Envlron. Scl. Technol. 1992, 26, 2005-2011
Long-Term Field Comparison of Ceramic and Poly(tetraf1uoroethene) Porous Cup Soil Water Samplers Claus Beler,’ Karin Hansen, Per Gundersen, and BJrarnR. Andersen
Laboratory of Environmental Sciences and Ecology/Groundwater Research Centre, Technical University of Denmark, Building 224, DK-2800 Lyngby, Denmark Lennart Rasmussent
Danish Forest and Landscape Research Institute, Department of Forest Health and Forest Ecosystems, Skovbrynet 16, DK-2800 Lyngby, Denmark The validity of porous cup soil water samplers to extract soil solutions is most often evaluated in short-term laboratory studies. Such laboratory studies differ significantly from the situation in the field. Therefore the aim of the present study was to compare ceramic and PTFE porous cup soil water samplers under natural conditions in a long-term field experiment. Soil solutions were sampled from two depths (E, horizon and BC horizon) in an acidified, sandy forest soil in Denmark. Taking the spatial variability of the forest into account, the mean concentrations of base cations, S042-, Cl-, H+, and nonpurgeable organic carbon (NPOC) obtained with the two sampler types are compared. In most cases the concentrations of solutes and NF’OC in the PTFE cups were higher than the concentrations in the ceramic cups. The differences in mean concentrations of K+, Mg2+,Na+, Cl-, S042-,and NO, obtained with the two samplers were generally within the variability of the PTFE samplers whereas the concentrations of Ca2+, H+, and NPOC obtained with the PTFE samplers were significantly higher than those obtained with the ceramic samplers. The difference in NPOC in the E, horizon is concluded to be caused by filtering. The difference in Ca2+may to some extent be ascribed to dissolution of “glass pellets” in the PTFE sampler matrix, whereas the difference in H+concentration between the sampler types remains unexplained.
Introduction Ecological investigations of natural soil processes often require sampling of soil water solutions. Obviously, the chemical composition of the sampled solution reliably should reflect the composition of the soil water solution if misleading conclusions are to be avoided. This requires that the soil water solution must not be changed by leaching or retention of elements by the sampler itself during sampling. Several methods exist for sampling of soil water solutions. Reviews are presented by Litaor ( I ) and Beier et al. (2). One of the most used methods is extraction of soil solution using porous cups made of ceramic, poly(tetrafluoroethene) (PTFE), polyethylene (PE), stainless steel, or glass and connected to a vacuum reservoir via a collection flask. Porous cups made of ceramic materials are widely used. Assessments of the validity of different soil water samplers have usually been based on short-term studies in the laboratory. The influence of the sampler has most often been evaluated by sampling of solutions of well-known concentrations of elements and compounds followed by comparison of the solute concentrations before and after passing the sampler (3-8).However, the conditions in the laboratory may in many respecta differ from the conditions
L.R.was involved in the development and manufacturing of the PTFE soil water sampler type used in this study. 0013-936X/92/0926-2005$03.00/0
Table I. Soil Characteristics for the Study Site at Klosterhede, Denmark horizon
0
Ea
Bh,
B,
C
+7-0 &10/18 10/18-30 30-40/70 40/70 depth (cm) 4.4 4.6 4.7 3.8 3.9 PH (HzO) 2.7 3.1 4.3 4.6 4.7 PH (CaW loss on ignition (%) 57 2.5 0.8 0.4 nd“ 4.0 2.0 0.0 clay (%) nd 5.4 4.8 2.0 silt (%) nd 41.0 35.6 12.0 fine sand (%) nd 49.6 coarse sand (%) 57.6 86.0 8.6 4.9 4.4 75 16 CEC (mmol,/kg) 14 22 22 31 11 base saturation (%)
“Not determined.
