Introduction

Nitrogen (N) is at the center of agronomy, climate change, and water quality conversations. Farms are striving to achieve higher yields and quality while reducing emissions and leaching losses by improving their nitrogen use efficiency. We need to understand how nitrogen behaves in our agroecosystems, how to evaluate the nitrogen budget, and ways to minimize nitrogen loss.

Understanding the Nitrogen Cycle

Nitrogen cycles continuously between inorganic N, organic N, and gaseous N on farm fields. Consider the soil as a piggy bank for nitrogen (N). After nitrogen is “deposited” through inputs, it can also be withdrawn from the soil via crop harvest, volatilization, denitrification, runoff, erosion, and leaching. In other words, the cycling forms of nitrogen follow the law of conservation of mass, meaning that within the boundaries shown in the nitrogen cycle (Figure 1), nitrogen input equals the sum of nitrogen changes in the soil and nitrogen removed from the soil. The nitrogen budget can be represented as: N input = N removal +/- Changes in N storage.

Fall and spring are the most vulnerable times for nitrogen leaching

Fall and spring are particularly vulnerable to nitrate leaching because that is typically when much of our groundwater recharge occurs. The months following harvest through the planting of the following year’s crop are also times when most row crop agricultural systems are void of vegetation to help with interception and/or water and nutrient uptake during the shoulder periods of the growing season. Cover crops, interplanting, and other strategies that promote living cover can help mitigate the risk of loss during these critical times.

With projections of higher precipitation in Wisconsin in the future, the risk of losing nitrogen through leaching also increases (Wisconsin Initiative on Climate Change Impacts, 2021). Under conventional tillage, 84% of drainage events occurred from January 1st to June 30th. They also found that close to 75% of N leaching loss occurred from April 1st to June 30th.  Intense drainage, high soil residual N from various inputs (e.g. fertilizer, crop residual, manure), and low nitrogen uptake from crops in early growth stages could result in high amounts of nitrate leaching in the spring. As a result, finding ways to secure residual soil inorganic nitrogen during the shoulder season will minimize the risk of nitrogen loss.

Recommendations for reducing nitrogen loss

Understanding nitrogen use in a budget may reveal opportunities to reduce unnecessary use of nitrogen inputs, which saves time and money and reduces contamination to groundwater. Make decisions about how much inorganic nitrogen to apply after considering your soil type, pre-planting soil nitrogen testing, manure, legume, and irrigation N credits if applicable. Concentrations of nitrate-N in irrigation water of a recent study were determined to be between 1 and 45 mg/L NO₃-N, which adds an additional 0.2 to 10.1 lbs of nitrogen per inch of irrigation water applied (Campbell et al., 2023). Nitrate added through irrigation water can be directly absorbed by plants. To simplify the creation of field-based nitrogen budgets, Dr. Kevin Masarik at the UW-Madison Extension has developed a web-based tool named the “Nitrate Leaching Calculator”, which allows users to input environmental conditions, N inputs, and outputs. With this tool, you can understand the potential nitrogen loss from fields under specific management practices and compare different management scenarios. You can find the Nitrate Leaching Calculator and other upcoming related training opportunities at UW-Madison Extension Agriculture Water Quality Program’s website. Below is an example of a nitrogen budget including one field’s environmental factors, inputs, outputs, and estimated leachable N. Leachable N here refers to the potential of nitrogen loss in pounds per acre from (Table 1).

Keep the pace in the relay race of nutrient cycling using cover crops

. Nitrogen is like a baton in a relay race among crops and soil (Figure 2). After N is removed via harvest, soil inorganic N levels remain high in some fields. In wet years with heavy snow and rainfall, the risk of nitrate leaching greatly increases. To retain nitrogen at the end of the growing season, use a cover crop that efficiently scavenges nitrogen, like winter rye. In a field research on cover crops and nutrient management in Wisconsin from 2014 to 2017, researchers found that winter rye’s average dry biomass is over 2,000 lbs per acre, while uptaking more than 50 pounds of N per acre  (Ruark et al., 2019). In the spring, nitrogen from terminated cover crops will return to the soil as inorganic nitrogen and continue in the nutrient cycling process. From a nutrient management perspective, be mindful of the tradeoff of using cover crops. Adjust manure N credit from the previous application depending on cover crops biomass. Read recommendations from this article for more information.

References

• Wisconsin’s Changing Climate: Impacts and solutions for a warmer climate. (2021). Wisconsin Initiative on Climate Change Impacts.
• Arriaga, F., & Cates, A. (2023). Soil Health and Water Quality. Soil Health Nexus.
• Masarik, K. C., Norman, J. M., & Brye, K. R. (2014). Long-Term Drainage and Nitrate Leaching below Well-Drained Continuous Corn Agroecosystems and a Prairie. Journal of Environmental Protection, 05(04), 240–254. https://doi.org/10.4236/jep.2014.54028
• Campbell, T. A., Masarik, K. C., Heineman, E. M., & Kucharik, C. J. (2023). Quantifying the spatiotemporal variability of nitrate in irrigation water across the Wisconsin Central Sands. Journal of Environmental Quality, 52(6), 1102–1114. https://doi.org/10.1002/jeq2.20521
• Ruark, M., Patton, J., & Siemering, G. (2019). Cover Crops, Manure, and Nitrogen Management (A4178). UW-Madison Extension.