Investigating the Fate of Nitrogen Fertilizer in Perennial Groundcover Corn Cropping Systems Using 15N Stable Isotope Labeling
Issue
Sustainable agricultural practices are crucial for meeting global food demand while protecting the environment. Nitrogen (N) fertilizer is often necessary for optimal plant growth but can lead to environmental issues if used inefficiently. Traditional cover crops can help mitigate these issues but have limitations such as economic constraints and potential yield reductions due to competition for resources.
Perennial grass groundcover (PGC) offers unique advantages, including continuous soil coverage, reduced soil disturbance, enhanced soil health, biodiversity, and ecosystem resilience. Eliminating the need for yearly replanting reduces economic risks. Nonetheless, a knowledge gap exists regarding N dynamics in PGC systems.
Objective
Objectives include evaluating corn N fertilizer use efficiency, investigating competition for N fertilizer between corn and PGC, and quantifying N fertilizer leaching below the root zone. Using 15N stable isotope labeling will allow for precise tracking of N fertilizer. 15N is a form of N that acts like a tracer, allowing us to follow its path after application to help measure how much N fertilizer the corn plants use, how much stays in the soil and how much might leach into the groundwater.Understanding N fertilizer fate in PGC systems will help develop management recommendations for farmers to maintain or increase productivity while reducing environmental impacts.
Approach
This research aims to improve N management in PGC systems using advanced techniques to track N fertilizer movement. The study involves planting corn into an established Kentucky bluegrass/perennial ryegrass groundcover and evaluating different PGC system approaches: conventional tillage, strip-tillage with chemically suppressed grass, and strip-tillage with non-suppressed grass. Each plot will receive three N treatments: no N, urea, and 15N labeled urea.
Plant and soil samples will be collected at various growth stages to analyze for 15N to determine N fertilizer content. Lysimeters will collect soil water samples following rainfall events to estimate N fertilizer leaching.
Project Updates
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
July 2025
Since our last report we have completed the processing of all plant and soil materials collected in year 1 of the project. These samples are awaiting 15N isotopic analysis, which will be completed once the appropriate instrument has been installed and is operational. Additionally, we have successfully established year 2 of the project by completing tasks including strip-tillage of PGC plots, conventional tillage of control plots, planting of corn, application of enriched and non-enriched nitrogen fertilizer, installation of suction cup lysimeters, sampling of soil pore water post precipitation events and plant and soil sampling at the V6 growth stage of corn.
Since our prior report we do not have new results relative to nitrogen. However, we have decided to analyze the collected corn samples for S content to evaluate the impact of PGC and N application on corn S uptake and N:S ratio. Preliminary Results for Corn Sulfur Uptake- no significant effect of PGC, regardless if the PGC was suppressed or unsuppressed, was observed on corn S uptake and any sampled growth stage (V6, VT, and R6). However, at all growth stages the treatments that received 200 kg/ha N fertilizer had significantly greater S uptake relative to the 0N treatments. This is likely due to the presence of N leading to healthier plants with more robust rooting systems which are capable of assimilating greater amounts of S. Corn N:S Ratio- For corn N:S ratio, there was a significant effect of PGC treatment at the V6 growth stage with the corn in the unsuppressed PGC treatment having a significantly greater N:S ratio than the conventionally tilled control but neither treatment was different from the suppressed PGC treatment. These results could be indicative of unsuppressed PGC inducing early season S deficiency in corn. However, this effect of PGC was no longer present at the VT or R6 growth stages where only the presence or absence of N fertilizer significantly influenced corn N:S ratio. This means that while PGC may induce early season S deficiency symptomology, the PGC itself is not outcompeting the corn for S, but rather is likely keeping soils cooler, limiting early season mineralization of S from organic matter.
Related activities and accomplishments
2 upcoming presentations - Updated results will be presented in an oral presentation at the Annual Tri-Societies meeting to be held Nov. 9-12 in Salt Lake City, Utah. This work will also be shared at the Midwest Cover Crops Council annual meeting to be held Feb. 10-11, 2026 in Dubuque, IA.
January 2025
Report summary for the project period:
During this timeframe, year 1 of the experiment was established, in-season plant, soil, and water samples were collected, harvest was completed and preliminary analysis of in-season plant samples was conducted. While a central focus of this project is to use stable isotope methods to evaluate nitrogen in perennial groundcover (PGC) systems, no isotope analysis has been conducted yet. We anticipate this analysis to begin late February or early March 2025 and subsequent reports will include these data. Preliminary results are as follows:
Corn biomass: In general, fertilizer treatment had a greater impact on corn biomass than groundcover treatment (Figure 1, see below). Specifically, the non-enriched urea (UN) had significantly greater biomass than 0N at all growth stages, and at VT the conventional tillage (CT) had greater biomass than the unsuppressed PGC (uPGC), but neither was different from the suppressed PGC (sPGC).
Corn plant population: No significant differences were observed in plant population between fertilizer or groundcover treatments (Figure 2).
PGC biomass: Sampling time did not have a significant effect on PGC biomass, so the presented data is averaged across sampling times. However, groundcover and fertilizer did have significant effects on PGC biomass (Figure 3). Specifically, the UN treatment had significantly greater PGC biomass than the 0N treatment and uPGC had significantly greater PGC biomass than sPGC.
Corn nitrogen recovery efficiency: No significant differences were detected in corn nitrogen recovery efficiency (NRE) at V6 or VT (Figure 4). However, there was a trend for lower NRE in PGC treatments at V6 but greater NRE in PGC treatments at VT.
Corn nitrogen uptake: No significant differences were observed in corn N uptake at V6 between fertilizer or groundcover treatments (Figure 5). However, at VT there was a significant effect of both fertilizer and groundcover (Figure 6). Specifically, UN had greater N uptake than 0N and sPGC treatment had significantly greater N uptake than uPGC, but neither was different from CT.
Corn grain yield: There was no significant effect of PGC on corn yield; however, UN did have significantly greater yield than 0N (Figure 7).
(File with figures, 1-7: roth_2024_inrcproject_1-25figures.pdf)
Related activities and accomplishments
To date, the project and preliminary results have been presented three times, including the Annual Tri-Societies Meeting, the RegenPGC Annual Meeting and an Agronomy Department Graduate Student Poster Session. Moving forward, the data from this project will be shared at the 2025 Annual Tri-Society Meeting, the 2025 RegenPGC Annual Meeting, the 2025 North Central Soil Fertility Conference and the 2026 Midwest Cover Crops Council Meeting.
Additionally, the data will be used in an MS thesis and submitted for publication in peer-reviewed journals.