in the field; e.g., redox potential, water saturation, and 0 2 / C 0 2relationships (I) can possibly affect the results. Further, it has been suggested that equilibrium between the soil solution and the sampler may not be obtained in short-term studies (9). Therefore, short-term tests may not always lead to reliable results compared to results obtained under field conditions. Soil solution extraction by porous cups has also been evaluated by laboratory comparison with saturated paste extracts of air-dried soil samples (IO). However, air-dried soil samples may differ significantly from the natural soil situation in the field, which may explain differences in the results obtained. Comparisons of tension soil water samplers with zero tension lateral flow collectors have been carried out in the field (II-I4), and results obtained by tension soil water samplers have been compared to sample results obtained by centrifugation of soil samples (9). Such comparisons suffer from the drawback that differences in the solutions sampled may not necessarily be due to artifacts caused by the samplers but will rather be due to sampling from different water pools. Recently, ceramic and PTFE porous cup samplers were found to obtain almost identical samples when installed next to each other in the mineral soil horizons (15). According to the above considerations, comparison of different porous cup samplers in long-term field experiments is important. The aim of the present study wm to compare the soil water solutions obtained with porous cup soil water samplers made of ceramic and PTFE, respectively, over a long time period under field conditions including organic and mineral soil layers.
Materials and Methods Study Site. The study was carried out in a 73-year-old Norway spruce (Picea abies) plantation located at Klosterhede, Lemvig in Western Jutland, Denmark. The stand is homogeneous, consisting of even-aged trees planted in rows and managed to be of nearly the same size. The soil
0 1992 American Chemical Society
Envlron. Sci. Technol., Voi. 26, No. 10, 1992 2005
is classified as a Typic Haplorthod (podzol) developed on a homogeneous sandy, nutrient-poor deposit of a glaciofluvial outwash plain. Selected soil characteristics are given in Table I. The study area is completely flat. Soil water was collected from four different plots in the stand. Three plots were covered by a roof below the canopy to enable manipulation of water and nutrient inputs to the soil (16). The treatments were as follows: (1) untreated control, receiving natural throughfall water; (2) summer drought, receiving natural throughfall below the roof during the rest of the year; (3) irrigation (optimal for tree growth) with deionized water and sea salt, receiving more water than the control plot during the summer months; (4) fertigation (fertilization and irrigation, optima! for tree growth), irrigation with deionized water and solutions containing all macro- and micronutrients. Soil Water Sampling. In each treatment plot, soil water was sampled on a monthly basis for a period of 2 years (December 1988 to December 1990) from the E, horizon (15 cm) and the BC horizon (55 cm) using two different types of soil water samplers: (1)ceramic cups (20-mm outer diameter, 50-mm porous length, pore size -. 1 pm) (P80, Staatliche Porzellan-Manufaktur, Berlin, Germany); (2) PTFE cups (21-mm outer diameter, 50-mm porous length, pore size -1.0 pm) (Prenart standard, Prenart Equipment Aps., Frederiksberg, Denmark). The PTFE is mixed with glass pellets to control the pore size. In the control plot, nine PTFE and nine ceramic samplers were installed in each depth. In the drought, irrigated, and fertilized plots, nine PTFE and three ceramic samplers were installed in each depth. The nine and three samplers, respectively, were installed in the plots so that they represented the distances close, medium, and far from the tree trunks. In each depth, the PTFE samplers were connected three by three to the same sampling bottle, so that each sample was a pooled mean of three samplers at different distances from the tree trunks. By this sampling strategy, three samples were obtained with the PTFE samplers from each depth and plot, whereas with the ceramic samplers, three samples were obtained from each depth in the control plot and one sample was obtained from each depth in the drought, irrigated, and fertilized plots. Before installation, each ceramic cup was flushed with 1 L of 0.1 M HCl and each PTFE cup was flushed with 1L of 1.0 M HC1. Hereafter, each cup was flushed with 2 L of demineralized water. The ceramic cups were mounted on a PVC pipe (20-mm diameter) and connected to the sampling bottles with PE tubing. The PTFE samplers were connected directly to the sampling bottles with PE tubing. Prior to installation, each