How rain-resilient are N practices?

Editor’s note: The following was written by Eseul Choi, Guilherme DePaula, Peter Kyveryga, and Suzanne Fey for the Iowa State University Center for Agricultural and Rural Development Agricultural Policy Review.

Farmers in the U.S. Corn Belt face a challenging balancing act: Applying enough nitrogen fertilizer to maximize crop yields while minimizing environmental harm from runoff and leaching.

Climate change complicates this balance, as more intense and frequent rainfall events lead to increased nitrogen loss from agricultural fields, resulting in lower yields and greater pollution.

As farmers adapt their management strategies, policymakers and researchers must ask: Which combinations of fertilizer practices and crop rotations best withstand these new rainfall patterns?

A new study published in Land Economics investigates this question using large-scale, on-farm experimental data from Iowa.

What we studied

To investigate the relationship between nitrogen management, rainfall and environmental risk, we analyzed two large datasets from the Iowa Soybean Association — a set of randomized on-farm nitrogen experiments and a broader survey dataset containing over 3,900 Corn Stalk Nitrate Tests.

These datasets provide unique field-level insights into how different combinations of fertilizer form, timing and crop rotation respond to excessive rainfall.

Our analysis focuses on the estimated change in the probability of nitrogen leaching under wet conditions. Because nitrogen leaching cannot be directly observed at scale, we follow agronomic best practices and use CSNT readings at the end of the season as a proxy.

We estimate how rainfall affects these leaching risks for various combinations of practices, including the use of anhydrous ammonia, urea ammonium nitrate, manure, different application timings (fall, spring, side-dressing), and rotation types (corn-corn and corn-soybean).

Key findings

Our analysis shows that excessive early-season rainfall significantly increases both the agronomic value and the environmental risk associated with nitrogen fertilization. As rainfall intensifies, the yield penalty from applying insufficient nitrogen rises, prompting farmers to apply more nitrogen. However, this also heightens the risk of nitrogen leaching into the environment.

To understand how different nitrogen management strategies perform under these conditions, we estimate for a wide range of practice combinations the increase in the probability of nitrogen leaching under excessive rainfall relative to normal rainfall.

These include differences in:

We find that management combinations vary widely in their resilience to excessive rainfall. Figure 1 summarizes our findings by ranking 10 common practice combinations by the estimated increase in nitrogen leaching probability when exposed to high rainfall.

We find the practices most resilient to the effects of excessive rainfall are the use of corn-soybean rotation, side-dressing or split N applications, and the use of urea ammonium nitrate.

Excessive rainfall affects these practices the least. However, they are not necessarily the most environmentally friendly. For example, UAN causes the highest marginal environmental damage under any weather condition because it contains more nitrate, which is especially prone to leaching. Yet under excessive rainfall, the probability of leaching increases more sharply for anhydrous ammonia than for urea ammonium nitrate.

The least resilient combination involves applying manure in the fall while using continuous corn rotation. This approach leads to a 77% increase in the risk of nutrient leaching during wet conditions.

Likewise, practices that include spring application of anhydrous ammonia and side-dressing in corn-after-corn fields also show considerable vulnerability to leaching.

In contrast, side-dressing with a corn-after-soybean rotation results in the lowest increase in leaching risk, about 32%, which is less than half the risk compared to the most vulnerable combinations.

These findings highlight important tradeoffs in nitrogen management practices. While methods such as spring or side-dressing applications may mitigate yield losses in wet years, others can significantly worsen environmental outcomes. For example, although manure is often seen as a more sustainable nitrogen source, its effectiveness diminishes sharply under heavy rainfall, particularly when applied in fall.

Understanding these interactions is crucial for developing climate adaptation strategies in agriculture. Our findings highlight the necessity for targeted policies and outreach that promote nitrogen management practices resilient to increasingly variable weather.