As soybean yields increase, the probability of a profitable yield response to nitrogen fertilizer is also increasing. Research funded by ISA to the coinvestigators found that biological nitrogen fixation accounts for only 50% of soybean nitrogen demand. Soybeans must capture the remaining 50% of nitrogen demand from the soil. This very important result demonstrates that high soil nitrogen availability is critical to achieve high-yielding soybeans. This research was presented at the ICM and ISA Research Conferences. On average, across 16 site-years represented in our ISA-funded study, the average soybean nitrogen uptake from the soil was 111 pounds nitrogen per acre. Moreover, the average nitrogen balance (that is, grain nitrogen harvest minus biological nitrogen fixation) was -30 pounds nitrogen per acre. Our findings were later replicated by University of Nebraska-Lincoln and Kansas State University researchers.

Continued research in this area addressing opportunities to increase soil nitrogen availability for soybean is critical because the potential for profitable nitrogen fertilization of soybeans is increasing for at least three reasons:
1) Wetter weather is expected to decrease nitrogen fixation. The fixation process is extremely sensitive to water availability; too little and too much soil moisture reduces fixation. Over the past decade, Iowa soils have become measurably wetter.

2) Total nitrogen demand by soybean increases with increasing yield. Yet the proportion of total nitrogen derived from fixation declines with increasing yield. Hence, as soybean yields increase, soybeans will become even more reliant on soil nitrogen: One bushel of soybeans requires 4.2 pounds of nitrogen. At 55 bushels per acre, soybeans require 231 pounds of nitrogen with 49% coming from fixation (114 lbs) and 51% coming from the soil (117 lbs). At 65 bushels per acre, soybeans require 271 pounds nitrogen with 45% coming from fixation (123 lbs) and 52% coming from the soil (148 lbs).

3) The effects of increasing corn yields and winter cover crop production prior to soybeans have uncertain effects on soybean nitrogen fertility. There may be less soil nitrogen for soybeans for at least the following reasons: i) Higher corn yields increase soil nitrogen uptake and leave less residual inorganic nitrogen for the following soybean crop; ii) Greater corn residue inputs and cover crop residue inputs reduce soil inorganic nitrogen pool size owing to the high carbon-to-nitrogen ration of corn residue, which promotes microbial immobilization of ammonium and nitrate; iii) Greater corn residue inputs keep the soil cooler and wetter, which slows microbial nitrogen mineralization (that is ammonium and nitrate production) from soil organic matter and manure. Alternatively, the high carbon-to-nitrogen ratio of corn residue and the nitrogen demand of cover crops may increase inorganic nitrogen retention in the soil and allow for greater nitrogen mineralization throughout soybean growth and production. Together, these results indicate that corn residue production, cover crop residue production, and soybean yield potential should be positively associated with the probability of profitable soybean yield response to nitrogen fertilizer: high corn residue and cover crop production, coupled with high soybean yield potential, are likely to generate conditions when nitrogen fertilization is profitable.

The objective of this project is to understand and predict the interacting effects of corn residue production, cover crop production, residual soil ammonium and nitrate levels, and modeled soybean yield potential on soybean nitrogen uptake, soybean response to nitrogen fertilizer, and soybean yield.
We will use a coupled measurement and modeling approach to address our objective. We will leverage the Iowa Nitrogen Initiative research trials that are conducted in a unique partnership between farmers, ISU, and ISA. The Initiative is supported by a wide range of agricultural organizations and will provide Iowa farmers with the latest nitrogen science to benefit productivity, profitability, and environmental performance. The program aims to generate engagement, transparency, and credibility to demonstrate the high efficiency of Iowa corn and soybean farmers and ensure continuous improvement in nitrogen management to ensure Iowa farmers remain the most efficient in the world. The Initiative was piloted in crop year 2021 and was expanded in crop year 2022 to include more than 60 scientifically robust, fully replicated experiments on portions of fields that are either historically high and stable yield zones or low and variable yield zones. Each trial includes a minimum of three nitrogen fertilizer rates (though 5 is common) in a minimum of five replicated plots. Iowa Soybean Association Research Center for Farming Innovation has led the deployment of these trials through funding from partner organizations directed through ISU.

These trials are an ideal place to conduct this work because they contain gradients of crop residue and residual soil inorganic nitrogen levels; most of the trials will be planted to soybeans in crop year 2023 and we estimate a minimum of 10 will be planted to a winter cereal rye cover crop.

We will leverage these experiments to understand how soybean responds to corn residue production, residual soil inorganic nitrogen levels, and winter cover crops. We will model soybean yield potential for each site and integrate these data with measurements of residual soil inorganic nitrogen level, cover crop production, corn residue production (crop year 2022) and soybean yield (crop year 2023). We will use the APSIM model, which is widely used by ISU and ISA scientists. Measurements will be conducted by ISU and ISA researchers.

