Graduate Studies

 

First Advisor

Arindam Malakar

Date of this Version

5-2024

Document Type

Article

Citation

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

Major: Natural Resource Sciences

Under the supervision of Professor Arindam Malakar

Lincoln, Nebraska, May 2024

Comments

Copyright 2024, Yvon Ukwishaka. Used by permission

Abstract

Long-term and excessive fertilizer application has continuously increased nitrate in the vadose zone-groundwater integrated system. Nitrate, a highly miscible and mobile inorganic nitrogen species, leaches through the vadose zone to groundwater, posing human health implications when continuously consumed. This study aims to evaluate the redox-based transformation of inorganic nitrogen species in a vadose zone under a row-cropped, sprinkler-irrigated field in central Nebraska. Undisturbed sections of deep vadose zone cores were packed in columns representing the entire vadose zone, and a groundwater table was simulated. The biweekly collected porewater and sediment representative samples were analyzed for nitrate-N and ammonia-N transformations. The average ammonia-N concentrations under rainfed (13.0 ± 2.8 µg g-1) were significantly higher than in sprinkler-irrigated systems (12.7 ± 2.5 µg g-1) and pre-experimental soil (4.6 ± 0.5 µg g-1, p-1) were significantly higher than in rainfed (2.2 ± 0.2 µg g-1) and sprinkler-irrigated systems (2.2 ± 0.3 µg g-1,p < 0.05). In biweekly analyzed porewater, the simulation beneath the sprinkler irrigation showed significantly higher rootzone nitrate-N concentration (NO3-N = 6.6 ±1.5 mg L-1, p3-N = 2.6 ± 0.5 mg L-1). At capillary fringe and groundwater, no significant difference in nitrate concentration was observed throughout all the sampling events (p > 0.05). Furthermore, at the root zone, the rainfed system porewater produced significantly higher ammonia-N concentration (NH4-N = 3.5 ± 0.9 mg L-1, p 4-N = 2.3 ± 0.6 mg L-1), with comparable concentrations in the capillary fringe. Interestingly, the sprinkler-irrigated systems had higher NH4-N levels than the rainfed system, whereas the NO3-N concentrations were comparable in both irrigation systems, and the trends were observed as dissolved oxygen fluctuated. This study suggests that differences in water volume input can impact nitrate transformation in the vadose zone.

Advisor: Arindam Malakar

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