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.
A recent study from the University of Illinois provides a new look at the sources and processes affecting the nitrogen load in tile drainage water, according to a news release.
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,” said lead study author Zhongjie Yu, assistant professor in the Department of Natural Resources and Environmental Sciences at Illinois.
“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.”
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.”
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.”
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.
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