Dissolved Organic Carbon Dynamics in a Riparian Aquifer: Effects of Hydrology and Nitrate Enrichment

The capacity of riparian soils to remove nitrate (NO-3) from ground water is well established, but the effects of ground water NO-3-enrichment on C dynamics are not well studied. We incubated horizontal cores of aquifer material extracted from beneath moderately well-drained (MWD) and poorly drained (PD) soils in a riparian forest in Rhode Island, USA for 132 d, and dosed (flow rate, 170 mL d-1; dissolved O2, 2 in PD and 5 mg L-1 in MWD cores) with ground water amended with either Br-, Br- + NO-3 (10 mg N L-1), or Br- + NO-3 + DOC (20 mg C L-1). The DOC was extracted from forest floor material and added during the first 56 d of the experiment. Addition of NO-3 had limited effect on CO2 production while DOC amendment had a significant effect in the PD but not in the MWD mesocosms. Total CO2 production (mg CO2–C kg-1 soil) was 6.3, 7.0, and 10.1 in the PD and 3.6, 4.0, and 4.5 in the MWD cores amended with Br-, Br- + NO-3, and Br- + NO-3 + DOC, respectively. Carbon balance (Cbal = DOCin - (DOCout + CO2–C) showed a net C retention of 8.0 mg C kg-1 soil in the DOC-amended MWD cores (equivalent to 50% of the DOC added), and a net C loss of 8.3 mg C kg-1 soil in similarly treated PD cores. The lack of C retention in the PD cores was ascribed to reductive dissolution of minerals implicated in DOC sorption. These findings underscore that there is marked variation in C dynamics in riparian aquifers that has the potential to influence the fate of NO3- and DOC in the landscape.