IDENTIFYING REACTIVE NITROGEN DYNAMICS IN THE DEEP VADOSE ZONE TO PROTECT GROUNDWATER QUALITY

• Sponsoring Institution: National Institute of Food and Agriculture
• Project Status: ACTIVE
• Funding Source: AFRI COMPETITIVE GRANT
• Reporting Frequency: Annual
• Accession No.: 1027886
• Grant No.: 2022-67019-37179
• Cumulative Award Amt.: $749,861.00
• Proposal No.: 2021-09154
• Project Start Date: Apr 1, 2022
• Project End Date: Mar 31, 2026
• Grant Year: 2022
• Program Code: [A1411]- Foundational Program: Agricultural Water Science

Recipient Organization
UNIVERSITY OF NEBRASKA
(N/A)
LINCOLN,NE 68583

Performing Department
Water Center

Non Technical Summary
Increased agricultural intensification has led to excess nitrogen inputs on soil and declined water quality in many regions of the United States (US). The US corn-belt states use large quantities of nitrogen fertilizer, and most of these regions contribute to continued pollution of local groundwater. Under typical conditions, crops take up ~47% of applied nitrogen, and the rest is lost through volatilization, run-off, and leaching. A substantial amount is leached to the vadose zone, which is the soil layer between the land surface and the water table. The vadose zone acts as a temporary nitrogen reservoir, storing the leached nitrogen. Globally, the vadose zone is estimated to store 605-1814 million tons of nitrogen as nitrate, primarily coming from applied fertilizers. While most vadose zone studies only measure nitrate, our past studies show the widespread occurrence of both nitrate and ammonium in deep vadose zone sediments. Moreover, the irrigation method (flood versus pivot) correlates with ammonium occurrence, suggesting that water input plays a critical underappreciated role in the fate of nitrogen beneath the land surface.A wide variety of biogeochemical reactions control the occurrence of specific nitrogen species in the vadose zone. However, inaccessibility of the vadose zone makes it challenging to study and identify these biogeochemical reactions, which can control the nitrogen movement to the groundwater resources. Further, these nitrogen species transformation reactions are linked with surface processes occurring in the agroecosystems such as irrigation practice and fertilizer types. This project will measure the occurrence of multiple nitrogen species, which will include inorganic and organic nitrogen species beneath gravity, pivot irrigated and dryland corn using deep coring, elucidate transformation pathways between different nitrogen species by comprehensive chemical analysis, and column experiments with isotope-labeled fertilizers and simulate reactive transport of nitrogen with appropriate modification of a well-developed USDA-ARS management model - Root Zone Water Quality Model (RZWQM2). The new knowledge from field and column experiments and the modified model can be used as a decision-support tool by regulatory agencies to show how proposed management practices mitigate groundwater pollution. Ultimately this work will support the development of proactive management practices to protect groundwater quality.

Animal Health Component

15%

Research Effort Categories

Basic

70%

Applied

15%

Developmental

15%

Classification

Knowledge Area (KA)	Subject of Investigation (SOI)	Field of Science (FOS)	Percent
102	0199	2000	75%
133	0210	2050	25%

Knowledge Area
102 - Soil, Plant, Water, Nutrient Relationships; 133 - Pollution Prevention and Mitigation;

Subject Of Investigation
0210 - Water resources; 0199 - Soil and land, general;

Field Of Science
2050 - Hydrology; 2000 - Chemistry;

