Graduation Term

2015

Degree Name

Doctor of Philosophy (PhD)

Department

School of Biological Sciences

Committee Chair

William L. Perry

Abstract

Watershed biogeochemistry throughout Midwestern agroecosystems have been altered through hydrologic manipulation and over-application of nitrogenous fertilizers. As a result, nitrate (NO3-N) export from subsurface drainage has negative impacts on local and downstream ecosystem health. Wetland installation has proven to be a viable option for targeted management where a large proportion of NO3-N is removed through the bacterially mediated process denitrification. For denitrification to maintain high rates under prolonged NO3-N saturation, a stable supply of labile dissolved organic carbon (DOC) is required. The focus of this dissertation was to study how the stoichiometry of agricultural wetlands limits denitrification within a controlled laboratory and field-scale applications.

Denitrification rates within wetlands that retain subsurface tile drainage were limited by the availability of DOC. The limitation of DOC became more evident under high NO3-N concentrations, suggesting that wetland sediments have an insufficient pool of labile DOC to maintain elevated rates of denitrification during seasonally intense NO3-N inputs. It was than hypothesized that terrestrial DOC contributions delivered by surface water would serve as an effective DOC subsidy for denitrifying bacteria. It was found that wetlands retaining drain tile or surface water exhibited similar changes in denitrification to 2:1 and 4:1 C:N ratios.

Bacterial production and denitrification were measured under low (1:1) and high (4:1) C:N ratios to test how bacteria allocate DOC at differing ratios. There was no change in denitrification in sediments incubated at the 1:1 ratio, while bacterial production significantly increased throughout the incubation period. The 4:1 ratio did result in a significant increase in denitrification rates and bacterial production, but bacterial production did not differ between the 1:1 and 4:1 treatments.

Changes in NO3-N removal and denitrification in response to DOC availability (1 mg/L and 10 mg/L) were observed throughout replicate 5 days. Denitrification rates increased significantly at both DOC concentrations throughout the study period. Reductions in NO3-N concentration were observed at low and high DOC availability, however significant reductions only occurred at the 10 mg/L DOC treatment wetlands.

The contribution cover crops have on WEOC in agricultural soils was tested by modeling spectral data from dissolved C fractions extracted from soil cores. Within the first years of cover crop implementation, the amount and types of WEOC did not differ between plots with or without cover crops. This also translated into similar rates of denitrification observed throughout the study period, suggesting that terrestrial denitrification was limited by C availability, similar to receiving aquatic systems.

Access Type

Dissertation-Open Access

DOI

http://doi.org/10.30707/ETD2015.Grebliunas.B

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