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Vincent Brahy, Scientific Collaborator, Fonds National de la Recherche Scientifique (FNRS) Université catholique de Louvain (UCL), Faculté des Sciences Agronomiques Bruno Delvaux, Département des Sciences du Milieu et de l'Aménagement du Territoire Unité des Sciences du sol, 2/10, Place Croix du Sud B-1348 Louvain-La-Neuve, Belgium
Abbreviations: PCAPS, passive capillary samplers
The paper by Goyne et al. (2000) focuses on artifacts caused by the collection of soil solution with passive capillary samplers (PCAPS). They conclude that using PCAPS is not suitable for aqueous geochemical studies of dilute soil solutions, mainly because acid-washed PCAPS reduced Al concentrations and increased pH and concentrations of Ca, Na, and Si relative to zero-tension samplers. We believe that the authors cannot attribute unequivocally these differences in solution chemistry to leaching and weathering of PCAPS fiberglass wicks because they sampled two different categories of soil solutions, one at zero tension and the other one at 5.4 kPa (PCAPS). The two samplers collected different fractions of soil solution with different residence times in the soil and thus chemical composition (Marquès et al., 1996). We believe also that the concluding statements by Goyne et al. (2000) cannot be extrapolated for all soil geochemical studies. The PCAPS technique can be valid to collect soil solutions, as reported by us in forest acid soils on loess in Belgium (Brahy et al., 2000; Brahy, 2000). In our study (1995–1998), all samples from the two first sampling dates were discarded because some of them presented large pH values (6.1–8.3) and large Na+ concentrations (1.1–8.2 mmolc L-1). These values rapidly decreased in the subsequent samples and reached values similar to the ones presented in Table 2 (in Brahy et al., 2000). After 4 mo in our forest soils, the PCAPS seemed to interfere very little with the pH, the organic compounds, and the major cations and anions of the solution. The following observations support this assessment; (i) Although we can not compare the composition of the soil solutions extracted with distinct sampling techniques, the concentrations we measured were quite similar to those measured in other European loessic soils under forest (Bredemeier et al., 1990; Van der Salm and De Vries, 2000). (ii) In our soil solutions, the concentration of Si, Al, and the sum of the concentrations of alkali and alkali-earth cations (Na++ K++ Ca2++ Mg2+ mmolc L-1) were positively and strongly correlated with the concentration of NO-3 (R = 0.66, 0.92, and 0.78, respectively; n = 100). It is well known that in acid brown forest soils, the production of nitric acid has a major impact on the dissolution of aluminosilicates and the mobilization of Al, alkali, and alkali-earth cations (Berthelin et al., 1990). The fiberglass wicks do not adsorb or desorb NO-3 (Holder et al., 1991). Therefore, the correlations we measured support the fact that PCAPS interfered very little with Al, Si, Na, Ca, K, and Mg. (iii) In addition to soil solution samplers, we used cation-exchange resin and test-vermiculite inserted in situ to study soil weathering processes. The ion accumulation on the resin and the transformation of the test-vermiculite are both in excellent agreement with the composition of the soil solutions (Brahy et al., 2000; Brahy, 2000). For example, the relatively large Mg amount sorbed by the resin in the AB horizon of the podzolized Cambisol is consistent with the large Mg concentration in the solution (Fig. 1)
. This large Mg concentration has been related to the weathering of Mg-bearing phyllosilicates depleted of Al interlayers in complexing environments (Brahy et al., 2000). We believe that the major discrepancies between our results and the results presented by Goyne et al. (2000) could be because of both the kind and the duration of the fiberglass wicks pretreatment. Goyne et al. soaked the wicking material in 0.01 M HNO3 and changed the solution every 24 h for 10 d until the pH stabilized at pH 2. In our study, we did not use strong acids. We washed the wicks with deionized water and we changed the rinsing water every day until its electrical conductivity reached very small values (<2 µS). The whole procedure took 
2 mo. Goyne et al. (2000) postulated that their acid treatment did not destroy the integrity of the fiberglass rope. This might not be the case as they noted that harsh cleaning treatments with more concentrated acid solutions resulted in complete wick dissolution. Most probably, a strong acid pretreatment of the fiberglass wicks could increase the sensitivity of the wicks to weathering and contamination of the soil solutions. This could increase the requisite duration for an optimal cleaning of the wicks, in the laboratory and in the field. In conclusion, we believe that the concluding statement by Goyne et al. (2000) should be more nuanced in that PCAPS might be useful to monitor the composition of dilute soil solution after a suited pretreatment of the fiberglass wicks. Therefore, in agreement with Goyne et al. (2000), we also believe that it is necessary to develop and to investigate alternative pretreatments to improve the cleaning of the fiberglass rope prior to use PCAPS in acid forest soils.
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