sampler was wetted in distilled water. The samplers were installed in the soil in holes made by a steel rod of the same diameter as the sampler. To improve capillary contact between the sampler and the soil, a slurry of demineralized water and soil (1:1ratio) from the installation depth was poured through a pipe to the bottom of the hole before the sampler was inserted. The samplers were connected to a tensiometer-controlled vacuum system (2,17),where the vacuum was constantly kept at a level slightly higher than the soil tension causing a constant difference between the soil tension and the sampler vacuum. In this case, the sampling vacuum was kept at -0.1 X lo5Pa above the soil tension, which yielded 10-20 mL of soil water per sampler per day during wet soil conditions. During dry soil conditions, the vacuum system was cut off because the samplers would lose the capillary contact. 2006
Envlron. Scl. Technol., Vol. 26, No. 10, 1992
Sampling bottles were kept cool in soil pits in order to avoid any change of concentrations due to microbial activity. After installation, the samples were discarded for 3 months prior to the study period. Hereafter, samples were collected on a monthly basis. Each soil solution sample was analyzed for H+ (pH meter), Ca2+,Mg2+,and A13' (AAS), Na+ and KS (AES), and SO4'- and C1- (IC). Selected samples were analyzed for nonpurgeable organic carbon (NPOC), which in this case at low pH is assumed to be similar to total organic carbon (TOC). The results were analyzed in order to assess the validity of the two sampler types. Since only one sample is obtained by the ceramic samplers at each depth in the three treated plots, it is not possible to perform an analysis of variance (ANOVA). Therefore, the results obtained by the two methods are presented as the mean value of the three samples obtained by the PTFE samplers f l standard deviation and compared to the single sample from the ceramic cup samplers. Mean values are simple timeweighted means of the concentrations obtained in the samples regardless of the volume sampled (i.e., no volume weighting). Hypothesis. Installation procedure, porous area, soil depth, placement in the soil, applied vacuum, and sampling procedure were similar for all samplers. Further, both types of samplers work according to the same principle of extraction of soil water from the soil pores through small pores in the porous material of the sampler. The samplers differ in respect to construction material and pore size, which may cause differences in their filtering ability. Finally, they may differ in respect to the sampling direction as the porous area of the ceramic sampler is extended around the tip of the sampler where the PTFE sampler is solid. However, the tip area of the sampler is relatively small compared to the total sampler area, and the strongest forces are needed when water is sampled from below the sampler. Therefore, the amount of water entering from below the ceramic sampler is much less than from the sides, and the differences in sampling direction between the two sampler types are expected to be negligible. So, assuming the samplers are similar in respect to the basic principle of installation and sampling, we expect that differences in sample concentrations caused by the samplers will be due to the sampler material or the pore size.
Results The mean soil water concentrations and the standard deviation for the total period obtained with the PTFE and the ceramic samplers in each experimental plot are shown for all measured substances in Table 11. The monthly mean concentrations and standard deviations for ea2+are shown for each soil horizon and treatment in Figure 1. Generally, a substantial variation in the soil solution concentrations of all substances was seen for the yearly as well as the monthly mean Concentrations obtained with the PTFE samplers, especially in the control plot. The concentration levels were almost the same in the E, and the BC horizon except for Ca2+and H', whereas the concentration levels of most substances differed between treatments with at least a factor of 2 from the lowest to the highest concentration. The mean concentrations obtained with the two sampler types often differed, and in both horizons, the highest concentrations were usually obtained with the PTFE CUPS. The difference in the annual mean concentrations of K+, Mg2+,Na+, Cl-, S042-pand A13+ obtained with the two samplers was generally small and in most cases within the standard deviation of the PTFE samplers, whereas the
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Dec 88
Jun 89
Des-89
Sampler 1ypc:
of
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I - Ceramic
1. mean c c m e m a h s (Irmd,/L) and standard d e v i a h s of Ca2+obtained with the experimental twtment and ihe soil horizon.
