PARENT SESSION
Oral Session 1: Aquatic Ecology I: Rivers, Nutrients, and Water Quality.
Presiding: D Kashian
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room A 105.

What carbon sources support groundwater microbial activity in riparian forests?

Gurwick, Noel^*,1, Groffman, Peter², Stolt, Mark³, Gold, Arthur³, McCorkle, Daniel⁴, Kellogg, D.³, ¹ Program in Biogeochemistry, Ithaca, NY, USA² Institute of Ecosystem Studies, Millbrook, NY, USA³ University of Rhode Island, Kingston, RI, USA⁴ Woods Hole Oceanographic Institution, Woods Hole, MA, USA

ABSTRACT- A major question in riparian research is the source of energy to support subsurface microbial denitrification. Because the supply of labile carbon frequently limits microbial activity in the subsurface, identifying the relative importance of carbon sources to this ecosystem helps explain spatial heterogeneity of denitrification rates. We have investigated the importance of buried, carbon-rich soil horizons, deep roots and dissolved organic carbon as potential carbon sources to support groundwater denitrification in riparian forests in Rhode Island. We used field observations, laboratory soil incubations, and in-situ groundwater incubations to evaluate these sources at four sites in different geomorphic settings. In particular, we measured the ¹⁴C-DIC signature of ambient groundwater and groundwater that had been degassed, re-introduced into the well, and incubated in-situ. Buried horizons formed by alluvial processes appear to be an important source of carbon in the subsurface, as shown by active respiration in laboratory incubations; greater microbial biomass in buried carbon-rich soils compared to surrounding carbon-poor soils; and the presence of very old carbon (>1,000 ybp) in DIC 225 cm beneath the surface. DIC collected from shallower wells showed no clear evidence of ancient carbon. Roots also appear to be important, creating hotspots of carbon availability and denitrification in the generally carbon poor subsurface matrix formed by glaciofluvial processes. Dissolved organic carbon did not stimulate denitrification in aquifer microcosms in the laboratory, suggesting that this was not an important carbon source for denitrification in our sites. Determining which carbon source is fueling denitrification has practical implications. Where buried horizons are the key source, surface management of the riparian zone will likely have little direct influence on groundwater denitrification. Where roots are the key source, changes in the plant community are likely to influence denitrification capacity in the subsurface.

Key words: root production, biogeochemistry, riparian forest, denitrification