Comparison of chemical composition of soil solutions collected by zero-tension plate lysimeters with those from ceramic-cup lysimeters in a forest soil

We analysed the chemistry of solutions collected from soil by zero-tension plate lysimeters and cup lysimeters connected to a constant suction (600 hPa) under Douglas fir in the Beaujolais mountains (France). The chemistry of zero-tension lysimeters' (ZTL) and tension lysimeters' (TL) solutions differed enormously: TL solutions were most concentrated in Si, NH4+-N, NO3-N, Cl-, Mg2+ and Al3+ and TOC, whereas Ca2+ and K+ concentrations were greater in ZTL solutions. Organic matter (OM) greatly influenced the solution chemistry in both ZTL and TL. The chemistry of ZTL solutions was affected mainly by OM mineralization in the forest floor and upper soil horizon, and that of TL solution seemed to be related to destabilization of humified compounds under the new vegetation. Nitrification was important: residual nitrification led to excess protons neutralized in weathering or ion exchange reactions, mobilized cations, mainly Al3+. Selectivity coefficients calculated both for ZTL and TL solutions formalized the differences between the two types of solutions. Nevertheless, these coefficients remained most often within the same order of magnitude indicating relations between them. Solutions from the two devices provided different information and should not be used for the same objectives. Solutions from ZTL are suited for ecosystem input-output budgets, whereas TL solutions are more useful when equilibrium between the solution and solid phase or when plant nutrition are considered.