Impact of Temperature and Hydrological Residence Time on the Fate and Transport of Iron and Organic Carbon in Subalpine Wetlands

Abstract:

Wetlands contain one third of the planet’s soil carbon (C) and are characterized by markedly different chemical and physical environments than terrestrial ecosystems. The hydrologic residence time and temperature in wetlands influences their redox conditions and thus biogeochemical reaction rates. In these environments, transformation and movement of C and iron (Fe) are closely linked due to the sorption of organic C by solid Fe(III)-phases. Therefore, changes in Fe biogeochemical cycling will influence the size and turnover rate of soil C pools, which could negatively impact water quality and C storage.

We examined the effects of hydrologic residence time and temperature on reduction of autochtonous Fe(III)-oxides. Fe(II)-export rates, used as a lower bound for bulk Fe(III)-reduction rates, and dissolved organic carbon (DOC) export rates were measured on intact mineral soil cores using flow-through reactor experiments exposed to temperatures based on mean annual conditions (6°, 12°, and 18°C). Soils were from a slope and a depressional subalpine wetland (USDA Fraser Experimental Forest, CO, USA), characterized by different hydrologic residence times, were compared.

In the depressional wetland we observed the shallower soil depths have higher overall Fe(II)-export rates than the deeper soil depths. As temperature increases, Fe(II)-export rate increases, with a more than doubling in magnitude from 12 to 18° C. In comparing sites that are continuously inundated to those that are seasonally inundated, surprisingly we see higher Fe(II)-export rates in the seasonally inundated sites for all temperatures and depths. In the slope wetland we observed an opposite trend with depth and with temperature, with Fe(II)-export rates declining by almost an order of magnitude between 6 and 12°C.

In addition, our results showed a positive correlation between Fe(II)-export rates and DOC export rates suggesting Fe(III)-reduction exerts a biogeochemical control on water quality. Overall, knowing Fe(III)-reduction and DOC export are sensitive to changes in temperature and hydrologic residence time provides important insight into how subalpine wetland systems may respond to the predicted increase in soil temperatures and dramatic precipitation events accompanying climate change.