Keywords

deep vadose zone

reactive nitrogen dynamics

groundwater vulnerability assessment

agroecosystem

Goals / Objectives
The long-term goal of this study is to integrate experimental and modeling approaches to evaluate the importance of reactive nitrogen (Nr) dynamics in the deep vadose zone beneath agriculturally intensive ecosystems. A better understanding of deep vadose Nr dynamics will provide the basis for proactive measures to protect the groundwater quality from nitrogen (N) pollution. The N input rate has doubled to increase food production, which has altered the global N cycle. The surge in N input and the cascading effect of Nr has resulted in a decline in the quality of groundwater and surface water used for drinking water supplies, specifically in agriculturally intensive areas (AIAs). Sustainable management of agroecosystems requires maintenance of the supporting natural resources and ecosystem services, which is a key aspect of the Bioenergy, Natural Resources, and Environment (BNRE) program area of NIFA and the primary goal of the study.Reactive nitrogen (Nr) are biologically and chemically reactive N compounds in the biosphere and includes inorganic reduced forms (e.g., NH3, NH4+), inorganic oxidized forms (e.g., NO2-, NO3-), and organic compounds (e.g., urea, amines, proteins, nucleic acids) of N. Food and fiber production is the main anthropogenic activity, which is expected to contribute 215 million tons-N/yr of Nr by 2050. It is estimated that 90% of increased Nr originates from synthetic fertilizer or N fixed in agricultural fields. Crops take up 42 to 47% of total applied N and the rest of it is lost to the environment. Nitrate, from fertilizer, is a key component of the Nr input in AIAs. Nearly all previous research has primarily focused on N cycling and subsequent loss in the root zone (~1 m or ~3 ft depth). One major missing N piece, critical for understanding the N cycle, and until recently has been considered insignificant, is N in the deep vadose zone. The chemical composition, reactions, transformation mechanisms, and fate of Nr in the vadose zone-groundwater system, as well as the influence of surface agroecosystem practices on dissolved Nr dynamics in the deep vadose zone, are largely unknown.The vadose zone, the layer from the land surface to the water table, is a reservoir of transient porewater and reactive nitrogen species. The vadose zone, through which solutes and water percolate, is connected and integrated with the groundwater. Globally, the vadose zone is estimated to store 605-1814 million tons-N of nitrate, and North America is considered to have the highest nitrate storage per unit area of the vadose zone. The depth of the vadose zone is considered to be the most significant factor affecting the nitrate concentrations in groundwater. Most estimates of Nr in the vadose zone only consider nitrate. However, in our recent vadose zone studies at different AIAs in Nebraska, we quantified inorganic Nr, and found similar concentrations of reduced (ammonium) and oxidized (nitrate) Nr in deep sediment cores down to ~35 m (115 ft). Further, the inorganic Nr species - ammonium and nitrate, presented different concentrations in the deep vadose zone, which was significantly (p<0.05) influenced by surficial irrigation practices. For example, pivot or sprinkler irrigated sites (n=22 site and 1054 samples) contained higher quantities of ammonium and lower amounts of nitrate than gravity or furrow irrigated sites (n=9 and 283 samples), which contained more nitrate and lower ammonium. The total inorganic Nr concentrations were similar at both types of irrigation sites. Our preliminary study suggests that it is equally important to quantify other forms of stored Nr in the vadose zone and understand their reaction dynamics as a consequence of surficial processes for predicting N losses.We hypothesize that irrigation methods, fertilizer types applied to soil, and mobile carbon transport from the root zone control N reaction dynamics in the deep vadose zone. These inputs beneath AIAs can influence transformation between oxidized to reduced Nr species. Quantifying the impact of surface processes, which include irrigation types - pivot/sprinkler, gravity/furrow, and no irrigation as well as fertilizer type, which has changed over the past four decades, in Nr transformation in the deep vadose zone can lead to developing simple and easy-to-adopt management practices, which will promote nitrate reduction reactions in the vadose zone, for example, the study will identify the production of slow-moving Nr species, such as ammonium formation. Several missing pieces of N that can regulate Nr dynamics are predicted to occur in the deep vadose zone. In the proposed study, our group will integrate experimental and modeling approaches to address the following research objectives:Link surface agricultural processes such as irrigation water application volume (pivot, gravity, or dryland) and fertilizer type (anhydrous ammonia, UAN), and dissolved organic carbon loss below the root zone with the occurrence of different Nr species in the deep vadose zone.Identify physicochemical and biological processes occurring in the vadose zone, which impacts the presence of Nr species.Integrate experimental data in a reactive transport model to understand the dynamic nature of N profile in the vadose zone with response to changing management practices at the land surface and risk assessment of the stored Nr species.Develop surface management strategies to favor N-reaction producing slow-moving Nr species, which will prolong N stay or reduce N loading in the vadose zone and protect groundwater quality.These research objectives will help us quantify important Nr species in the vadose zone under intensive agricultural practices and link their occurrence with surficial processes occurring in AIAs. Extensive vadose zone sample collection is needed to quantify the impact of surface processes - irrigation water application volume and fertilizer type application on Nr dynamics under different soil types and climatic conditions. To precisely elucidate the impact of these surface processes on Nr reactive transport and transformation, controlled column experiments with collected vadose zone sediments will be carried out to identify the abiotic and biotic processes predicted to occur in the vadose zone. Controlled column experiments will also help mitigate field sampling variability. The generated data and modified root zone water quality model will be useful for proper N budgeting, and groundwater N pollution vulnerability assessment. The missing pieces of N identified in the vadose zone from the study will also help other states such as California, Iowa, and most of the Midwest, which are affected by N pollution.

