Relationship between total dissolved organic carbon and SO 4−2 in soil and waters

Abstract

Soluble organic matter in forest soils appears to play a decisive role in the mobility of SO 4 −2. In this laboratory study, a forest Haplorthod soil was leached to investigate the relationship between total dissolved organic carbon (TOC) and SO 4 −2. The leaching solutions were distilled water (DW), sulfuric acid (S), sulphuric acid and nitric acid SN, (SN1, 1:1 M and SN2, 1:2 M). The pH of the acid solutions was 3.7. Pour chemical amendments were used, control (C), lime (L), calcium phosphate (P) and lime and calcium phosphate (LP). After leaching, the soil columns (48 columns) were sampled and analyzed. Evaluation of the leachate data consistently showed that the TOC was negatively or positively correlated with the mobility of SO 4 −2 when the EC was >100 or <100 μS/cm, respectively. However, results of the extracted soil solution showed that at any given depth the TOC was positively correlated with SO 4 −2 (EC < 100 μS/cm). The positive relationship between TOC and SO 4 −2 can be attributed to competition between TOC and SO 4 −2 on the exchangeable positive charge sites. In contrast, the negative relationship between the TOC and SO 4 −2 was due to increasing salt content that caused the TOC to precipitate. Sulfate, then, becomes the dominant soluble anion and is present at a higher concentration than TOC. A negative correlation between TOC and SO 4 −2 in waters is often found in data from natural stream and lake waters. It is apparent that the salt effect and competition mechanism between TOC and SO 4 −2 provide an explanation for the increasing TOC levels in natural water, especially in areas known to be affected by with high sulfate deposition.