Dr Chamindra Lakmali Vithana

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.

This article provides a general introduction to acid sulfate soil materials including those in Muthurajawela, a marsh of high ecological significance on the western coast of Sri Lanka. This peat bog is identified as one of the country's two acid sulfate soils landscapes. It covers ~3000 ha in area and contains a 3 to 7 m thick peat layer. Here we review the acid sulfate soils landscape of Muthurajawela marsh providing a broad description of the marsh, including its environmental setting, soil/peat characteristics along with the threats it faces. Furthermore, we briefly discuss the biogeochemistry of coastal acid sulfate soils (CASS) and the relationship to Muthurajawela. Finally, we discuss potential impacts of acid sulfate soil materials on the Muthurajawela marsh, and thereby, identify gaps in the understanding of the marsh's geochemistry. Our study identified that future investigations are required to assess the acidity status in the marsh, partitioning behaviour of trace elements in acid sulfate soil materials and to assess the impact of salt water intrusion and extreme weather events on acid sulfate soil materials.
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•Muthurajawela marsh on the west coast is a saline peat bog with acid sulfate soils.•The marsh is being severely exploited by anthropogenic activities.•World acid sulfate soils research body lacks Sri Lankan contribution.•Acidity status and trace elements partitioning behaviour in the marsh are unknown.•Proper management is needed to prevent occurrence of acid sulfate soil hazards.