Sorption retards remediation of clayey sulfuric soils with straw-derived dissolved organic matter

When sulfidic soils become drained, oxidation of pyrite can cause acidification and formation of iron (Fe) oxyhydroxy sulfate phases such as jarosite. Remediation via re-establishment of reducing conditions requires submergence and addition of biodegradable organic carbon (OC) to stimulate activity of reducing bacteria. Addition of straw-derived dissolved organic carbon (DOC) has been shown to induce rapid microbial reduction in sandy sulfuric (pH <4) soils. In clayey sulfuric soil, DOC may be less efficient because of limited availability for microbes due to its sorption to reactive minerals. We tested the possible effect of sorption on the remediative potential of straw-derived DOC using a set of incubation and sorption experiments, and used solid-state ¹³C-NMR spectroscopy for the chemical characterization of OC. The tested materials were a clayey, jarosite-containing sulfuric soil (pH 3), and artificial model soils composed of synthesized jarosite either mixed with quartz powder or quartz powder + clay minerals. The results showed that addition of DOC from wheat straw induces reduction conditions varying with soil sorptivity. For the model soils, DOC sorption was little, and DOC additions of 0.8 mg OC g⁻¹ were sufficient to achieve permanently reducing conditions and an increase in pH to >6.0. In the natural sulfuric soil, much higher DOC additions were needed (1.8 mg OC g⁻¹) to facilitate continuous reducing conditions, but pH increased only to values no higher than 5.0–5.5. The natural soil revealed strong sorption of added DOC. Sorption preferentially reduced the proportion of proteins, while the proportion of lignin components, which can hardly be used by microorganisms under reducing conditions, remained relatively high in solution. Thus, high DOC additions were required to overcome the sorption-induced limitations in OC availability. The results suggest that wheat straw-derived DOC is a promising approach also for remediation of clayey sulfuric soils; however, OC additions need to be adjusted to compensate for possible sorption.