The Missing Flux in a 35S Budget for the Soils of a Small Polluted Catchment

A combination of cosmogenic and artificial ³⁵S was used to assess the movement of sulfur in a steep Central European catchment affected by spruce die-back. The Jeze?í catchment, Kru?né Hory Mts. (Czech Republic) is characterized by a large disproportion between atmospheric S input and S output via stream discharge, with S output currently exceeding S input three times. A relatively high natural concentration of cosmogenic ³⁵S (42 mBq L^-1) was found in atmospheric deposition into the catchment in winter and spring of 2000. In contrast, stream discharge contained only 2 mBq L^-1. Consequently, more than 95% of the deposited S is cycled or retained within the catchment for more than several months, while older S is exported via surface water. In spring, when the soil temperature is above 0 °C, practically no S from instantaneous rainfall is exported, despite the steepness of the slopes and the relatively short mean residence time of water in the catchment (6.5 months). Sulfur cycling in the soil includes not just adsorption of inorganic sulfate and biological uptake, but also volatilization of S compounds back into the atmosphere. Laboratory incubations of an Orthic Podzol from Jeze?í spiked with 720 kBq of artificial ³⁵S showed a 20% loss of the spike within 18 weeks under summer conditions. Under winter conditions, the ³⁵S loss was insignificant (<5%). This missing S flux was interpreted as volatilized hydrogen sulfide resulting from intermittent dissimilatory bacterial sulfate reduction. The missing S flux is comparable to the estimated uncertainty in many catchment S mass balances (±10%), or even larger, and should be considered in constructing these mass balances. In severely polluted forest catchments, such as Jeze?í, sulfur loss to volatilization may exceed 13 kg ha^-1 a^-1, which is more than the current total atmospheric S input in large parts of North America and Europe.