We will communicate the results of this work at the ISA and ICM research conferences and we will publish two extension publications. One will report the measured effects of previous corn yield, nitrogen fertilizer input, and residual inorganic nitrogen levels on soybean yield and yield potential. We will publish a second extension publication that addresses how these effects interact with cover crop production to affect soybean yield potential and soybean yield gap. This analysis will allow us to calculate probabilities of soybean response to nitrogen fertilizer and identify potential corn residue management strategies that can boost subsequent soybean yield. Finally, these analyses will add great value to the Iowa Nitrogen Initiative by explicitly integrating the effect of corn nitrogen fertility management on soybean management and production.

1) We will determine how corn and cover crop residue levels impacts soybean yield and nitrogen dynamics. It is possible that corn and cover crop residues can be better managed to benefit soybean yield.
2) We will determine how the interaction among increasing corn yield, increasing soybean nitrogen demand from the soil, and a wetter climate will impact soybean yield response to nitrogen fertilizer. Using previous corn yield, soil nitrogen levels, and modeled soybean yield potential, it may be possible to forecast the probability of a profitable soybean yield response to nitrogen fertilizer.

Update:
In this reporting period, we successfully acquired and began to clean 2023 soybean yield data from 2022 Iowa Nitrogen Initiative research plots. We worked with farmer participants in the 2022 Iowa Nitrogen Initiative who deployed completely randomized and replicated nitrogen fertilizer rate trials to corn. Typically, these trials included five nitrogen fertilizer rates each in five unique replicated plots for a total of 25 plots per trial. In trials that were rotated to soybean in 2023, we acquired the 2023 soybean yield data. We were able to acquire these data from approximately 20 farmers (note the 2022 Iowa Nitrogen Initiative was a small-scale pilot year).

Also in this reporting period we have preliminary analyses of these soybean yield data. There is a slight but apparently significant trend between nitrogen fertilizer rate to the 2022 corn crop and 2023 soybean yield.

In the next reporting period, we are working to continue cleaning of the 2023 soybean yield data, analyzing the data, and interpreting the data. If our preliminary interpretation is confirmed and there is a positive relationship between 2022 N fertilizer rate to corn and 2023 soybean yield, we will determine why using statistical and process-based models. There are a number of potential causes of a positive relationship. Surplus residual nitrogen could increase soil fertility or greater corn residue levels from higher corn yield could have improved soil conditions (e.g., greater moisture). In the next reporting period, we will be have more information.

Updated February 10, 2025:
This project aimed to understand if and how corn yield and the optimum N rate to corn affects soybean yield. As soybean yields have increased, soybean N demand from soil has increased; more N is harvested in soybean than soybeans fix from the atmosphere. Hence, we hypothesized that residual soil N following corn is positively associated with soybean yield. We acquired soybean yield data from 18 independent trials. Across the 18 trials, there was slight positive but statistically insignificant relationship between the economic optimum N rate to corn in 2022 and the 2023 soybean yield. Within each trial, there was no relationship between corn yield or N fertilizer rate and the subsequent soybean yield. As the Iowa Nitrogen Initiative continues to collect data on corn yield and N fertilizer rates, we will continue to work to understand how soybean yield interacts with previous and following corn yield and management.

In this reporting period, we completed the cleaning and analysis of 2023 soybean yield data from 2022 Iowa Nitrogen Initiative research plots. We worked with farmer participants from the 2022 Iowa Nitrogen Initiative who deployed completely randomized and replicated nitrogen fertilizer rate trials to corn to collect 2023 soybean yield data from the locations of the 2022 N trial in corn. Typically, these trials included five nitrogen fertilizer rates each in five unique replicated plots for a total of 25 plots per trial.

We acquired soybean yield data from 18 independent trials. Across the 18 trials, there was slight positive but statistically insignificant relationship between the economic optimum N rate to corn in 2022 and the 2023 soybean yield. Within each trial, there was no relationship between corn yield or N fertilizer rate and the subsequent soybean yield.

Although the relationship between the economic optimum N rate to corn and soybean yield was not significant, there are some reasons to expect a positive relationship between the economic optimum N rate to corn and soybean yield. We will continue to investigate this relationship in future work through the Iowa Nitrogen Initiative. In particular, greater soybean yields require more N from the soil, leaving less residual soil N after soybean harvest. Analyses from the Iowa Nitrogen Initiative demonstrate that residual soil nitrate is one of the most important drivers of the economic optimum N rate to corn and residual soil nitrate is included in our new N-FACT decision support system. In the future, we should be exploring the relationship between soybean yield and the following economic optimum N rate to corn (opposite of the timeline of our approach in this work). As we grow relationships with farmer partners in the Iowa N Initiative, we expect more opportunities to collect these data. A second area of future work that has been prompted by this project is the need for an improved understanding of factors that affect any potential relationship between corn yield, nitrogen fertilizer requirements of corn, and soybean yield. The data analysis conducted herein indicate future data to include in our analyses.

This project will provide soybean farmers with improved information for decision making about potential nitrogen fertilizer applications to soybeans. As soybean yields continue to increase, the probability that soybeans will yield will have an economic response to nitrogen fertilizer input increases. However, the probability of such a response is likely a complex outcome of residual soil nitrogen levels, previous corn yield and residue management, and cover crop management. This project will provide information about these factors interact to affect soybean nitrogen nutrition so that farmers can better anticipate under what conditions soybeans will have an economic response to nitrogen fertilizer.