Degree Type

Dissertation

Date of Award

2018

Degree Name

Doctor of Philosophy

Department

Natural Resource Ecology and Management

Major

Environmental Science

First Advisor

Thomas M. Isenhart

Abstract

Nitrate leaching from Midwestern agricultural fields is an important environmental issue, as this nitrate can enter streams and cause local drinking water issues as well as increase the size of the annual benthic hypoxic zone in the Gulf of Mexico. Tile drainage has been linked to the majority of the nitrate loading to the Gulf of Mexico. There are many edge-of-field practices that can help reduce the amount of nitrate exported from a field. This dissertation primarily focuses on riparian buffers as an edge-of-field practice to remove nitrate, both from tile water with saturated riparian buffers (SRBs) and from groundwater with traditional riparian buffers. The first study aimed at determining the mechanism for nitrate removal in SRBs. Soil cores were obtained from three SRBs, and the acetylene inhibition method was used to determine in situ denitrification rates. Average cumulative denitrification accounted for between 3.7 and 77.3% of the total nitrate removed via SRBs. These rates appeared to vary by SRB depending on the time since vegetation establishment. The second study looked at potential substrate limitations for denitrification in SRBs. A denitrification potential experiment, where nitrate and carbon were added both separately and together, was used to determine limitations. The SRB planted the same year as it was saturated, BC-2, appeared to have a carbon limitation, potentially due to lack of perennial vegetation root turnover. All three SRBs varied in maximum denitrification potential rates, determined by adding both nitrate and carbon, especially at greater temperature incubations. The buffers with the longest established vegetation had larger maximum denitrification potential rates, thus indicating that a soil property, hypothesized to be increased soil aggregation, seems to be responsible for elevated denitrification rates. The third study used a 21 year sampling well data set from three riparian buffers to determine how nitrate removal changes over time. Two buffers, RS and ST, both former cropland sites, saw enhanced nitrate removal rates after 6 to 10 years of perennial vegetation establishment. RN did not see enhanced nitrate removal rates over time, but rather had elevated rates throughout the study, potentially due to having been managed as pastureland since at least 1981. Overall, riparian buffers, both traditional and saturated, appear to show great promise for nitrate removal, especially with established vegetation.

Copyright Owner

Tyler Groh

Language

en

File Format

application/pdf

File Size

116 pages

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