparametric Spearman statistic. Differences between depths as well as between concentrations of water extracts and KCl extracts were compared with the nonparametric Wilcoxon-matched-pairs test. All tests, except for the Nemenyi test, were performed using STATISTICA 6.0 software (StatSoft, 2003). We used the split-line model described by McDowell and Sharpley (2001b) to identify change points of the effect of DPS on other variables. The model consists of two linear regressions separated by a critical value of the independent variable. Change points are regarded as significant if the slopes of the two linear relationships are significantly different. The model was fitted using the nonlinear regression procedure of the SPSS 9.0 software (SPSS, 1999). The level of significance for all tests was defined as p < 0.05.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Dissolved P was the parameter that reacted most sensitively on fertilization. We observed significant effects for the 0- to 30-cm depth at all sites and for the 30- to 60-cm depth of the Dülmen (old) experiment. In the Dülmen (old), Dülmen (new), and Nienburg experiments, the treatments receiving the highest P input had significantly higher concentrations of dissolved P compared with the controls. At Hamburg, Treatment 3 receiving 22 kg P ha–1 yr–1 as Thomaskalk had significantly lower concentrations of dissolved P than Treatment 5 receiving 44 kg P ha–1 yr–1. In subsoils, differences between treatments were less pronounced. Generally, concentrations of dissolved P decreased sharply with increasing depth. For the 60- to 90-cm depth, they were often close to the detection limit.
Water-extractable colloidal P concentrations tended to increase with increasing P fertilization in the 0- to 30- and 30- to 60-cm depths, but there was no significant positive effect detectable except for the 30- to 60-cm depth of Treatment 4 at Nienburg receiving 42 kg P ha–1 yr–1 (Table 2). In contrast, at Nienburg fertilization with 63 kg P ha–1 yr–1 showed significantly smaller concentrations of colloidal P than the control in the 0- to 30-cm depth. Whereas concentrations of Pox, dissolved P, and DPS decreased with increasing depth, the concentrations of colloidal P did not change. As a consequence, the fraction of colloidal P increased from 28 ± 17% of water-extractable P for topsoils to 94 ± 8% for the 60- to 90-cm depth.
No significant differences between effects of organic and mineral fertilizers could be detected for any soil P fraction or DPS.
Relating Dissolved and Colloidal Phosphorus Concentrations to Degree of Phosphorus Saturation
Dissolved P concentrations were significantly correlated with DPS in the 0- to 30- and 30- to 60-cm depths (Fig. 1)
. No correlation was detected for the 60- to 90-cm depth. Fitting the split-line model to the data from the Dülmen experiments for the 30- to 60-cm depth, we identified a significant change point of DPS = 0.11 ± 0.01.
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The electrical conductivity decreased with increasing soil depth from 62 ± 14 µS cm–1 in the 0- to 30-cm depth to 39 ± 13 µS cm–1 in the 30- to 60-cm depth and 26 ± 7 µS cm–1 in the 60- to 90-cm depth. The difference between the 0- to 30- and 60- to 90-cm depths was significant. Conductivities were small because the sandy soils were sampled in January after most solutes had been leached. Exchangeable Ca2+ concentrations decreased significantly from topsoils to subsoils (839 ± 273 mg kg–1 in the 0- to 30-cm depth, 361 ± 101 mg kg–1 in the 30- to 60-cm depth, and 312 ± 64 mg kg–1 in the 60- to 90-cm depth). Similarly, K+ concentrations were significantly higher in the plow layer (84 ± 21 mg kg–1) than in the 30- to 60-cm depth (51 ± 16 mg kg–1) and the 60- to 90-cm depth (57 ± 18 mg kg–1). Concentrations of exchangeable Mg2+ were approximately 5% of the Ca2+ concentrations in the plow layer (42 ± 18 mg kg–1). In the 60- to 90-cm depth they were similar to concentrations in the 0- to 30-cm depth (40 ± 18 mg kg–1), while concentrations in the 30- to 60-cm depth were significantly smaller (28 ± 11 mg kg–1).
