Addition of organic matter influences pH changes in reduced and oxidised acid sulfate soils

• Organic matter addition stimulated pH increase during submergence of ASS materials.
• In the following dry period the pH remained higher in amended soils.
• The pH decrease in the dry period was smaller at 40% WHC than at higher water contents.
• The pH increase in the second wet period in amended soils was smaller than in the first wet period.

Acid sulfate soils (ASS) are wide-spread in coastal and inland wetlands. When sulfidic ASS are exposed to oxygen, sulfuric acid is generated which can threaten wetlands and surrounding ecosystems. Organic matter plays an important role in ASS, as energy source for sulfate reducing bacteria during submergence and by stimulating competition for oxygen between oxidation of iron sulfide and utilisation by decomposers during dry periods. The aim of this study was to assess the effect of organic matter addition, as a potential management strategy, on pH changes in ASS in submerged and dry periods. Three ASS, namely sulfuric, hypersulfidic and hyposulfidic soils, collected from one profile in a wetland were used unamended or amended with 10 g C kg− 1 as finely ground wheat straw. The soils were exposed to a submerged (wet) period, a dry period, followed by another wet period. In the first wet period (10 weeks), the pH increased only in the amended soils, which was accompanied by a strong decrease in redox potential. To investigate the effect of water content during the dry period on pH, the soils were rapidly dried to 40, 60, 80 or 100% of water holding capacity (WHC) at the start of the dry period. This water content was maintained during the dry period. The pH decrease during the 10 week dry period was greater in amended than in unamended soils and greater at 60, 80 or 100% than at 40% of WHC. At the end of the dry period, the pH was higher in amended than in unamended soils and greater at 40% of WHC than at the higher water contents. In the second wet period (16 weeks), the pH increased only in the amended soils. The pH increase was accompanied by a decrease in redox potential in the amended soils. The water content in the previous dry period did not influence pH in the second wet period in the unamended soils, but in the amended soils, the pH was higher in soils previously maintained at 40% of WHC than that maintained at higher water contents. At the end of the second wet period, the pH was higher in amended than in unamended soils. This study shows the ameliorative effect of organic matter addition in ASS. Organic matter addition can improve energy supply for sulfate reducers which results in an increase in pH during the wet period and lead to a higher pH in the oxidation period. The smaller pH increase and redox potential decrease in amended soils in the second compared to the first wet period suggest that OM decomposition was lower in the second wet period likely because rapidly decomposable compounds had been utilised in the previous wet and dry periods and only recalcitrant OM remained. Therefore OM may have to be added repeatedly for sustained amelioration of ASS.