Project Methods
Nebraska Natural Resources Districts (NRDs) are tasked to make management decisions to preserve water quality. We will work closely with NRD managers to identify vadose zone sampling locations according to irrigation practices. This collaboration will identify the missing nitrogen (N) pieces of the deep vadose zone through three tasks, which will complete the four objectives of the study.Task#1, Field Sampling: The soil cores will be collected from 23 NRDs throughout the state of Nebraska in years 1 and 2. In each NRD we will collect one core each from gravity/furrow irrigated sites, pivot/sprinkler irrigated sites, and non-irrigated sites (dryland). Further, a control site vadose zone sample will be collected from Nine-Mile Prairie on the northwest edge of Lincoln, which is a pristine and completely undeveloped prairie grassland in eastern Nebraska. In total, 70 locations will be sampled across the entire state under different irrigation water management. The sample location will be selected so that we maximize to revisit previously sampled locations, and samples will be collected to the water table, along with the corresponding groundwater sample. These samples will cover different climatic conditions, soil types, vadose zone thickness, and agricultural practices. The sampling locations will be agricultural sites, with preferentially similar cropping systems. Intact cores will be separated for column experiments. The entire dataset will help us quantify different organic and inorganic Nr species in the collected vadose zone samples and link their occurrence with surficial processes. The N-components measured separately will be mass-balanced with total N to identify any missing or unidentified N-component in the vadose zone. The sediment mineralogical composition, the oxidation state of N, carbon, iron, and microbial data will help identify relevant abiotic and biotic Nr transformation processes.Task#2, Column Experiment: In years 2, and 3, a series of large laboratory-scale column experiments will be conducted to evaluate two variables - irrigation water volume input and fertilizer type effect on the presence of Nr species in the collected vadose zone sediments. This task will address objective 2 of the proposed study and link it with field-collected data. For the column experiments, the vadose zone sampling sites will be grouped into agricultural intensive areas (AIAs) of Nebraska, and control from the Nine-Mile Prairie site, making two sets of column experiments. In each set, 18 PVC columns, 2 m (10 ft) long with 15 cm (6 in.) internal diameter, will be packed with vadose zone sediments matching the depth to the water table. The top 1 m of the columns will be packed only with the top 1 m of collected sediment samples to represent the root zone and will be lined with a plexiglass mesh layer to prevent any root movement below 1 m. The rest of the 1 m column will take a scaled-down approach to represent the entire intermediate vadose zone profile (root zone to the water table) until the water table. The condensing of the vadose zone will help in studying Nr dynamics within the time period of the project.The 18 sets of columns for AIAs vadose zone sediments will be divided into three groups based on irrigation practices above the vadose zone, gravity, pivot, and dryland. The columns will be packed based on the lithology of the collected sediments. The gravity columns will be packed with vadose zone sediments from gravity sites, the pivot columns with sediments of pivot sites, and the dryland column will be composed of dryland sediments. These three irrigation type groups (n=6) will be sub-divided into two groups for different fertilizer type applications - one set will receive 15N labeled anhydrous ammonia (historically used fertilizer) (n=3) and other 15N labeled UAN (n=3), currently used fertilizer. The control columns prepared using sediment from the Nine-Mile Prairie site, will also be divided into three groups of irrigation type and further sub-divided to fertilizer type and studied similarly. Corn will be planted in each of the columns to simulate organic carbon loading coming from crops in the root zone of AIAs and quantify its impact on the intermediate vadose zone. Precipitation, as synthetic rainwater, will be provided to each column experiment set as per the historical data from Nebraska and Prairie region.Task#3, Model Simulations: This task will address objectives 3 and 4 of the proposed study, and utilize the data generated in Task #1 and #2. The vadose zone Nr data will be simulated in the Root Zone Water Quality Model (RZWQM2), which is a well-established USDA-ARS model to simulate management practices. The model will elucidate Nr movement and transformation through the deep vadose zone and collaborator Green will provide his support for achieving this task. Because transport of ammonium to deep soils in the root zone and below is not expected using RZWQM2 as currently parameterized, the source code of the model will be accessed to incorporate Nr transformation pathways that are identified in the proposed study. The soil physicochemical data, as well as Nr species concentration, will be the primary input for the model. The RZWQM2 can accommodate only ten layers so the model will be calibrated and validated for the top 10 m (~ 33 ft) of the vadose zone to quantify the influence of surface and root zone processes as an input to the vadose zone. The calibrated model will simulate 'what if' scenarios of management practices, such as irrigation types, fertilizer types, fertilizer application timing, switching to fertigation or chemigation, and plant population density to simulate changes in the Nr-species form and concentration in the vadose zone, which will project Nr reactive transport beyond the experimental time scales. The potential for future groundwater N pollution from the total N present in the vadose zone will be carried out using a statistical method such as classification and regression trees. Groundwater N pollution vulnerability assessment will simulate the possibility of different management changes to protect the groundwater quality. The column and vadose zone N-data in the proposed study will be used to calibrate and validate the hazard potential of N. Changes in management practices, specifically, irrigation practices and fertilizer type will be assessed and linked to the potential of N-leaching to the groundwater. The SSURGO soil database will be mined to simulate this vulnerability assessment on other locations of the US with irrigated agriculture.Results will be analyzed statistically using regression models and graphed using Origin Pro graphing program where appropriate. The design of column experiments will be completely randomized with independent replicates, which will also allow for the analysis of significant differences between the factors. Interactions between irrigation practices and fertilizer types will be evaluated using statistical models. The integrated approach will address objective 4 of the proposed study, where simple proactive agricultural practices will be framed by evaluating model simulations and experimental outcomes. These processes will be developed as management strategies to protect groundwater quality. The generated Nr dynamics data under different soil conditions, irrigation practices, and fertilizer use can be used for proper N-budgeting in AIAs of other states, such as Iowa, California, and Idaho, where N pollution of groundwater sources is a critical challenge.