monthly mean concentrations of all measured substances occasionally exhibited differences between the two sampler types exceeding the standard deviation. However, no systematic differences were detected, since none of the sampler types was constantly sampling the highest concentrations. The highest monthly as well as yearly mean concentrations of H+ and Ca2+were sampled by the PTFE samplers, especially in the E, horizon, which may indicate a systematic difference between the sampler types. The mean concentrations of Ca2+obtained with the ceramic samplers are generally low (-40 pmol,/L) and do not
and me ceramic samplers as a f~m
exhibit any large differences between treatments or horizons,whereas the mean concentrationsobtained with the PTFE samplers in the E. horizon are generally higher (loCrsO0pmolJL) and exhibit large differences between the different treatments, Also, in the BC horizon, the PTFEsamplers sample the highest Ca2+concentration hut the differencebetween the two sampler typea is small. The monthly Ca2+concentrations obtained with the ceramic samplers are in many cases within the standard deviation of the PTFE samplers (Figure 1). Similarly, HC concentrations are generally highest in samples obtained with the PTFE samplers in both horizons (Table 11). Envlron. Scl. Technol., Vol. 26, No. 10, 1992 2007
Table 11. Mean Concentrations (pmol,/L) of Solutes Obtained by PTFE and Ceramic Soil Water Samplers for the Total Study Period as a Function of Treatment and Sampling Depth treatment drought irrigation PTFE ceramic PTFE ceramic
control
K Ca Mg Na C1 SO4 A1 H
K Ca Mg Na c1
so4 A1 H (I
PTFE
ceramic
134 f 71" 436 f 87 700 f 302 2405 f 1098 2942 f 1345 696 f 353 565 f 173 163 f 43
28 30 369 1502 2066 316 522 122
62 f 55 209 f 84 406 f 42 1639 f 232 1979 f 103 443 f 29 328 f 72 85 f 21
119 f 57 85 f 13 565 f 190 2771 f 885 3481 f 1082 742 f 339 886 f 429 80 f 11
33 17 691 3069 3830 660 890 45
57 f 33 98 f 44 368 f 13 1827 f 146 2224 f 63 412 f 75 388 f 60 56 f 7
E, Horizon 36 14 f 5 36 115 f 94 372 288 f 41 1932 1623 f 382 2285 2489 f 641 334 110 f 23 655 836 f 370 54 130 f 40
fertigation PTFE ceramic
30 22 192 1154 1770 62 473 100
132 f 61 377 f 89 474 f 106 1048 f 460 1984 f 354 116 f 38 558 i 256 135 f 6
177 38 304 377 1073 221 463 85
34 38 284 1508 2202 472 748 53
75 f 53 100 f 79 479 f 134 1944 f 1788 2684 f 1811 527 f 364 805 f 545 70 f 15
76 34 275 371 879 351 593 52
BC Horizon 23 29 321 1556 2067 334 528 48
32 f 9 44 f 7 292 f 127 1706 f 577 2655 f 912 228 f 36 1047 f 451 83 f 13
Standard deviation.
-
I 200,
0
50
100 (PPm) NPOC(PTFE)
200
150
0 0
200
400
600
800 I
Ca (FTFE) (pmol(c)/liter)
200 150
T 4 :/1
0 0
50
100
150
200
H (FTFE) (pmol(c)/liter) Figure 2. Monthly mean concentrations (pmol,/L) of H+ and Ca2+ obtained with the PTFE and the ceramic samplers plotted against each other.
The monthly mean concentrations of H+ and Ca2+obtained with the two sampler types are plotted against each other in Figure 2. The resulta confirm the indications that Ca2+and, to some extent, H+ are sampled at the highest concentrations by the PTFE samplers as the monthly sample concentrations fall below the 1:l line. Sample concentrations of NPOC from the two sampler types are presented in Figure 3. In the E, horizon, the NPOC concentrations obtained with the ceramic samplers were significantly lower than those observed with the PTFE samplers. In the BC horizon, the two sampler types were sampling more evenly.
Discussion Comparison of NPOC. The difference in NPOC in the two sampler types, especially in the E, horizon, may possibly be caused by filtering of organic molecules by the ceramic cups, although the increased filtering by the ce2008 Envlron. Scl. Technol., Vol. 26, No. 10, 1992
Ea-horizon
0
BC-horizon --
1 1line
Figure 3. Monthly mean concentrations of NPOC (ppm) of selected samples obtained with the PTFE and the ceramic samplers plotted against each other.
