The soils of the Midwest are considered some of the most fruitful globally, largely due to the comprehensive tile drainage systems that effectively eliminate excess moisture from agricultural land. However, it’s not just water that makes its way through these tile drains. Nitrogen travels alongside the soil water, reaching drainage ditches, streams, and eventually the Mississippi River Basin. Here, it contributes to significant algal blooms and low-oxygen conditions that adversely affect aquatic ecosystems in the Gulf of Mexico.
A new research study from the University of Illinois Urbana-Champaign offers fresh insights into the sources and dynamics of nitrogen found in tile drainage water. This research uncovers a surprisingly large and persistent “legacy” pool of nitrogen, challenging the widespread belief that nitrogen quickly moves through tile drainage systems as a temporary result of fertilizer application and microbial action.
“The legacy effect refers to the delay between when nitrogen becomes available in the soil and when it is washed away into waterways. For instance, if you apply nitrogen fertilizer this year, it won’t show up in downstream water immediately. This lag has been observed in various systems, but previous researchers were unsure of its causes or how significant it was,” explained Zhongjie Yu, the lead author and an assistant professor within the Department of Natural Resources and Environmental Sciences (NRES) in the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois.
To identify the sources of nitrogen in drainage water, the research team first needed to distinguish between nitrates coming from different origins. They collected samples of tile drainage from a corn-soybean field weekly for three years and measured the nitrate levels. Additionally, soil, crop residue, and fertilizer samples were gathered to evaluate nitrogen concentrations and the naturally occurring stable isotopes of nitrogen and oxygen that make up nitrate molecules. By utilizing advanced laboratory equipment, previous researchers linked slight differences in heavier nitrogen (15N) and oxygen (18O) isotopes to various nitrogen sources and the microbial processes of nitrification and denitrification.
“We can think of nitrogen and oxygen isotopes like a fingerprint that helps identify the sources of nitrate and track how microbes recycle it,” Yu noted. “Different sources exhibit different isotope ratios, similar to how humans possess unique fingerprints.”
Yu pointed out that nitrates from inorganic fertilizers have a lower isotope ratio, indicating fewer heavier nitrogen and oxygen atoms compared to nitrogen derived from organic soil sources.
The research team also analyzed soil samples in the laboratory, incubating them to study how microbial nitrogen cycling affects nitrate isotopes. With the data gathered from both the field and laboratory studies, they were able to trace nitrate sources over time and across different cropping systems.
“Our findings indicate that the original isotope ratios of nitrate closely resembled those of ammonia fertilizer and nitrogen from soybean biomass and remained consistent when no new fertilizer was introduced,” Yu mentioned. “This implies a substantial legacy pool of nitrate within the soil and a time delay between nitrogen’s addition to the system and its subsequent export as nitrate through tile drainage.”
He added that when new fertilizer was applied as anhydrous ammonia to corn, a significant shift in the isotopic signature, indicating the influx of new nitrogen, was detected in the tile drainage water, especially following rainfall. Nevertheless, this new nitrogen signal often faded quickly, with the legacy signature reappearing within days to weeks.
This pattern aligns with findings from study co-author Richard Mulvaney’s team at NRES, who, in a series of studies, traced nitrogen uptake in corn plants using labeled isotopes. They discovered that less than half of the nitrogen from fertilizers is absorbed by plants; most of the nitrogen absorbed by corn actually comes from the soil. Consequently, the remaining fertilizer nitrogen is likely lost through tile drainage or converted into a reactive form stored in the soil, leading to prolonged nitrogen release.
Yu stated that recognizing the legacy effect is crucial for informing management strategies and influencing how policymakers assess the effectiveness of practices aimed at reducing nitrogen loss.
“We often anticipate immediate results from management changes regarding nitrogen loads. However, even if we cease nitrogen fertilizer application for a year, we may still observe significant losses from the system for several years,” he cautioned. “It’s not simply a matter of reducing nitrogen input to solve the problem instantly.”
According to Yinchao Hu, the study’s first author and a doctoral student, nitrate loss from corn fertilizer peaked during high tile drainage discharge events, implying that proactive management could be advantageous when rains are anticipated.
“If we can manage fertilizer application during periods of significant discharge, it may help reduce nitrogen pollution,” she said. “Additionally, if we have sufficient forecasts for impending rain events, farmers could adopt adaptive measures such as temporarily closing the tile drainage.”