Progress 04/01/24 to 03/31/25

Outputs
Target Audience: Our target audience includes producers, natural resource managers, scientists, graduate students, and undergraduate students concerned with water quality. We aim to provide knowledge that ensures proactive management practices to prevent groundwater quality depreciation. In this reporting cycle, PD Dr. Malakar and graduate students involved in the project presented the research at multiple conferences. PD Malakar shared research findings with local Natural Resources Districts and peers. We disseminated information via newsletters to researchers, faculty, and the general public, outlining the project's scope and findings. Graduate students shared the project with the general public through East Campus Discovery Days at the University of Nebraska-Lincoln. We collaborated with the extension and outreach centers to collect vadose zone cores. Over the past year, we collected soil cores from fourNRDs and shared project details with them. PD Dr. Malakar also taught two special graduate courses related to groundwater quality and environmental geochemistry, further educating students on groundwater quality issues and management practices. By engaging these audiences, we aim to foster understanding and promote best practices to prevent nitrate contamination and protect groundwater resources.? Changes/Problems: We experienced several challenges for having approvals from landowners, which shifted the timeline of our project based on the seeding and harvesting schedule. After getting the approval, we immediately began field work which involved clearing utilities. Challenging weather conditions also impacted on the planning as our field sampling scheduled for mid of January involves cold temperatures and strong winds and making the process more difficult. Additionally muddy fields coupled with snow added complexity to our tasks. Overall, these challenges have made us more adaptive and the unwavering support from our advisor has enabled us to plan and execute our tasks more efficiently. We had a great time engaging with the landowners who are enthusiastic about this project. Their excitement to learn the project objectives is encouraging focusing on nitrate and other reactive nitrogen dynamics. In this semester our field operations have been extended to 4 NRDs and we have many to cover in the upcoming sessions. What opportunities for training and professional development has the project provided? Three graduate and twoundergraduate students got the opportunity to learn and get trained from this project. The graduate student presented preliminary findings at the American Chemical Society Spring 2024 National Symposium. Dr. Malakar, a new faculty member leading the project, has gained experience in managing the project and fostering new collaborations. One student graduated and is working in Nebraska with the Department of Health and Human Services and another graduate student finalized his committee and also submitted his research proposal. How have the results been disseminated to communities of interest? Ongoing results have been presented in weekly research group meetings and discussed with the Co-PDs. One journal article has been published, and two manuscripts are under review, while two more manuscripts are under preparation. Results were also presented at the ACS Fall 2024 Conference and local conferences. NRDs have been informed about new findings through individual meetings. What do you plan to do during the next reporting period to accomplish the goals? In the next reporting period 2025-2026, we plan to finish NRDs coring, analyzing soil cores that were collected before and interacting with other NRDs with their data sets and comparing the results.We also plan to complete analysis of remaining samples from the concluded greenhouse experiment. Our focus will be primarily on the RZWQM2, where we plan to update the source code with the new nitrogen transformations pathways identified in greenhouse experiments.

