Properties and Distribution of Acid Sulfate Soils in Freshwater Wetlands During Drying-Wetting Cycles on Norfolk Island

Abstract

Freshwater inland wetland systems around the world have been significantly affected by human activities, particularly disturbances (excavation) as well as drying (drought) and wetting (flooding) scenarios. These impacts often result in lowered water table levels, which can have severe environmental impacts, particularly if acid sulfate soils (ASS) are present. If such soils are acidified, it can cause degradation of ecosystems and damage to infrastructure and the leaching of acidity and metals into receiving waters. This situation was identified on Norfolk Island following an extended run of dry years from 1970 to 2020. In the wetter years that followed the properties of the ASS changed depending on the severity of disturbance. The transformations that took place were investigated through description and sampling to 2 m deep on three occasions from 2020 (dry), 2021 (wet) to 2020 (flooding). These organic-rich wetlands with ASS span a range in severity of disturbance from cattle pugging (moderate), soil erosion, drainage, infill and cultivation (high), dam construction (very high), sheetflood erosion (severe) and deep excavation of drains (extreme). The study aimed to determine and evaluate the temporal and spatial variability of ASS in wetlands impacted by changes in the five anthropogenic disturbance categories across this drying-wetting cycle. Soil properties measured included pH, reduced inorganic sulfur (S_Cr), titratable actual acidity (TAA), retained acidity (RA), acid neutralizing capacity (ANC) and mineralogy using X-ray diffraction (XRD) analyses. Using this information, five descriptive soil-landscape models were constructed and supported by colour photographs. The aim was to enable ready understanding of ASS changes taking place. The models describe the variety of ASS materials, water movement and soil properties that can represent both reversible and irreversible changes. More specifically, the descriptive soil-landscape models illustrate how prolonged drying of hypersulfidic organic soils leads to the formation of sulfuric organic soils. These models also show how successive wetting and flooding in moderate to very highly disturbed wetlands transforms Sulfuric organic soils to Hypersulfidic and Monohypersulfidic organic soils under anoxic conditions (reversible changes). In contrast, these models also depict how wetting and flooding of in severe and extremely disturbed wetlands do not change Sulfuric organic soils (irreversible change) due to their being permanent disconnection from the water table. The descriptive soil-landscape models also illustrate how occurrences and formation of the iron-rich precipitates are significantly affected by drying-wetting cycles and anthropogenic disturbance. The following precipitates were prominent: (i) schwertmannite and ferrihydrite in ponded water and coating vegetation, (ii) iridescent floating films of schwertmannite and ferrihydrite on water, and (iii) ammoniojarosite coating vegetation on sulfuric organic soils. If acidification occurs, these mineral assemblages serve as visual indicators of ASS hazards, which can threaten water resources by reducing water quality and cause corrosion damage to heritage structures. Based on these findings a summarized soil-landscape explanatory model and predictive ASS models are presented in a companion paper. This article aims to assist land managers to better understand and respond to these diverse wetlands.