Addition of wheat straw to acid sulfate soils with different clay contents reduces acidification in two consecutive submerged-moist cycles

Abstract

Acid sulfate soils (ASS) are wide-spread in wetlands and experience distinct biogeochemical changes during submerged and moist periods. During submerged periods, under reducing conditions, sulfate reduction can result in a pH increase whereas moist periods induce sulfide and ferrous iron oxidation, leading to acidification which can pose a threat to the environment. A laboratory incubation experiment was carried out to determine the effect of organic matter (OM) amendment on pH and redox potential over two successive submerged-moist cycles in different types of ASS (Sulfuric, Hypersulfidic and two Hyposulfidic soils). The four soils also differed in clay content (10, 15, 23, 38% referred to as C10, C15, C23 and C38) and were collected in a wetland at Banrock Station, South Australia. The experiment was conducted over 24 weeks with each submerged and each moist period lasting 6 weeks. The soils were unamended or amended with 10 g C kg−1 finely ground wheat straw. During the submerged periods, the soils were maintained under a layer of water, in the moist periods they were kept at 100% of water holding capacity. The soil pH increased in both submerged periods, particularly in amended soils with low clay content (C10 and C15). Changes in pH during submerged and moist periods were greater in soils with low clay content (C10, C15) than those with high clay content (C23, C38). The effect of wheat straw addition on pH at the end of submerged and moist periods did not differ between the two submerged-moist periods, with a higher pH in amended than unamended soils. In submerged periods, the redox potential (Eh) decreased more strongly in amended soils than in unamended soils. It was 100–200 mV from week 2 onwards in amended soils whereas the Eh was 300 to 400 mV higher without amendment except in C38 where it was 100–200 mV in both amendment treatments. In the moist periods, the pH decreased more strongly in amended than in unamended soils, particularly in C10 and C15 but remained higher than in unamended treatments. This study showed that wheat straw addition maintains its ameliorative effect on soil pH for at least two submerged-moist cycles, but the ameliorative pH effect depends on clay content, being greater in soils with low clay content.