Date of Graduation

12-2016

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

J. Thad Scott

Committee Member

Jason M. Taylor

Second Committee Member

Lisa Wood

Third Committee Member

Mike Daniels

Keywords

Biological sciences, Earth sciences, Agricultural ditches, Best management practice, Denitrification, Nitrogen cycling, Nutrient loading

Abstract

Nitrogen (N) derived from fertilizer application in agricultural systems may contribute to significant environmental impacts, including eutrophication of fresh and coastal waters. Rice cutgrass (Leersia oryzoides) can significantly enhance denitrification potential in agricultural ditch sediments, but relationships with known drivers are not well understood. To address this, I examined effects of nitrate (NO3-) availability on dinitrogen gas (N2) and NO3- fluxes seasonally in Chapter 2. Denitrification rates were measured as N2 flux from intact vegetated sediment cores using Membrane Inlet Mass Spectrometry (MIMS). Michaelis-Menten models were developed from observations to mathematically describe N2 fluxes across the spring, summer, and fall seasons. Summer N2 models exhibited the highest Vmax and K, with N2 fluxes peaking near 20 mg m-2 h-1. In all seasons, percent NO3- retention peaked at 1 mg L-1, before decreasing with increasing NO3- concentrations, except summer where maximum retention was maintained from 1-5 mg L-1 before declining at higher concentrations. Denitrification rates were strongly correlated with NO3- uptake rates by vegetated sediments in spring (r2 = 0.94; p < 0.0001) and summer (r2 = 0.97; p < 0.0001), but low NO3- uptake resulted in virtually no net denitrification in fall and winter. Sediments vegetated with cutgrass immobilized a significant fraction of NO3- entering them and permanently removed up to 30-40% of immobilized NO3- through denitrification during the growing season. I then applied models developed in Chapter 2 to existing datasets from experiments conducted at two different scales: mesocosms and experimental ditches (Chapter 3). Both models estimated similar peaks in net N2 fluxes from mesocosm data. Additionally, estimates of areal N2 production from the mesocosm study were similar to those predicted via mass balance in a previous study. Model application to the experimental ditch study highlighted differences between weired and non-weired ditches; however, estimates from linear regression model did not reflect trends previously reported in the literature. Further exploration into model application is necessary to determine the utility of both models, but both models may be useful in informing more complex models of N movement in agricultural watersheds to help land managers quantify the benefits of BMP implementation.

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