Date of this Version
A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science
Under the supervision of Professor Bijesh Maharjan
Lincoln, Nebraska, November 2023
Two species of mint, Mentha piperita (peppermint) and Mentha spicata (spearmint), are cultivated across the U.S. Mint is an aromatic herb and perennial crop grown for several aromatic oils, among which menthol and carvone are the primary oils for peppermint and spearmint, respectively. Currently, farmers in the Nebraska Panhandle use different nitrogen (N) rates and various N fertilizers in mint production because there is no information available regarding the optimum rate and source of N fertilizer for mint production in the area. Therefore, optimized N management in mint production could increase aromatic oil yield and quality and reduce greenhouse gas emissions (nitrous oxide; N2O). Nitrous oxide is a major greenhouse gas and the single most important ozone-depleting emission. Increasing N2O emissions from agriculture are linked to soil management and the application of N fertilizers. The objective of this research was to assess the effects of fertilizer N sources (PCU; polymer-coated urea and urea) and rates on dry matter yield and oil concentration in irrigated peppermint and spearmint. Additionally, the experiment aimed to assess the effects of fertilizer N rates and sources on N2O emissions in peppermint production. The experiment was conducted in 2022-2023 at the University of Nebraska Panhandle Research and Extension Center near Scottsbluff, Nebraska. The experimental design was a randomized complete block with four replicates. The main factor was N treatment, which included the control, urea, and PCU surface applied at different rates, resulting in five different soil available N levels (spring soil test N + applied N). In 2022, dry matter yield ranged from 3.33 to 3.98 Mg ha-1 in peppermint and 2.33 to 2.63 Mg ha-1 in spearmint. Only spearmint dry matter yield was significantly affected by N rates and N sources in 2022. In 2023, dry matter yield ranged from 7.56 to 13.28 Mg ha-1 in peppermint and from 10.41 to 13.43 Mg ha-1 in spearmint. Mint dry matter yield was greater in 2023 than in 2022, as 2022 received less N than 2023 and had plant establishment issues. In 2023, greater peppermint dry matter yield was obtained from PCU and urea at 248 kg N ha-1 , while greater spearmint dry matter yield was obtained from 161 kg N ha-1 of soil available N with PCU which showed the best results compared to urea. Oil concentrations in peppermint leaves in the fertilized treatments were greater than in the control but they did not vary by applied N rates or sources, while spearmint oil concentrations did not show any response across N rates and sources. The urea treatments had greater N2O emissions than PCU across all N rates in both years, except for the lowest N rate in 2022. Data reported an incremental trend of N2O emission with an increment of soil available N in the urea treatments but not in the PCU in both years. N2O emission from PCU did not differ from control in both years. The results suggest that the N application can be optimized to improve the performance of sustainable mint production using advanced fertilizer technology such as PCU. However, more site-year data would be necessary to determine the optimum N rates.
Advisor: Bijesh Maharjan