Purpose

Acid sulfate soils (ASS) are common in wetlands and can pose an environmental threat when they dry because oxidation of pyrite may cause strong acidification. Addition of organic matter can stimulate sulfate reduction during wet periods and minimize acidification during dry periods. However, the effect of the organic amendment may depend on its composition.

Materials and methods

Three wetland acid sulfate (sulfuric, hypersulfidic, and hyposulfidic) soils collected from different depth in one profile were used. The soils, unamended or amended with 10 g C kg^?1 as glucose, wheat straw, pea straw, or Phragmites litter, were incubated for 18 weeks under flooded conditions (“wet period”) followed by 10 weeks during which the soils were maintained at 100 % of maximum water-holding capacity (“dry period”).

Results and discussion

During the wet period, the pH decreased in the control and with glucose to pH 3–4, but increased or was maintained in residue-amended soils (pH at the end of the wet period about 7). In the dry period, the pH of the control and glucose-amended soils remained low, whereas the pH in residue-amended soils decreased. However, at end of the dry period, the pH was higher in residue-amended soils than in the control or glucose-amended soils, particularly with pea straw (C/N 50).

Conclusions

Amendment of acid sulfate soils with plant residues (particularly those with low to moderate C/N ratio) can stimulate pH increase during flooding and reduce acidification under oxidizing conditions.