Agronomy and Horticulture, Department of

 

First Advisor

Javed Iqbal

Second Advisor

Daniel Snow

Date of this Version

5-2024

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Doctor of Philosophy

Major: Agronomy and Horticulture (Soil and Water Sciences)

Under the supervision of Professors Javed Iqbal and Daniel Snow

Lincoln, Nebraska, May 2024

Comments

Copyright 2024, Arshdeep Singh. Used by permission

Abstract

Increasing groundwater nitrate (NO3-N) contamination has raised significant environmental and health concerns in the irrigated sandy soils of Nebraska. Four studies were conducted to evaluate the impact the impact of 1) static (NE Yield Goal) vs. dynamic nitrogen (N) recommendations tools (Maize-N, Canopy Reflectance Sensing, Granular, and Adapt-N), 2) three N (optimum, suboptimum, and low) and irrigation rates (farmer’s full irrigation (FIT), 80% FIT, and 60% FIT), 3) conventional N sources vs. enhanced efficiency fertilizers (EEFs), and 4) multiple N splits (2, 3, 4, and 5-N splits) on agronomic (maize yield), environmental (NO3-N leaching), and economic returns (return to N (RTN) and RTN after considering environmental costs (RTNEnv). The first study indicated that the static Nebraska Yield Goal outperformed all the dynamic N tools by predicting the N rate and yield closer to EONR. At the same time, Maize-N recommended N above- and Canopy Reflectance Sensing, Granular, and Adapt-N below the EONR range. Environmentally, N tools did not significantly affect NO3-N leaching. Economically, all tools were equally effective in determining RTNEnv. In the second study, a reduction in N rate by 25% in the suboptimum (202 kg ha-1) and 50% in low N rate (135 kg ha-1) reduced NO3-N leaching by 24% (7 kg NO3-N ha-1) and 51% (15 kg NO3-N ha-1), maize grain yield by 8% (14.5 Mg ha-1) and 11% (14.0 Mg ha-1), and RTN by $215 ha-1 and $298 ha-1, respectively. The 80% FIT had significantly higher grain yield and RTN, and lower NO3-N leaching by 13-21% than 60% FIT and FIT. In the third study, the use of EEFs (i.e., urea with urease and dual (urease and nitrification) inhibitors) in a single pre-plant application substantially decreased NO3-N leaching with the levels approaching control treatments in both years, and had the same or better crop yield, and improved economic returns than conventional Urea-UAN split and pre-plant Urea. Lastly, we did not observe any benefit of increasing the number of in-season N-split applications in protecting groundwater quality or improving maize yield in a dry year. Overall, these findings will help producers and policymakers decide the right nitrogen recommendation tools, nitrogen source, and irrigation rates for improving agronomic, economic, and environmental performance.

Advisors: Javed Iqbal and Daniel Snow

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