Sulphate Flux from an Upland Forested Catchment in South-Central Ontario, Canada

Net SO4 export from forested catchments will delay the response of downstream surface waters to decreases in S-deposition. In the Muskoka-Haliburton region of south-central Ontario, net SO4 export from wetland-dominated catchments is largely explained by release of stored S from swamps, although similar net export from upland catchments requires an alternate explanation. To investigate possible sources and controls of SO4 export in upland systems, SO4 input-output budgets were calculated for a small (3.45 ha) upland, forested catchment in Haliburton County over an eight yr period. Temporal patterns of SO4 concentrations in soil leachate were significantly correlated with patterns in stream water, but expected relationships with deposition were weak. Instead, SO4 concentrations in surface soil leachate (LFH) appeared to be related to changes in soil moisture, and SO4 concentrations in LFH leachate increased dramatically in the summer, when warm dry periods were broken by rainstorms. Higher SO4 concentrations in organic surface soils were translated vertically down the soil profile; however, SO4 concentrations at the base of the soil profile (B-horizon) were much less variable than in the LFH, and appear to be modified by processes occurring in the mineral soil, likely adsorption/desorption reactions. Deposition is the major source of SO4 to catchments in this region, but SO4 export in stream water exceeded input via bulk deposition by at least 40% in every year of record. Net SO4 export from upland catchments appears to be a result of SO4 release through mineralization of organic compounds and desorption from subsurface mineral soil.