Impacts
What was accomplished under these goals? Field Sampling: The coring activities were paused during the growing season of 2024 and resumed later in the Fall. In August 2024, a new graduate research assistant was hired and reinitiated the on-field sampling events. We identified the coordinates from the Nebraska vadose zone interactive map, focusing on the type of irrigation, i.e., dryland, pivot, and gravity. From April 2024 to March 2025, we contacted four additional NRDs (Lower Platte South, Nemaha, Lower Big Blue, and Papio-Missouri River) across Nebraska, which brings the total to 12 already done NRDs. This collaboration with NRDs enables us to identify appropriate sampling locations and focus on previously cored sites to effectively track changes in the vadose zone nitrate levels over time. We successfully collected 259 ft of soil cores from NRDs along with fresh groundwater. Controlled Column Experiments: As reported in our last reporting cycle about our plan, we conducted our finalgreenhouse column experiments using the nine cores from cropland (3 from dryland, 3 from Pivot-irrigated, 3 from gravity-irrigated cropland areas) to understand nitrogen dynamics and transformation, and link surface management practices to subsurface nitrogen transformation. The soil cores were packed into 18 soil columns, maintained at natural vadose zone temperatures, and planted with corn after adding 15N-labeled fertilizer. Various treatments combining different fertilizers and irrigation methods were tested. Water and gas samples were collected and analyzed for biochemical properties, including pH, dissolved oxygen, major ions, nitrate, ammonium, and nitrogen isotopes. Gas samples were analyzed for CO2, N2O, and CH4. After corn maturity, plants and soil columns were stored and analyzed for nitrogen concentration and soil properties. Nitrogen Simulation: Graduate students trained in the USDA Root Zone Water Quality Model (RZWQM2), along with the source code, to simulate nitrogen species movement and transformation in the soil and vadose zone. We have already started working on the GUI version of the model and, with the source code in hand, he also plans to review it.

Publications

• Type: Peer Reviewed Journal Articles Status: Published Year Published: 2025 Citation: Borah, P., Borah, G., & Malakar, A. (2025). Mitigating nitrate contamination in groundwater: A comprehensive review of in-situ approaches. Groundwater for Sustainable Development, 28, 101406. https://doi.org/10.1016/j.gsd.2025.101406
• Type: Other Journal Articles Status: Published Year Published: 2024 Citation: Ray, C., & Malakar, A. (2024). Nitrate contamination in Nebraskas Ogallala aquifer. Science, 386 (6719) 280. https://doi.org/10.1126/science.adr3796
• Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Kumar, C., Ukwishaka, Y., Snow, D.D., Miller, D., Ray, C., Fleisher, D., Timlin, D., Reddy, V. and Malakar, A. (2024) Impact of irrigation and fertilization practices on reactive nitrogen dynamics in the deep vadose zone: Insights for sustainable groundwater quality management. Oral Presentation  ACS National Symposium and Expo Fall 2024, 18  21 August 2024, 4104982, Denver, Colorado.
• Type: Conference Papers and Presentations Status: Published Year Published: 2024 Citation: Dushimeyesu, J., Miller, D., Ray, C., Timlin, D., Fleisher, D., Reddy, V. and Malakar, A. (2024) Nitrous oxide production in undisturbed subsoil: A column experiment. Oral Presentation  ACS National Symposium and Expo Fall 2024, 18  21 August 2024, 4110156, Denver, Colorado

Progress 04/01/23 to 03/31/24

Outputs
Target Audience:Our target audience includes producers, natural resource managers, undergraduate and graduate students, and postdoctoral researchers concerned with water quality. We aim to provide knowledge that ensures proactive management practices to prevent groundwater quality depreciation. PD Dr. Malakar was invited to present the research to the Nebraska Corn Board, the state corn commodity board, and the Water Resources Advisory Panel. Additionally, we disseminated information via newsletters to researchers, faculty, and the general public, outlining the project's scope and findings. We collaborated with the extension and outreach centers at the University of Nebraska to discuss findings and did vadose zone sampling. Over the past year, we collected soil cores from six NRDs, covering various irrigation patterns, including gravity, pivot, and rainfed systems. PD Dr. Malakar also taught an undergraduate course related to the vadose zone in the Fall 2023 semester, further educating students on groundwater quality issues and management practices. We aim to foster understanding and promote best practices to prevent nitrate contamination and protect groundwater resources by engaging these audiences. Changes/Problems:We faced challenges in recruiting a full-time Ph.D. student and encountered difficulties in vadose zone sampling due to harsh weather conditions and geological obstacles. Additionally, obtaining DNA from the samples proved challenging, necessitating modifications to the DNA extraction method and subsequent re-extraction What opportunities for training and professional development has the project provided?The project has provided training and professional development opportunities through student involvement and stakeholder collaboration. One postdoc, two graduate, and two undergraduate students were trained through the project. The graduate students communicated with research collaborators and Natural Resources Districts and conducted sampling as needed. The undergraduate student developed a research proposal, received funding from the University of Nebraska-Lincoln's Undergraduate Creative Activities & Research Experiences (UCARE) Program, and graduated with a senior thesis and distinction. She will start as a master's student in the fall of 2024. The other undergraduate is getting trained to do soil analysis. The new graduate student started in August and led the greenhouse experiment efforts. PD Malakar presented preliminary findings at the American Chemical Society Fall 2023 National Symposium. PD Malakar, a new faculty member leading the project, has gained experience in managing the project and fostering new collaborations. PD Malakar has fostered cooperation with local stakeholders, including Natural Resources Districts and the Nebraska Corn Board, through the project. The postdoc also got an opportunity to learn geochemical simulations through the project. How have the results been disseminated to communities of interest?Ongoing results are presented by the graduate students in weekly research group meetings. The data generated has been discussed with the PDs. The PDs meet every other month to discuss the progress of the project. Two journal articles are going through internal review, and results from the concluded greenhouse experiment are being formulated for another research article. Results were also presented in the fall 2023 meeting of ACS. One of the graduate students presented research at the Nebraska Water Center conference, the Water Integrated Cropping System conference, and the Daugherty Water for Food Global Institute Conference and won multiple awards. PD Malakar shared his findings with the Nebraska Corn Board and the Water Resources Advisory Panel. What do you plan to do during the next reporting period to accomplish the goals?Planned activities for the next reporting period include conducting extensive column experiments, expanding field sampling efforts, and finalizing model simulations. We plan to conduct extensive column experiments with cores from Twin Platte, Central Platte, and Lower Elkhorn Natural Resources Districts (NRDs) to understand the changes in vadose zone reactive nitrogen biogeochemistry due to intensive agricultural practices. We will continue to collaborate with NRDs to collect vadose zone cores, and the plan is to collect cores from eight NRDs in the next reporting year. We will have another full-time GRA working on the project starting in the fall of 2024. We will finalize the transfer of the RZWQM2 source code to simulate and incorporate our findings, where our USDA-ARS collaborator will train the current GRA.