Effects of Ionic Strength and Electrolyte Composition on the Release of Colloids
Masking the effect of ionic strength and exchangeable cations by adding 0.01 M KCl as background electrolyte increased the electrical conductivity of the extracts to 1.43 mS cm–1 (Table 4). Increasing the total electrolyte concentration reduced the optical density and the concentrations of colloidal P, Fe + Al, and C. This effect was most pronounced for soil samples from the 60- to 90-cm depth. As a consequence, the optical density, the concentrations of colloidal P and Fe + Al, as well as the ionic strength and pH, were significantly smaller for samples from the 60- to 90-cm depth compared with samples from the 0- to 30-cm depth. The zeta potential was significantly larger for samples from the 60- to 90-cm depth than for samples from the 0- to 30-cm depth.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The concentration of water-extractable dissolved P decreased sharply from topsoil to subsoil, which is in accordance with depth profiles of dissolved P concentrations reported by Siemens et al. (2004) for sandy soils of northwestern Germany. It is noteworthy that DPS never exceeded the critical value of 0.25 in the subsoils, which indicates leaching of dissolved P concentrations larger than 100 µg L–1 potentially enhancing eutrophication of surface waters is unlikely at all sites. The critical value of 0.25 has been identified as tolerable for similar sandy soils of the Netherlands (Breeuwsma and Silva, 1992; van der Zee and van Riemsdijk, 1986, 1988; van der Zee and de Haan, 1994).
Relating Dissolved Phosphorus Concentrations to Degree of Phosphorus Saturation
We found a significant change point of P saturation for samples from the 30- to 60-cm depth of the Dülmen Podzol, above which concentrations of dissolved P increased sharply [Dülmen (old) and Dülmen (new) experiments; Fig. 2]. Principally, the value of this change point as well as the slopes of the two linear regressions below and above the change point are soil specific and depend on the soil's sorption capacity, the soil's P affinity, and experimental conditions such as the considered range of soil P or the soil to solution ratio (Koopmans et al., 2002). In fact, the increase in dissolved P concentrations with increasing DPS in the 0- to 30-cm depth at Nienburg seems to be small compared with the increase at the other sites (Fig. 2, 0–30 cm). Furthermore, part of the scatter of the relationship between DPS and dissolved P in samples from the 30- to 60-cm depth might be caused by sampling varying fractions of Podzol B and C horizons when taking samples from a fixed depth increment. However, the change point of DPS of approximately 0.10, which was determined for the 30- to 60-cm depth of the Dülmen site, has some relevance for the other sites as well as for the other soil depths, as pooling the data from all sites and depths resulted in a similar change point of approximately 0.10. The apparent robustness of the change point value under given experimental conditions allowed several authors to derive common DPS values as indicators of P leaching for sets of soils (Celardin, 2003; Maguire and Sims, 2002; McDowell et al., 2002) or even combinations of topsoil and subsoil horizons from different soils (Nair et al., 2004).
The critical DPS value of 0.1 that we found is smaller than the critical value of 0.25 (van der Zee and de Haan, 1994) and also smaller than the value of 0.20 that was reported by Nair et al. (2004) for sandy soils from the Suwannee River basin, Florida. In the case of the Dutch reference value, this difference might be attributed to the different ways that were used to identify critical values of P saturation. Whereas a description of the sorption process was used in the Netherlands by van der Zee and de Haan (1994) to identify a P saturation corresponding to a critical dissolved P concentration of 100 µg P L–1, we used a statistical model to separate two regions of P saturation without defining a critical target concentration of dissolved P. Overall, our findings confirm the results of Maguire and Sims (2002), McDowell et al. (2002), Nair et al. (2004), and Siemens et al. (2004), which suggest that DPS is the most important factor controlling the concentration of dissolved P in noncalcerous soils of temperate and subtropical climates.
