The relationship between soil heterotrophic activity, soil dissolved organic carbon (DOC) leachate, and catchment-scale DOC export in headwater catchments

Dissolved organic carbon (DOC) from terrestrial sources forms the major com ponent of the annual carbon budget in many headwater streams. In high-eleva tion catchments in the Rocky Mountains, DOC originates in the upper soil ho rizons and is flushed to the stream primarily during spring snowmelt. To id entify controls on the size of the mobile soil DOC pool available to be tra nsported during the annual melt event, we measured soil DOC production acro ss a range of vegetation communities and soil types together with catchment DOC export in paired watersheds in Summit County, Colorado. Both surface w ater DOC concentrations and watershed DOC export were lower in areas where pyrite weathering resulted in lower soil pH. Similarly, the amount of DOC l eached from organic soils was significantly smaller (p < 0.01) at sites hav ing low soil pH. Scaling point source measurements of DOC production and le aching to the two basins and assuming only vegetated areas contribute to DO C production, we calculated that the amount of mobile DOC available to be l eached to surface water during melt was 20.3 g C m(-2) in the circumneutral basin and 17.8 g C m(-2) in the catchment characterized by pyrite weatheri ng. The significant (r(2) = 0.91 and p < 0.05), linear relationship between overwinter CO2 flux and the amount of DOC leached from upper soil horizons during snowmelt suggests that the mechanism for the difference in producti on of mobile DOC was heterotrophic processing of soil carbon in snow-covere d soil. Furthermore, this strong relationship between over-winter heterotro phic activity and the size of the mobile DOC pool present in a range of soi l and vegetation types provides a likely mechanism for explaining the inter annual variability of DOC export observed in high elevation catchments.