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Soil Solution Sampled with and without Tension in Arable and Heathland Soils

Dep. of Agricultural Sciences, Section of Soil, Water and Plant Nutrition, Royal Veterinary and Agricultural Univ., Thorvaldsensvej 40, DK-1871 FC, Denmark

Niels Christensen

National Environmental Research Institute, P.O. Box 358, DK-4000, Denmark

*Corresponding author.

ABSTRACT

The composition of soil solution is of importance in many aspects of soil science. It is therefore of interest to reach an understanding of when to apply a certain sampling method in preference to others. For this reason we evaluated both zero-tension and tension samplers for use in long-term monitoring of soil solutions sampled fortnightly in a sandy arable soil plot from 1987 to 1990 and in a sandy calluna heathland plot from 1986 to 1990. Compared with tension solution, the zero-tension solution in the arable plot was more influenced by the soil solution composition from the upper fertilized plow layer, and in the heathland plot the zero-tension solution was more influenced by the composition of rainwater. On both plots the temporal variability in the amount of nonsorptive solutes, as well as systematic differences in the amount of sorptive solutes sampled by these techniques, give support to the notion that zero-tension samples represented the flux concentrations, whereas the tension samples were approximations of resident concentrations. In conclusion, if the objective of sampling soil solution is to gain insight in internal processes in a specific soil layer, tension methods are most appropriate. On the other hand, if the objective is to study the loss of solutes from a given layer, zero-tension methods should be used. Using less mobile water as a basis for the estimate of leaching processes may lead to biased results, especially when considering sorptive solutes. The estimates of nonsorptive solute leaching may also become biased, dependent on the coincidence of high drainage with biased concentration measurements.

Joint contribution of the Section of Soil, Water and Plant Nutrition, RVAU, and the Dep. of Terrestrial Ecology, National Environmental Research Institute, Denmark.

Received for publication May 29, 1992.