The role of organic matter in ameliorating acid sulfate soils with sulfuric horizons

• Organic matter addition to sulfuric soils significantly increased soil pH.
• Organic matter addition to sulfidic soils partially prevented acidification.
• Organic matter decomposition transformed sulfate to sulfide by anaerobic bacteria.
• Organic matter data used for management decisions in environment sensitive areas.

Acid sulfate soils with sulfuric horizons (pH < 4) can exert a range of negative impacts on the ecology and productivity of soils. The primary treatment for these soils is to raise the pH of the sulfuric horizons using lime. Although often effective, this treatment can be expensive and not well suited to large areas. In this laboratory study, we have investigated the possible use of plant organic matter (OM) to ameliorate: (i) “sulfuric soils” [produced by the oxidization of clayey sulfidic material (pH > 4) samples to form “sulfuric horizon material” (pH < 4)] and (ii) “neutralized sulfuric soils” [produced by the neutralization of peaty sulfuric horizon material (pH < 4) with alkaline sandy loam]. The advantage of this approach is that organic matter is readily available and inexpensive. The experimental treatments used leaf material from Phragmites australis as the source of organic matter, which was either incorporated into the two manufactured soils or applied to the surface. After 6 months of incubation under either aerobic or anaerobic soil conditions, pH, Eh and sulfate content were measured. The results showed that incorporation of OM into the sulfuric soil significantly increased soil pH, the extent depending on the moisture level. Changes in pH and sulfate content were correlated with Eh. Application of OM to the “neutralized sulfuric soil” was only partially effective in preventing acidification. It was concluded that the decomposition of OM by aerobic bacteria results in oxygen depletion, which then favors metabolic conversion of sulfates to sulfides by anaerobic bacteria. The results of this study have important implications for the broad scale management of acid sulfate soils.