ramic cups in the E, horizon contradicts recent findings on the same location by Beier and Hansen (15),who compared the same sampler types under controlled field conditions. However, in their study, soil solutions were extracted by PTFE samplers with a pore size of 5 pm compared to the 10 pm used in this study, indicating that a change in pore size from 5 to 10 pm may be important for the collection of organic molecules in the soil depth and site investigated here. If the filtering of organic molecules causes the observed difference in NPOC, one might expect a difference in NPOC concentrations in both horizons. However, the soil solution concentration of NPOC and the size of the organic molecules are most likely higher in the E, horizon compared to the BC horizon, and therefore, filtering influences the sample concentration in the BC horizon to a minor extent. One has to consider the possibility of filtering in connection with the purpose of the sampling. When budget studies are performed, the organic bound ions should be accounted for, whereas in concentration and process studies (e.g., Ca/M ratios, toxicity to roots), only the free ions in the solution should be accounted for. This research area needs more attention. Comparison of Substance Concentrations. There are differences between concentrations of most ions in samples obtained with the two sampler types but except for Ca2+and H+ the differences are generally not significant, taking the large variability in the soil solutions into account. Since none of the concentrations of the substances is constantly higher in one of the samplers, the
differences are not likely to be ascribed to the samplers themselves but rather to the variability in the soil. In contrast, similar PTFE cups were recently reported to contaminate soil solution samples with Mg and Na (and Si and Fe not analyzed here) (8). However, the reported contaminations in the range of 0-20 pmol,/L (8) are far below the soil solution concentrations in this study, and such a possible small contamination would therefore not be detected. With respect to K+, the result of this study indicating no difference between the two sampler types agrees with results reported by Maitre et al. (8). (a) Calcium. The difference between concentrations of Ca2+obtained with the two types of samplers may be caused either by leaching of Ca2+from the PTFE cups or by retention by the ceramic cups. Recently, Maitre et al. (8) found the PTFE samplers to contaminate a citrate buffer test solution (pH 4.25) with Ca2+to a level of -300 pmolJL, which was suggested to be caused by washout of unbound Ca2+and/or acid dissolution of CaO from the "glass pellets" in the sampler matrix. If leaching from the sampler matrix is substantial, the variability of the samples would be expected to be relatively low and the concentration level to be equally high in all treatments. However, the mean concentrations of Ca2+obtained with the PTFE samplers differ between treatments in the E, horizon by a factor of 5 and concentrations are generally 3-10 times as high in the E, horizon compared to the BC horizon no matter which treatment, indicating that the Ca2+concentration is determined by the soil solution and not by the sampler. Alternatively, leaching of Ca2+could be caused by exchange with protons, but this would cause the proton concentration in the PTFE cups to be lower compared to the ceramic cups, which is opposite to the observations. Furthermore, the change in proton concentration between treatments and horizons is much less than the change in Ca2+concentration. Therefore, leaching of Ca2+from the PTFE sampler matrix may contribute to the difference observed in the Ca2+ concentrations between the two different sampler types, although leaching is not likely to be the only explanation. Retention of Ca2+by the ceramic cups could be caused by filtering of large organic molecules, which to some extent could be indicated by the lower concentrations of NPOC, especially in the E, horizons. A possible connection between soil solution content of organic molecules and base cations is tested by plotting base cation concentrations against the NPOC concentrations (Figure 4). There is no clear dependency between NPOC and Na+, K', Mg2+,and Ca2+. Furthermore, the Ca2+concentrations obtained with the ceramic samplers are generally below 40 pmol,/L, and the Ca2+concentrations are in the range of 40-600 pmol,/L from the PTFE samplers. If this substantial difference (up to a factor of 15) between the Ca2+ concentrations obtained with the two types of samplers is caused by a filtering of organic molecules, a difference would be expected to appear for the other cations as well. However, there seem to be no connections between the filtering of Na', Ca2+,Mg2+,and K+, and no support can be found for possible filtering of organic Ca2+containing molecules. On the basis of the above considerations, no clear conclusions concerning the reason for higher Ca2+concentrations in the PTFE samplers can be drawn. (b) Protons. The sampling of different H+ concentrations with the two sampler types contradicts the findings in ref 15. The difference could be explained by H+ absorption in the PTFE matrix during the washing procedure prior to installation and subsequent release during sampling. However, this release would be expected to decrease
0
100
200
K (pmol(c)/liter)
300
0
2000
4000
6ooo
8000
Na (pmol(c)/liter)
Figure 4. Monthly mean concentrations (cLmol,/L) of Ca2+,Mg2+,K', and Na+ plotted against the NPOC concentration (ppm) independent of the sampler type.
