Role of SO4 adsorption and desorption in the long-term S budget of a coniferous catchment on the Canadian Shield

The hypothesis that SO₄ desorption can explain apparent long term net SO₄-S losses (5 kg·ha^–1·yr^–1 on average) at the Lake Laflamme catchment from 1982 to 1991 is examined. Field observations show that SO₄ concentrations in the soil solution are strongly buffered during percolation through the Bf horizon. In the Bf horizon, SO₄ exchange reactions between the adsorbed and aqueous compartments are rapid (hours). Most (60%) of the adsorbed SO₄ may be readily desorbed with deionized water. These observations and the presence of an important adsorbed SO₄-S reservoir in the Bf horizon (113 kg·ha^–1) as compared with annual wet SO₄-S deposition (7 kg·ha^–1), suggest that on the short-term, adsorption and desorption reactions can control dissolved SO₄ concentration in the Bf horizon. To examine whether SO₄ adsorption/desorption could explain long-term SO₄-S losses by the catchment, an aggregated Langmuir isotherm for the Bf horizon was used to calculate the catchment's resilience to changing SO₄-S loads. The results indicate that the soil should adjust rapidly (within 4 years) to changing SO₄-S loads and that SO₄ desorption alone cannot explain long-term net SO₄-S losses. Other possibilities, such as an underestimation of dry deposition or the weathering of S-bearing minerals also appear unlikely. Our results suggest a net release of SO₄-S from the soil organic S reservoirs (1230 kg·ha^–1) present in the catchment.