Document Type

Thesis - Open Access

Award Date

2026

Degree Name

Master of Science (MS)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Vander Luis Novaes Nunes

Abstract

Free-living nitrogen (N) fixation (FLNF) may represent a sustainable pathway to supplement crop N demand in agroecosystems, yet edaphic and management factors regulating this process under field conditions remain poorly understood in the Northern Great Plains. Using 15N2 direct method to quantify potential FLNF (pFLNF) and baseline FLNF (bFLNF), this thesis investigates how crop rotations (2-, 3-, and 4-year systems), tillage management (conventional vs. no-till), cover cropping, and soil properties affect the potential of free-living diazotrophs to supply N via FLNF in a long-term no-till corn-based system in Southeastern South Dakota. A two-year field study evaluated pFLNF across four growing season stages. Overall, conventional tillage (CT) consistently supported higher pFLNF rates than no-till (NT), primarily due to soil pH under CT, which frequently exceeded pH 6.0, a threshold associated with higher diazotrophic activity. Elevated exchangeable N suppressed pFLNF when driven by synthetic N fertilization, overriding the pH effect. Cover crop residue decomposition influenced lower exchangeable N rates, related to immobilization, allowing soil pH to emerge as a dominant pFLNF predictor. A comparison of the two measurements, potential vs. baseline, revealed a different pattern for bFLNF rates due to the no addition of carbon and water saturation during the assay.

Publisher

South Dakota State University

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Rights Statement

In Copyright