Date of Graduation

12-2011

Document Type

Thesis

Degree Name

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

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Scott, J. Thad

Committee Member

Haggard, Brian E.

Second Committee Member

Miller, David M.

Third Committee Member

Boss, Stephen K.

Keywords

Earth sciences; Biological sciences; Denitrification; Landscape ecology; Nitrogen retention; Nutrient cycling; Reservoir

Abstract

Studies coupling direct measurements of micro-scale nitrogen (N) cycle processes with ecosystem-scale flux estimates are needed to determine N retention hotspots within river networks, where up to 50% of terrestrial loading to aquatic systems is retained. This study examined the role of denitrification, a microbially-mediated reactive N removal pathway, in ecosystem-scale N retention in 3 small (< 1 km2), shallow flood-control reservoirs. Annual reservoir N retention was estimated through mass balance modeling of system inputs and outputs. Annual denitrification rates were estimated by combining multiple measurements of seasonal, habitat-specific dinitrogen gas (N2) fluxes. Annual reservoir N retention ranged from 14 - 19 g m-2 in the reservoirs, while reactive N removal through denitrification was 13 - 25 g m-2. Denitrification efficiency, or the portion of the retained N load that was denitrified, was high relative to other lentic systems and was > 100% at 2 sites. Previous lentic denitrification studies may have underestimated denitrification efficiency by not considering water column denitrification, which was 50% of total denitrification at one study reservoir. However, not all potential inputs, most importantly biological N2 fixation, were included in this study's mass balance model, which likely led to underestimation of N retention. This study's findings indicate that reservoirs are N sinks in the landscape, and that denitrification plays a major role in regulating long-term storage of both watershed and biologically-fixed N loads in lentic systems.

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