Impacts
What was accomplished under these goals? Field Sampling: From April 2023 to March 2024, we collaborated with six NRDs across Nebraska to determine optimal sampling locations, focusing on previously cored sites to track changes in vadose nitrate over time. We collected 805 feet of soil cores from 18 vadose zone coring sites, and fresh groundwater samples were also collected from various irrigation areas within the NRDs. Additionally, 21 deep vadose zone cores from Marie Ratzlaff Prairie and Homestead Historic National Park were collected, which provided baseline data on reactive nitrogen species. Controlled Column Experiments: We conducted the first controlled greenhouse column experiments using the 21 cores from native prairies to understand nitrogen dynamics and link surface management practices to subsurface nitrogen transformation. The cores were packed into 24 soil columns, maintained at natural vadose zone temperatures, and planted with corn. Various treatments combining different fertilizers and irrigation methods were tested. Water and gas samples were collected and analyzed for biochemical properties, including pH, dissolved oxygen, major ions, nitrate, ammonium, and nitrogen isotopes. Gas samples were analyzed for CO2, N2O, and CH4. After corn maturity, plants and soil columns were stored and analyzed for nitrogen concentration and soil properties. Approximately 148 soil and 75 soil pore water samples were collected from the concluded column experiment for reactive nitrogen species measurements. We also extracted 100 vadose zone soil samples for 16s and ITS DNA sequencing. Combining field and column experiments, 585 soil and water samples were analyzed at the Water Sciences Laboratory, and 875 gas samples were analyzed by our USDA-ARS collaborator in the current reporting year. Nitrogen Simulation: We have a material transfer agreement in place with USDA -ARS to access the USDA Root Zone Water Quality Model (RZWQM2) source code, but we have not received the complete source code yet. However, we have initiated simulations of nitrogen species movement in the soil and vadose zone, and the graduate student will be trained on the model in the 2024-25 reporting year. The preliminary risk assessment map, developed utilizing publicly available groundwater nitrate data in the previous reporting year, was used to identify vadose coring locations in Eastern Nebraska.

Publications

• Type: Conference Papers and Presentations Status: Published Year Published: 2023 Citation: Ukwishaka, Y. and Malakar, A., (2023). Redox transformation of inorganic nitrogen species impacts on Uranium mobilization in the vadose zone. Oral Presentation  ACS National Symposium and Expo Fall 2023, 13  17 August 2023, 3925859, San Francisco, California
• Type: Journal Articles Status: Published Year Published: 2023 Citation: Malakar, A., Ray, C., DAlessio, M., Shields, J., Adams, C., Stange, M., Weber, K. A., Snow, D. D. (2023) Interplay of legacy irrigation and nitrogen fertilizer inputs to spatial variability of arsenic and uranium within the deep vadose zone. Science of the Total Environment 897, 165299.
• Type: Conference Papers and Presentations Status: Published Year Published: 2023 Citation: Ukwishaka, Y., Kaiser, M., Snow, D. D., Miller, D., Ray, C., & Malakar, A. (2023). Redox driven transformation of nitrogen species across the vadose zone: Insights from column experiments [Poster session]. Nebraska Water Center Conference, Omaha, NE.
• Type: Conference Papers and Presentations Status: Accepted Year Published: 2023 Citation: Ukwishaka, Y., Kaiser, M., Snow, D. D., Miller, D., Ray, C., & Malakar, A. (2023). Redox driven transformation of nitrogen species across the vadose zone: Insights from column experiments [Poster session].2023 Water for Food Global Conference, Lincoln, NE

