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Author ORCID Identifier
https://orcid.org/0000-0002-1531-5342
Date Available
8-31-2027
Year of Publication
2026
Document Type
Doctoral Dissertation
Degree Name
Doctor of Philosophy (PhD)
College
Agriculture, Food and Environment
Department/School/Program
Plant and Soil Sciences
Faculty
Hanna Poffenbarger
Faculty
Arthur Hunt
Abstract
Nitrogen (N) is an essential nutrient for plant growth and development, as it is a structural component of proteins and part of other important functions in the plant, such as enzymes. Wheat (Triticum aestivum L.) is one of the most widely cultivated crops worldwide and a major staple food source. Because wheat grain is protein-rich, N fertilization is critical for achieving both high yields and desirable grain quality. In productive regions of the United States, average N application rates range from 100 to 145 kg ha⁻¹. However, N fertilization is also associated with environmental concerns, as crops typically recover only about 42% of the applied N. The remaining N may be lost through leaching, denitrification, and gaseous emissions, contributing to environmental degradation.
Foliar fertilization has been proposed as a strategy to improve plant development at advanced growth stages when nutrients are directed to sink organs such as new leaves, flowers, and grains; being associated with increased grain protein concentration in several cereal crops. In addition, it may enhance nutrient availability while reducing total fertilizer requirements and environmental losses. Nevertheless, foliar fertilization can lead to phytotoxicity, and unabsorbed fertilizer remaining on leaf surfaces may still be washed off and contribute to environmental contamination. The use of nanoscale biocompatible N carriers represents a potential alternative. Nanoscale materials offer high surface area, reactivity, adhesion, and potential for controlled nutrient release, which may improve nutrient uptake efficiency while minimizing environmental losses and phytotoxicity.
This dissertation evaluated the potential of novel N fertilization strategies—foliar application and nanoparticle-based carriers—to maintain or increase wheat yield and grain quality while reducing environmental N losses. Specifically, the objectives were to: (1) assess the effectiveness of foliar N and nanocarriers in sustaining yield and reducing residual soil inorganic N in soft red winter wheat under controlled greenhouse conditions; (2) evaluate the effects of methods of N application (soil vs. foliar) and types of fertilizers (traditional vs. nanoparticle-based) on wheat productivity, grain quality, and N dynamics under field conditions over two wheat growing seasons; and (3) determine the impacts of these fertilization strategies on N₂O emissions, soil inorganic N, and nitrate leaching over two years of winter wheat field production.
Chapter 1 provides a general introduction to the concepts addressed in this dissertation, presenting a literature review and discussing relevant published studies. It also offers an overview of the main objectives covered in each of the subsequent chapters.
Chapter 2 comprised two greenhouse experiments conducted in Lexington, KY, during the winter–spring of 2023. The Foliar N Rate study evaluated four N rates (125, 250, 500, and 1000 mg N pot⁻¹) distributed at different proportions between foliar and soil application (0%, 40%, 60%, 80%, and 100% foliar), plus an unfertilized control. The Nanoparticle study compared urea and urea combined with a nanocarrier at two N rates (150 and 300 mg N pot⁻¹), applied either to soil or foliage, including unfertilized and nanocarrier-only controls. Measurements included grain dry mass, aboveground dry mass, root:shoot ratio, necrotic leaf area, and soil inorganic N. In the Foliar N Rate study, grain dry mass increased with N rate up to an estimated optimum of 644 mg N pot⁻¹, beyond which no additional gains were observed. Yield response was similar across foliar percentages, although necrotic leaf area increased with greater foliar N proportions. In the Nanoparticle study, yield mass increased with N rate, and application method had no significant effect. Foliar application reduced residual soil inorganic N compared with soil application in the Foliar N Rate study. These results indicate that foliar N can sustain yield while reducing soil inorganic N accumulation in soil.
Chapter 3 evaluated eight field treatments, including traditional urea and 28% urea ammonium nitrate (UAN) applied to soil or foliage, and nanoparticle formulations of both fertilizers applied foliarly, at three N rates (0, 56, and 112 kg N ha⁻¹). Grain yield was unaffected by application method but was greater with urea than with UAN. Soil-applied urea resulted in greater grain protein concentration and total N content than foliar-applied urea. At rate 112 kg N ha⁻¹, Fe-doped hydroxyapatite urea nanocarrier (FeHAU) increased grain yield by an average of 17% relative to traditional foliar urea and both foliar-applied UAN and UAN soil treatments. The use of both nanocarrier treatments, FeHAU and UAN NANO N+, also increased grain protein concentration and total N content compared with traditional foliar fertilizers. Overall, foliar application of traditional N fertilizer did not provide consistent advantages over soil application under the conditions of this study. However, nanocarrier-based fertilizers enhanced wheat productivity, grain protein concentration, and crop N content when applied foliarly, indicating their potential to improve the efficiency of foliar N fertilization.
Chapter 4 assessed effects of N application method (foliar and soil) and fertilizer type applied to the foliage (nanoparticle-based and traditional) on environmental losses. There were two years of winter wheat cultivation, at the recommended rate (112 kg N ha-1). Nitrous oxide emissions were monitored during the wheat growing season and the subsequent double-crop soybean season using static chambers. Nitrate leaching was assessed using lysimeters containing ion-exchange resin installed at 40 cm depth and collected annually. Aboveground N content was also evaluated as it is an important sink if N from the agricultural system. In the first wheat season, foliar applications of both urea and UAN significantly reduced cumulative N₂O emissions during the wheat season compared with soil applications. These differences were not significant on an annual basis. No significant treatment effects on cumulative N2O emissions were observed in the second wheat season and second year. Nitrate leaching was 58% higher for UAN than for urea, regardless of application method for both years. It was also significantly higher in the Fe-doped hydroxyapatite urea nanocarrier treatment compared to UAN, NANO N+ but there was no significant difference relative to the traditional treatments of foliar urea and foliar UAN. Wheat plants took up significantly less N with the traditional urea fertilizer applied to the foliage treatments compared to the soil-applied urea and Fe-doped hydroxyapatite urea nanocarrier treatments in the second year. Overall, foliar N application has the potential to mitigate seasonal N₂O emissions compared with soil application, but it is highly sensitive to interannual weather variation. Nitrogen source exerts a stronger influence on NO3- leaching than the use of nanocarriers.
Finally, Chapter 5 synthesizes the findings of this dissertation and highlights the complementary potential of alternative N fertilization strategies. While nanocarriers demonstrated promising agronomic benefits, foliar N application emerged as a potential approach for reducing selected environmental impacts of N fertilization. Ultimately, the path toward more sustainable wheat production may not lie in a single solution, but in understanding where, when, and how each strategy can be most effectively applied—aligning the right source and method with the needs of the crop and the conditions of the environment.
Digital Object Identifier (DOI)
https://doi.org/10.13023/etd.2026.417
Archival?
Archival
Funding Information
National Science Foundation (NSF) grant No. 1122905-464772 and the Small Grain Growers Foundation (no grant number)
Recommended Citation
Pacheco Inoue, Leticia, "Evaluation of Wheat (Triticum aestivum) Response to Foliar Fertilization, Soil Fertilization, and Nano Fertilization in a Controlled Environment and in the Field" (2026). Theses and Dissertations--Plant and Soil Sciences. 213.
https://uknowledge.uky.edu/pss_etds/213
