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
0%
Research Effort Categories
Basic
70%
Applied
15%
Developmental
15%
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.