Progress 04/01/22 to 03/31/23

Outputs
Target Audience:Producers, natural resources managers, and undergraduate students concerned with water quality will rely on this knowledge to help ensure proactive management practices are taken to prevent future groundwater quality depreciation. We have contacted extension and natural resources personnel within the state, describing the project's scope. PD Malakar presented the project to the Natural Resources Districts Managers Meeting in the Fall of 2022. We also informed researchers, faculty, and the general public through newsletters about the general scope of the project. We have discussed our findings and devised plans to implement the project sampling needs with our collaborators in the extension and outreach centers of the University of Nebraska. We contacted individual NRDs (seven in year#1) to collaborate in identifying sampling locations. PD Malakar has also developed undergraduate course to reach out to students. Changes/Problems:We had a problem recruiting a full-time Ph.D. student for the project. The selected student could not join due to long visa queues during COVID-19. We would like to have a full-time Ph.D. student this year. Vadose zone sampling in cropland is limited post-harvest and before the beginning of the growing season. Harsh weather has reduced our chances of collecting planned cores in year#1 of the project. Additionally, the selected prairie location was hard to core due to glacial till, and we have identified new sites where we are waiting for approvals. What opportunities for training and professional development has the project provided?This project has provided training opportunities for one graduate and one undergraduate student. The graduate student communicates with research collaborators and can conduct sampling as needed. The undergraduate student developed a research proposal, for which he received Undergraduate Creative Activities and Research Experience (UCARE) program funding from the University of Nebraska Lincoln. The graduate student working on the project presented the preliminary findings at the American Chemical Society Spring 2023 National Symposium, where the abstract was selected for oral presentation. The project is led by a new faculty member, Dr. Malakar, who ensures periodic meetings with the co-PDs, another aspect of professional development in managing the project. Part of the project was reaching out to stakeholders to promote the project, which has also helped the PD foster new collaborators and professional development. As a new faculty member, it took the initial effort to set up the laboratory, which was completed this year. How have the results been disseminated to communities of interest?Ongoing results are presented by the graduate student in weekly research group meetings. The data generated has been discussed with the PDs. The PDs meet every other month to discuss the progress of the project. Results from last year's experiments are currently being written for two journal articles. Results will be presented in the fall meeting of ACS, where the graduate student submitted an abstract. What do you plan to do during the next reporting period to accomplish the goals?We are collecting cores from the prairie region to conduct the large column experiment this summer. The reason for choosing a prairie location is to eliminate past agriculture practices that can lead to nitrogen species in the vadose zone. We will have 18 columns divided into three groups (n=6); one will be gravity, another pivot, and the last will be rainfed irrigation. Each irrigation group will be sub-divided into two (n=3), one will receive labeled ammonium (historical fertilizer), and another will receive labeled nitrate (most prominent fertilizer). The labeled nitrogen will be tracked in the column experiment. Additionally, corn will be grown on top of the columns. Field sampling is planned in six NRDs in the Fall of 2023 and six NRDs in the Spring of 2024. We will have a full-time GRA in the project. We are finalizing the DTA for RZWQM2 and will be able to start simulating our data.

