Nitrogen is a tricky nutrient. Plants need nitrogen to grow, develop, and reproduce—too little, and crops won't survive. The ability to create and apply plant-available nitrogen made our modern industrial-scale farms would not be possible. But too much nitrogen can cause other problems. Nitrogen-ladedn runoff from agricultural production is a major cause of pollution in the Mississippi River Basin and contributes to large dead zones in the Gulf of Mexico.
Illinois and other Midwestern states have set goals to reduce nitrogen load through strategies that include different land management practices, but it can be challenging to know which practices are effective, especially in the face of climate change. A new study from University of Illinois researchers, published in Journal of Environmental Management, uses computer modeling to estimate how those practices may be affected by potential changes in the climate, such as increased rainfall.
"The goal was to test whether those land management practices are useful in reducing nitrogen load in the water under different climate scenarios," says Congyu Hou, a doctoral student in the Department of Agricultural and Biological Engineering and the lead author on the study.
Using field data on soil properties, land use, land management practices, and weather patterns from the Willow Creek Watershed in Oklahoma, the researchers estimated surface runoff and nitrogen load at the field-scale level. Their model included 12 land management practices and 32 climate projections for the years 2020 to 2070, yielding a total of 384 scenarios.
"While modeling is commonly used to estimate nitrogen load, most projections use average climate data. This study expands on that practice by including all possible climate predictions," says Maria Chu, assistant professor of agricultural and biological engineering in the College of Agricultural, Consumer and Environmental Sciences. Chu is Hou's advisor and a co-author of the study.
Hou says the study, "Field-scale nitrogen load in surface runoff: Impacts of management practices and changing climate," serves as a test case for the type of modeling simulation the researchers employed. "We use a field-edge model that divides the watershed into smaller cells, for a total of 5,911 cells. The model calculates the edge of the field, focusing on how much nitrogen is transported away from the field."
The research also tested a new index of how to measure the soil's ability to hold nitrogen. They found that even without a reduction in application, nitrogen load can be reduced simply by redistributing land uses, Hou explains.
For example, their findings indicated that crop rotation practices do help reduce nitrogen loss. They also found that splitting fertilizer application between spring and fall generally is more beneficial and that fertilizer application rate is the most critical factor in determining both the amount and probability of high nitrogen load.
Hou cautions that the study's findings are preliminary. "With modeling, you're still far away from practical application," he says. "The model is the first step, then comes a small-scale field test, then small-scale tests at different locations. If those work, you can eventually expand to a larger region."
In addition to expanding the scale of modeling, Chu's research group looks at other aspects of nitrogen pollution. For example, while this study focuses on surface runoff, another study looks at nitrogen load in groundwater.
"We're also looking at the different components of nitrogen—that is, nitrate, nitrite, and ammonia—to determine which one is most critical to address," Chu says.
Producing enough food to feed a growing global population is vital, but so is the continued health of our water sources. Limiting nitrogen runoff under various climate scenarios while maintaining crop yields is a challenge, but the researchers hope their model and analysis method can be used to examine tradeoffs between environmental welfare and the economic benefits of using nitrogen fertilizer.