Midwestern soils are among the most productive in the world, thanks in part to extensive tile drainage systems that remove excess water from crop fields. But water isn't the only thing flowing through tile drains. Nitrogen moves along with soil water into drainage ditches, streams, and ultimately into the Mississippi River Basin, where the nutrient contributes to massive algal blooms and hypoxic conditions that impact aquatic life in the Gulf of Mexico.
- Lauren Quinn
A recent study from the University of Illinois Urbana-Champaign provides a new look at the sources and processes affecting the nitrogen load in tile drainage water. The study reveals an unexpectedly large and stable "legacy" pool of nitrogen, adding nuance to the common belief that nitrogen pulses rapidly through tile drainage systems as a transient reflection of fertilizer input and microbial activity.
"The legacy effect relates to the time lag between when nitrogen is made available in the soil environment to its loss to waterways. For example, if you have a nitrogen input via fertilizer this year, it won't be reflected in offloads downstream immediately. This lag has been found in many systems, but previous researchers didn't know what caused it or how large its magnitude was," said lead study author Zhongjie Yu, assistant professor in the Department of Natural Resources and Environmental Sciences (NRES), part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois.
To understand the origin of nitrogen in drainage water, the research team first had to differentiate nitrate derived from various sources. They collected tile drainage samples from a corn-soybean field on a weekly basis over three years and measured nitrate. They also collected soil, crop residue, and fertilizer samples to analyze nitrogen concentrations as well as naturally occurring, stable isotopes of nitrogen and oxygen, the two elements that make up nitrate molecules. Using sensitive laboratory equipment, previous researchers associated slight variations in heavier nitrogen (15N) and oxygen (18O) isotopes with various nitrogen sources and the microbial nitrogen cycling processes of nitrification and denitrification.
"We can think of nitrogen and oxygen isotopes as a fingerprint to identify the sources of nitrate and how it's being recycled by microbial processes," Yu said. "Different sources have different isotope ratios, just like humans have different fingerprints."
Yu added that nitrate derived from inorganic fertilizer has a lower isotope ratio, with fewer heavy nitrogens and oxygens, than bulk soil organic nitrogen sources.
The research team also brought soil samples into the lab and incubated them to learn how microbial nitrogen cycling affects nitrate isotopes. With both the field and lab data, the researchers could trace nitrate sources through time and across cropping systems.
"Our results show that the original isotope ratios of nitrate were similar to those of ammonia fertilizer and soybean biomass nitrogen and did not vary over time when there was no new fertilizer input to the system," Yu said. "This suggests a large legacy pool of nitrate in the soil and a time lag between when nitrogen is added to the system and when it is exported as nitrate in tile drainage."
He added that when new fertilizer was added as anhydrous ammonia to corn, a large shift in the isotopic signal, reflecting the new nitrogen, was recorded in tile drainage water, especially when rain events followed the application. However, this new nitrogen signal was often short-lived, with the legacy signal reemerging within the following days to weeks.
The pattern lines up with results from study co-author and NRES professor Richard Mulvaney's group. In a series of studies, that group used labeled isotope techniques to trace nitrogen uptake in corn plants, finding that less than half of fertilizer nitrogen is used by the plants; instead, corn took up most of its nitrogen from the soil. The remaining fertilizer nitrogen, according to the new results, is likely lost in tile drainage or converted into a reactive fraction stored in the soil, leading to the release of nitrogen long-term.
Yu said the evidence of a legacy effect can inform management and impact how policymakers evaluate the success of nitrogen loss reduction practices.
"Often, we expect to see immediate effects of management changes in nitrogen load. However, even if we stopped applying nitrogen fertilizer for a given year, we might still see significant loss from that system for a few years," he said. "It's not like if we reduce nitrogen input, it can solve everything immediately."
The study's first author, doctoral student Yinchao Hu, added that nitrate loss derived from corn fertilizer was strongest during high tile-drainage discharge events, suggesting that a little management foresight could be beneficial when rain is in the forecast.
"If we can control application during periods of high discharge, that may help us to reduce nitrogen pollution," she said. "Or if there are sufficient forecasts for rain events, farmers can take adaptive measures and temporarily close the tile drainage."
The study, "Deciphering the isotopic imprint of nitrate to reveal nitrogen source and transport mechanisms in a tile-drained agroecosystem," is published in JGR Biogeosciences [DOI:10.1029/2024JG008027]. The research was supported by the USDA National Institute of Food and Agriculture [project no. ILLU-875-983] and the Illinois Nutrient Research and Education Council [project nos. 2021-4-360649-46 and 2014-5-360847-320].
