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Effects of Forest Floor Leachate on Sulfate Retention in a Spodosol Soil

The Swedish Univ. of Agric. Sci., Dep. of Ecology and Environ. Res., Box 7072, 750 07 Uppsala, Sweden.

^* Corresponding author.

ABSTRACT

Because of the involvement of several different mechanisms, sulfate retention in forest soils is a rather complicated process. We have investigated the effect of the dissolved substances from the forest floor of a Haplorthod soil (A0 horizon) on SO^2–₄ mobility in the upper mineral soil (A1/A2 horizon). Sulfate retention (SO^2–₄ retained in a solid phase) was estimated using two different solutions—distilled water solution (DWS) and forest floor leachate (FFL). Different concentrations of SO^2–₄ (0.00–2000 mg S L^–1) were added to both extractants as Na₂SO₄-³⁵S. The results show that sulfate retention by the mineral soil (A1/A2) increased linearly with increasing SO^2–₄ concentration in the DWS containing up to 500 mg S L^–1, whereas retention was zero when the soil was equilibrated with the FFL containing <250 mg S L^–1. However, retention started to increase linearly when the FFL contained more than 250 mg S L^–1. These data indicate that in the range of SO^2–₄ found under field conditions, SO^2–₄ retention was completely inhibited by the leachate solutions. These data also suggest that the organic ligands in the FFL were competing with SO^2–₄ on the exchangeable positive charge sites. Moreover, they may have an inhibitory effect on the growth of some SO₄-Al mineral compounds, mainly jurbanite and/or basaluminite.

Key Words: Dissolved substances from forest floor • Fe and Al oxides • Organic ligands exchange • Ionic strength • Acidic deposition impact