Impacts
What was accomplished under these goals? IMPACT DESCRIPTION Nitrogen-containing fertilizers are one of the sources of nitrate contamination in groundwater. From the total applied nitrogen fertilizer, crops take up 42-47%, and the rest is mainly lost to volatilization or the vadose zone and the water table through leaching. Irrigation practices, cropping management, fertilizer application rate, and timing all influence nitrate loss to leaching. With more than 90% (9.5 million acres) of land irrigated using the High Plains-Ogallala Aquifer system, Natural Resource Districts (NRDs) in Nebraska have been responsible for implementing regulations to control nitrate losses to groundwater. Our past vadose zone study correlated the occurrence of different nitrogen species, such as ammonia and nitrate, with above-ground irrigation. For example, pivot or sprinkler irrigation vadose samples contained significantly less nitrate and more ammonium than gravity or furrow irrigation systems. In the NIFA-funded project, we combine new in situ vadose zone measurements to identify the unique distribution of nitrogen species depending on surface irrigation practice. The experimental data is used in mathematical models to evaluate nitrogen occurrence and dynamics in Nebraska's vadose zone and identify why surface irrigation practice impacts the form of nitrogen in the deep vadose zone. In this study, we collaborate with NRDs to determine the best sampling locations and provide the selected sites' historical irrigation, cropping, and fertilizer data. The project has three goals: 1. Field sampling: NRDs are contacted to help determine the most suitable sampling sites, focusing on previously cored locations to help characterize how vadose nitrate changes over time. Cores will be processed and analyzed for nitrate, ammonia, and intermediate nitrogen species, including organic nitrogen, to generate a catalog and quantity of nitrogen stored in the vadose zone. 2. Controlled column experiments to follow and identify labeled nitrate pathways and transformations beneath the root zone. 3. Nitrogen simulation will be carried out on USDA Root Zone Water Quality Model "RZWQM2," which will be modified and used to evaluate nitrogen species (forms), movement, and transformation in the soil and vadose zone. The study will generate a large amount of data that will be used to develop a risk assessment map for nitrate and delivered to the stakeholders. The data from the project will pave the way for better technologies and management techniques to protect groundwater throughout the US. This study will also provide a better understanding of nitrogen interactions in simulation models to protect groundwater quality and understand nitrogen dynamics. In the first reporting period from April 2022 to March 2023, we conducted interviews to select a full-time Ph.D. student for the project, and we performed the following activities related to each goal: 1. Major activities completed / experiments conducted. Goal#1 of field sampling: We contacted NRDs across the Eastern part of the state to identify coring sites across three irrigation systems - pivot or sprinkler, gravity or furrow, and rainfed. Post-harvest in Fall 2022, we collected vadose zone cores from two NRDs covering pivot and dryland irrigation practices. Goal#2 of controlled column experiment: A short column experiment was successfully conducted to ensure our unique design using undisturbed vadose zone cores is feasible. In the column experiment, we collected cores from one of the NRDs and a pivot irrigated site. The experiment factor was sprinkler irrigation versus dryland. Two sets were prepared from undisturbed cores, and one received only rainwater (n=3) based on the rain data from the sampling location. The other set received rainwater and irrigation water as used by the grower in that field (n=3). Pore water was collected from the root zone and the capillary fringe. The groundwater table was maintained using groundwater collected from the sampling location. Continuous dissolved oxygen levels were measured at the capillary fringe, which is the connection point of the vadose zone and the groundwater table. The nitrogen species will be quantified in porewater, pre-soil, and post-soil samples. The experiment went through a complete growing season. The samples are being analyzed now. Goal#3: We are working on getting the source code of RZWQM2. A preliminary risk assessment map was developed for Eastern Nebraska to identify vadose zone coring locations where previous data is unavailable. The preliminary risk assessment map utilized publicly available nitrate data in the groundwater. 2. Data collected. We are starting to process the collected cores from the field sites, which is planned for the Summer of 2023. In the concluded column experiment, we collected ~148 soil samples for measuring nitrogen species, which were extracted for microbial analysis. We also collected ~50 soil pore water samples to identify nitrogen transformation reactions. 3. Summary statistics and discussion of results. The preliminary results from the column experiments clearly showed that irrigation water input mobilizes nitrate from the soil, though no external nitrate was added to the columns. We did find significantly higher nitrate from the porewater of the root zone beneath irrigated system than the rainfed system. We also observed spikes in dissolved oxygen levels at the capillary fringe during irrigation events. The risk assessment map has provided information on probable locations where we plan to collect vadose zone cores in the Fall of 2023.

Publications

• Type: Journal Articles Status: Accepted Year Published: 2023 Citation: Li, L., Shields, J., Snow, D. D., Kaiser, M., & Malakar, A. (2023). Labile carbon and soil texture control nitrogen transformation in deep vadose zone. Science of The Total Environment, 878, 163075.
• Type: Journal Articles Status: Accepted Year Published: 2023 Citation: Beegum, S., Malakar, A., Ray, C., & Snow, D. D. (2023). Importance of snowmelt on soil nitrate leaching to groundwaterA model study. Journal of Contaminant Hydrology, 255, 104163.
• Type: Conference Papers and Presentations Status: Accepted Year Published: 2023 Citation: Ukwishaka, Y., Kaiser, M., Snow, D. D., Miller, D. N., Ray, C., Malakar, A., Redox driven transformation of nitrogen species across the vadose zone: Insights from column experiments, ACS Spring 2023 Indianapolis, IN, March 26-30.