over time and should be accompanied by a significant consumption of base cations, which is not seen. A slight H+ consumption in the ceramic samplers along with leaching of A13+,as suggested by Raulund-Rasmussen (9), or leaching of base cations could be another explanation. However, this is in contradiction to the findings since the solute concentrations of Ca2+and AP+ obtained with the ceramic samplers are usually lower. (c) Aluminum. The PTFE cups contain no weatherable aluminum compounds, and therefore, the PTFE samplers are assumed not to influence the concentration of aluminum by leaching. In the acidic soil investigated, a possible contamination of the soil solution by the ceramic cups should expectantly cause significantly higher concentrations in those samples compared to the PTFE samples. However, the highest aluminum concentrations are generally observed in the samples obtained with the PTFE samplers (Table 11,Figure 5), except in the drought plot, but the difference in aluminum concentration between the two sampler types is usually within the standard deviation of the PTFE samplers. These results agree with findings by Hendershot and Courchesne (It?), who found no leaching of A13+from ceramic cups in less acid soil solutions (pH 5.5-6.0). On the contrary, the present study contradicts the study of Raulund-Rasmussen (9,19), who found that the type of ceramic cups also used in this study may leach aluminum because of proton-induced dissolution of the cup material. To investigate the possibility that gibbsite or another Al compound Cjurbanite, kaolinite) governs the AP+ concentration, a plot of pH - 1/3pAl vs pH 1/2pS0, from the BC horizon of the control plot is shown in Figure 6. The data points are separated in yearly categories in order to indicate a possible time dependence. Figure 6 indicates that gibbsite or jurbanite controls the soil water concentrations of H+, AP+, and S042-, but this control is not changed during the sampling period. Further, if the environment around the cups is changed, this change is similar for both types of samplers. Plots for the three treated plots, which is not included in the figure, showed similar results. This result agrees with recent fmdings (15). Comparison of Soil Water Samplers. Comparison of soil water samplers under field conditions is made
+
Environ. Sci. Technol., Voi. 26, No. 10, 1992 2009
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5. Monrmy concenlratkm @d&) of A P obtained wkh the PTFE and the caramic samplers in the dmerent experimental treat-
ments.
AI(OH)3 (pK-33 9)
......AIOHS04 (pK-17 2)
L.,
5.4
5.6
5.8
6
6.2
pH+UZpS04
-
+
Flpun E. plot of monthly mean calculated pH '/@AI against pH '/@O, in the Bc horizon ofthe mnbd plot as a function of the year [Rst year Ls (1) and SBmnd year Ls (2)]. Unes fa AI(% and M O , are indicated.
