Nitrogen 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.

Nitrogen (N) cycles continuously between inorganic N, organic N, and gaseous N on farm fields. Consider the soil as a piggy bank for nitrogen. After nitrogen is “deposited” through inputs, it can also be withdrawn from the soil via crop harvest, volatilization, denitrification, runoff, erosion, and leaching.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 like this:

N input = N removal +/- changes in

N storage

Times of higher risk

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 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 parts of the country in the future, the risk of losing nitrogen through leaching also rises. Under conventional tillage, 84% of drainage events occurred from January 1 to June 30. It has also been found that close to 75% of nitrogen leaching loss occurred from April 1 to June 30. Intense drainage, high soil residual nitrogen from various inputs (such as fertilizer, crop residual, and 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.

Consider the budget

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, preplanting soil nitrogen testing, manure, legume, and irrigation nitrogen 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 pounds of nitrogen per inch of irrigation water applied. Nitrate added through irrigation water can be directly absorbed by plants.

To simplify the creation of field-based nitrogen budgets, Kevin Masarik at the University of Wisconsin-Madison Division of Extension has developed a web-based tool named the “Nitrate Leaching Calculator,” which allows users to input environmental conditions, nitrogen 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 Division of Extension Agriculture Water Quality Program’s website.

Table 1 is an example of a nitrogen budget, including one field’s environmental factors, inputs, outputs, and estimated leachable nitrogen. Here, leachable nitrogen refers to the potential of nitrogen loss in pounds per acre.

Stay on track

Keep pace in the relay race of nutrient cycling by using cover crops. Nitrogen is like a baton in a relay race among crops and soil (Figure 2). After nitrogen is removed via harvest, soil inorganic nitrogen levels remain high in some fields.

In wet years with heavy snow and rainfall, the risk of nitrate leaching intensifies. To retain nitrogen at the end of the growing season, use a cover crop like winter rye, that efficiently scavenges nitrogen.

In a field research study 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 pounds per acre, while uptaking more than 50 pounds of nitrogen per acre. 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 trade-off of using cover crops. Adjust manure nitrogen credit from the previous application depending on cover crops biomass.