Assessment of the acidification risk of the acid sulfate soil materials in a tropical coastal peat bog:Muthurajawela Marsh, Sri Lanka

Abstract

Coastal regions in tropical countries encompass a diverse set of highly productive ecosystems with underlying acid sulfate (AS) soil materials. Muthurajawela Marsh is a tropical, saline, peat bog on the western coast of Sri Lanka and is known to contain AS soil materials. It is a critically important coastal wetland ecosystem and provides a multitude of benefits and services to the surrounding environment and the people in the area. At present, the AS soil materials in the marsh are at risk of exposure due to development activities in the surrounding areas. In this study, net acidity (NA) was quantified using an acid base accounting approach which includes retained acidity (RA) in addition to actual and potential sulfidic acidity (AA and PSA). The NA and the other soil characteristics were investigated in three soil profiles down to 1.5 m, along a north–south transect. All sites contained hypersulfidic soil materials as confirmed by field pH (pHF) > 4, field oxidation pH (pHFOX) < 4 and sulfide content > 0.01%. Net acidity values ranged from 23 to 4000 mol H+ t-1which was above the recommended management requirement value for peat and medium clay soils (i.e. 18 and 36 mol H+ t−1). At the northern site (S1), PSA was the main contributor to the NA, indicating future risk if the site were to become exposed to air. Both AA and RA were major contributing fractions at the middle (S2) and southern (S3) sites, with a possible imminent acidity discharge. All sites lack inherent buffering capacity, consequently, acidity can be released from the oxidation of the AS soil materials leading to the potential impact on the marsh ecosystem. The findings of this study indicate that human activities should be carefully managed to minimize the hazards that can occur due to exposing AS soils in the marsh.