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

Abstract

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 Na 2 SO 4 ‐ 35 S. 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 SO 4 ‐Al mineral compounds, mainly jurbanite and/or basaluminite.