Sources and controls of sulphur export in Precambrian Shield catchments in south-central Ontario

A series of studies was undertaken at Plastic Lake-1 (PC1) to determine the sources and controls on S cycling in small headwater catchments on the Precambrian Shield in south-central Ontario. Two observations were made about the S cycle in this region: (1) all streams exhibit highly coherent temporal patterns in SO₄ concentrations and export, and (2) most catchments exported more SO₄ in stream water than is received in bulk deposition during the past 2 decades. Synchronous temporal patterns in annual SO ₄ concentrations in both upland and wetland-draining streams were related to changes in climate, specifically those factors that determine catchment dryness. The number of days with no stream flow or stream flow below a critical threshold was a good predictor of the average stream SO₄ concentration in a particular year. Sulphate chemistry in the PC1 outflow is highly dependent on processes occurring in a conifer Sphagnum swamp located immediately upstream of the chemical sampling station. Hydrologic inputs to the swamp during the summer determine whether S is retained or released from peat on an annual basis. Drying and re-wetting of Sphagnum-derived peat caused a substantial increase in soluble SO₄ in laboratory experiments, which was slightly enhanced at higher temperature, but alternating moisture conditions had no immediate effect on Sphagnum. Despite large inter-annual changes in SO₄ release, over the long-term (i.e. 20-years) SO₄ inputs and exports from the swamp are in approximate balance. In contrast, the upland portion of PC1 ( i.e. PC1–08) consistently exports more SO₄ than is input in bulk deposition in every year of record. Even when inputs are increased to account for potential underestimates in dry deposition or weathering, the majority of catchments in this region exhibit net export in many years. Two internal sources are suggested to account for negative budgets: desorption and mineralization. Adsorption/desorption reactions respond directly to changes in SO₄ input concentration, and lysimeter data indicate the importance of these processes for buffering short-term changes in SO₄ concentration in LFH percolate. Desorption may be the primary direct response of upland soil to decreasing SO₄ inputs in deposition and may substantially extend the period of net SO₄ export in catchments that have large adsorbed SO₄ pools such as PC1. However, the adsorbed pool may be sustained by continuous net release from mineralization, and should also be considered in budget calculations. Mineralization was shown to be responsive to drying and re-wetting events and temperature, although results varied among different materials. Sulphate release from mineral soil did not appear to be influenced by changing moisture, temperature or deposition chemistry in laboratory experiments, although adsorption/desorption reactions may have largely masked small changes in SO₄ release via mineralization. The magnitude of organic S storage in mineral soil indicates that this pool could be an important source of export over the long-term. (Abstract shortened by UMI.)...