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COMMENTS & LETTERS TO THE EDITOR

Response to "Comments on ‘Artifacts Caused by Collection of Soil Solution with Passive Capillary Samplers’"

Soil Science Program, Dep. of Agronomy The Pennsylvania State University 116 ASI Bldg. University Park, PA 16802

* Corresponding author (kwg106{at}psu.edu)

Abbreviations: PCAPS, passive capillary samplers • ZTS, zero-tension samplers

We thank Drs. Brahy and Delvaux for their interest and comments regarding our manuscript. Like these authors, we maintain that passive capillary samplers (PCAPS) have potential utility in geochemical studies. The abstract in Goyne et al. (2000) states that "the PCAPS used in this study are not suitable for aqueous geochemical studies of dilute soil solutions." Further, the conclusions state that "they can be problematic when used to collect dilute soil solutions for complete geochemical investigations". We stand by these statements and can, in fact, attribute our results unequivocally to weathering of the PCAPS wicks. We do not imply that other PCAPS must be likewise problematic. But our caution to others employing this technique without critical evaluation is that they can be.

As we stated in the paper, we constructed all of our PCAPS from the fiberglass rope used by Holder et al. (1991) and Boll et al. (1992) (no. 1381 from Pepperell Braiding Co., Pepperell, MA). We did not evaluate the suitability of alternative fiberglass wicking material. Since Brahy et al. (2000) do not report the source of the fiberglass wicking material that they utilized, a direct comparison between PCAPS used in their study and ours may not be advisable.

Although we agree that PCAPS and zero-tension samplers (ZTS) may collect different fractions of soil solution with different residence times and chemical composition (Marques et al., 1996, Goyne et al., 2000), this was not the causative factor in our study. The tension applied by our PCAPS was 5.4 kPa versus 60 kPa for suction cup samplers in Marques et al. (1996). Prior research at our study site involved a chemical comparison between B horizon soil solutions sampled with ZTS and ceramic suction cup solution samplers evacuated to 60 kPa (Swistock et al., 1990). These authors found that despite the large difference in tensions, there were no significant differences in the solution concentrations of H⁺, NO^-₃, Cl^-, Na⁺, and Al_T collected with the two types of samplers. In contrast, we observed (Table 2) large significant differences for H⁺, Na⁺, Al_T, alkalinity, and other ions for both A and B horizons, but still no difference for NO^-₃, which is evidently not a component of the wick material (Goyne, 1998; Goyne et al., 2000). The alteration occurred despite the fact that the tension exerted by our PCAPS was <0.1 that of the suction cup samplers used by Swistock et al. (1990).

It is possible that prewashing the wicks with 0.01 M HNO₃ increased the sensitivity of the wicks to further weathering in the field. However, our preliminary laboratory studies indicated that (i) washing the material with deionized water for 1–2 wk was insufficient for removing soluble materials that would later contaminate acidic soil solutions and (ii) wick dissolution effects appeared to diminish with time in the field (on the time scale of months).

It is apparent that researchers should be aware of potential artifacts in order to minimize them. In our case, direct comparison with other means of in situ soil solution collection proved to be a useful way to reveal problems. As mentioned in Goyne et al. (2000), we believe that alternative sources of fiberglass wicking material and cleaning procedures should be investigated to diminish unwanted effects.

Received for publication April 2, 2001.

REFERENCES

• Boll, J., T.S. Steenhuis, and J.S. Selker. 1992. Fiberglass wicks for sampling of water and solutes in the vadose zone. Soil Sci. Soc. Am. J. 56:701–707.[Abstract/Free Full Text]
• Brahy, V., H. Titeux, and B. Delvaux. 2000. Incipient podzolization and weathering caused by complexation in a forest Cambisol on loess as revealed by a soil solution study. Eur. J. Soil Sci. 51:475–484.
• Goyne, K.W. 1998. Comparisons of soil solution chemistry and quantity from soils formed on contrasting geologic units. M.S. thesis. The Pennsylvania State University, University Park.
• Goyne, K.W., R.L. Day, and J. Chorover. 2000. Artifacts caused by collection of soil solution with passive capillary samplers. Soil Sci. Am. J. 64:1330–1336.[Abstract/Free Full Text]
• Holder, M., K.W. Brown, J.C. Thomas, D. Zabcik, and H.E. Murry. 1991. Capillary-wick unsaturated zone soil pore water sampler. Soil Sci. Soc. Am. J. 55:1195–1202.[Abstract/Free Full Text]
• Marques, R., J. Ranger, D. Gelhaye, B. Pollier, Q. Ponette, and O. Goedert. 1996. Comparison of soil solutions collected by zero-tension plate lysimeters with those from ceramic-cup lysimeters in a forest soil. Eur. J. Soil Sci. 47:407–417.
• Swistock, B. R., J.J. Yamona, D.R. DeWalle, and W.E. Sharpe. 1990. Comparison of soil water chemistry and sample size requirements for pan vs. tension lysimeters. Water Air Soil Pollut. 50:387–396.

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