difficult by the high spatial variability in the mil solution concentrations, which at this study site is a result of the 2010 Envkon. Sci. Tedmol.. Vol. 26, No. 10. 1992
natural spatial variability as well as a strong systematic variability depending on the distance to the single tree trunk (Figure 7). The systematic variability is possibly caused by the tree uptakelexchange of ions from the soil solution as well as an interaction of the canopy with the atmospheric deposition leading to the highest concentrations of solutes in throughfall close to the tree trunk ( Z ( t 2 2 ) . The large spatial variability was demonstrated even within a treatment plot at this study site (15) recently, and it may be the reason for some of the differences hetween samples from ceramic and PTFE samplera. Despite this variability, it should be expected that a possible retaining or leaching of substances in any one of the sampler types would show up as lower or higher concentrations, respectively, in the influenced samples at all treatments and horizons. It should be noted, however, that such chemical changes will be detected best at low concentration levels as high concentration levels in the soil solutions probably will 'hide" the influence of the samplers. This problem is highlighted by the differences between the results of this study performed at "normal" soil solution concentrations and the results reported by Maitre et al. (8) obtained from solutions with very low initial concentrations. Studies comparing different types of soil water samplers have led to contradicting and confusing results, which are hardly interpretable. As examples, it was formerly found that ceramic cups sampled higher concentrations of SO.,%, Ca2+,Mg2+, Mn2+,and K+ compared to samples obtained with zero tension samplers (12), lower concentrations of H+, Ca2+,K+, and NaCcompared to centrifugation (81, and lower concentrations of H+ and SO:- compared to samples obtained with zero tension pans (11).Assuming that the zero tension and the centrifugation sampling methods cause no change of the samples, it might be concluded that the observed differences are caused by the ceramic sampler. However, such results are not directly comparable since the methods differ in various respects, and differences may be caused by factors other than the cup sampler itself; e.g., two different sampler types may sample different sources of water (11,lZ). Thus, it seems a general problem in comparing soil water samplers, especially POrous cups, that the methods or the equipment used differ in too many respects. In our opinion, it is therefore necessary to supply more detailed information on the method used than is generally the case today. This especially includes the precise type of sampler used and the principle and magnitude of the applied vacuum, as it is probably too unspecific to refer to the soil water sampler as just ceramic, glass, PTFE, etc. Cups made of the *samen material but delivered by different manufactmra can be immensely different in respect to leaching or retaining of substances, depending on han-
dling processes and the original material used as well as the actual pore size. Furthermore, the way of applying tension may be important to the results obtained. Here, the variable tension principle was chosen because we believe that this method w i l l provide the most reliable results in respect to plant-available soil solutions. But obviously, this method is likely to provide significantly different results from the sudden tension method, where a high tension is applied for a short time (few days) and when sufficient water has been sampled the sampler is left in the soil without tension until next sampling. However, it is not known what the relationship is between the soil solution sampled by different sampling methods or different tension principles and the soil solution actually present in the soil pores of interest. This area certainly needs more research. Conclusions
This study compared soil solution chemistry in samples obtained over 2 years by two different types of tension soil water samplers made of ceramic and PTFE, respectively. Both types were connected to a continuous soil tension regulated vacuum pump, and all installation and sampling procedures were similar for the two sampler types. The mean concentrations obtained with the two sampler types over the 2-year study period differ in many cases. The highest concentrations are most often found in the PTFE samplers, but the differences in soil water concentrations of K+, Mg2+,Na+, C1-, S042-,and A13+obtained with the two sampler types are generally within the variability found for the nine PTFE samplers. In contrast, the difference in concentrations of Ca2+,H+, and dissolved organic matter, measured as NPOC, generally exceeds the variability of the PTFE samplers. The difference in NPOC is most likely caused by filtering of organic molecules by the ceramic samplers, whereas no clear explanations can be given for the differences in the Ca2+and H+ concentrations. A possible connection between the filtering of Ca2+and organic molecules is not supported, and Ca2+may therefore rather be a result of simple leaching or dissolution of Ca2+from the PTFE matrix, although the observations are contradictory. No conclusion can be drawn about the H+ concentration. Any use of soil solution data should include a close evaluation of the type of sampler used in relation to the soil type and the main purpose of the investigation. Since no sampler today can be considered ideal in all cases, the use of two or more sampler types may be necessary for evaluating the results obtained with each sampler type and to increase understanding of the soil water chemistry. So, in relation to the total cost of most studies, a doubling of the field equipment for at least 1 year is cost effective and is recommended. Acknowledgments
We are very grateful to technicians Preben Frederiksen, Preben Jmgensen, Lisbet Thomassen, and Andreas Harder
for their substantial effort and skillful work in the field and the laboratory. Registry No. PTFE, 9002-84-0; K+, 7440-09-7; Mg2+,7439Cl-, 16887-00-6; Sod2-,14808-79-8; NO3-, 95-4;Na+, 7440-23-5; 14797-55-8; Ca2+,7440-70-2; H+, 12408-02-5.
Literature Cited (1) Litaor, M. I. Water Resour. Res. 1988,24,727-733.
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Received for review December 12, 1991. Revised manuscript received June 10,1992.Accepted June 19,1992. The study was financed by EEC (GrantsEV4V-0029-DK and STEP 0038),The Ministry of Energy, and The Danish Forest and Nature Agency.
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