Long-term field comparison of ceramic and poly(tetrafluoroethene) porous cup soil water samplers

Abstract

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.