Monolithic lysimeters as tools to investigate processes in acid sulphate soil

The oxidation of sulphidic materials upon drainage of acid sulphate (AS) soils causes episodic hazards to aquatic ecosystems worldwide. Reintroducing a high groundwater level, in order to turn the soil reactions back to reduction, is an option to alleviate the environmental loading from these soils. In boreal conditions, with low soil temperatures and consequently slow chemical and microbial reactions, feasible tools are needed to effectively investigate the impacts of this practice on soil processes. With this aim, ten lysimeters containing AS soil monoliths (1m high, 0.5m wide) were set up in Finland to investigate the pore water quality and leaching of elements in response to an elevated water table. In this paper, the design of the lysimeters is described and results confirming their suitability for the purpose are presented. This paper focuses on the possible shortcomings of lysimeters in AS soils, such as unnaturally rapid oxidation of the sulphidic C horizon, side-wall flow of water in the ripening soil, and the impaired functioning of probes over time in the extremely acid soil and in the sulphidic materials. Dye and chloride tracing revealed that no significant sidewall flow occurred in the lysimeters. The continuous Eh measurements functioned unimpaired during the two-and-a-half-year experiment. Reduced conditions were preserved in the Cg horizon throughout the experiment. In drained horizons, Eh varied according to the water management practice and the growth of reed canary grass roots, but in the water-saturated horizons, Eh mainly responded to long-term changes in soil chemistry, such as the reduction of iron. Soil temperature in deep soil horizons closely corresponded to typical boreal conditions in winter, but it was a few degrees Celsius higher in summer. Our results demonstrate that it is feasible to simulate processes dependent on water management in AS soils with monolithic lysimeters.