Colloidal Phosphorus and Degree of Phosphorus Saturation
The significant increase of colloidal P concentrations with increasing DPS (Fig. 2) might be related to (i) an increasing P concentration of individual colloidal particles or (ii) an additional release of colloids from the soil. In the first case, increasing P concentrations of individual colloids might be the consequence of increasing dissolved P concentrations and sorption equilibria. In fact, the concentration of colloidal P was correlated to the DPS of colloids, which reflected the bulk soil DPS (Table 3). However, water-extractable concentrations of colloidal P were also positively correlated to the optical density and concentration of colloidal Fe + Al (Table 3). Both correlations indicate that additional colloids were released by an increasing DPS of the bulk soil. Hence, both processes seem to contribute to the increase of colloidal P concentrations with increasing DPS. However, the fact that colloidal P was released from subsoils with small DPS and the fact that the relations between concentrations of colloidal P and DPS were not uniform for all depths show that DPS is not the only factor that controls the release of water-extractable colloidal P. Similarly, the fact that colloidal P concentrations were significantly smaller in the 0- to 30-cm depth receiving the highest P inputs compared with the control in the Nienburg experiment (Table 2) indicates that factors other than P addition might influence the concentration of colloidal P. Effects of other factors controlling the stability of colloidal suspensions like ionic strength or electrolyte composition might add considerable variability to the relation between DPS and water-extractable colloidal P, which reduces the suitability of the extraction of colloidal P with water to study the effect of P saturation or P accumulation on the risk of colloidal P leaching.
Effects of pH, Ionic Strength, and Electrolyte Composition on Concentrations of Colloidal Phosphorus
It is well known that pH, ionic strength, and electrolyte composition (in particular Ca2+) significantly influence colloid mobilization and stability of colloidal suspensions (Kretzschmar et al., 1993; Heil and Sposito, 1993; Kretzschmar et al., 1999). In our case, it is unlikely that the pH had a pronounced effect on the release of water-extractable colloidal P, because differences among pH values within a given depth increment were small and not significant. The significant decrease of ionic strength, Ca2+, and, to a lesser extent, Mg2+ concentrations from topsoils to subsoils might explain the high mobilization of colloids despite low DPS in subsoils. Furthermore, organic carbon increases the aggregation of soil particles in topsoils and may therefore influence the release of colloids (Goldberg et al., 2000). Higher concentrations of colloidal C in soil samples from the 0- to 30- and 30- to 60-cm depths compared with the 60- to 90-cm depth at a given DPS might reflect the aggregation of primary particles by organic matter in the topsoils (Table 4).
By adding KCl as background electrolyte, we masked the effect of divalent cations and low ionic strength on the release of colloidal P. Consequently, DPS became the most important factor for the release of KCl-extractable colloidal P as pooled concentrations of colloidal P were closely correlated to DPS for all depths (Fig. 3). Similar to our findings regarding water-extractable colloidal P, the increase of colloidal P concentrations might be related to the additional release of colloids as well as to an increasing P concentration of single colloids because the optical density and the degree of P saturation of colloids increased with increasing DPS of the bulk soil (Fig. 3). Generally, our results correspond to the findings of Zhang et al. (2003) and Siemens et al. (2004) that P additions or increasing P saturation induce the release of colloids and colloidal P from soils. However, in contrast to Siemens et al. (2004), who reported a nonlinear relationship between the concentration of KCl-extractable colloidal P and DPS, we found a rather linear relationship (Fig. 3) without a significant change point. In the sandy soils of this study this might reflect a more heterogeneous distribution of colloids with different characteristics in the sandy soils of this study, which are released at different degrees of P saturation.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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0.1. Water-extractable colloidal P is a significant, and in subsoils the dominating fraction of P that is potentially mobile. Concentrations of colloidal P increase with increasing DPS without showing a critical level of P saturation for the release of P-containing colloids, which means that mobilization of colloidal P might be already enhanced by P accumulation at low levels of P saturation. Furthermore, the release of colloidal P is facilitated by multiple factors including high degree of P saturation, small concentrations of exchangeable Ca2+, and small total electrolyte concentrations. Consequently, extraction methods masking the effects of electrolyte concentration and composition by using a background electrolyte are superior to extractions with water for studying the effect of P accumulation on colloidal P in soils. |
ACKNOWLEDGMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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