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 negative 30 pounds nitrogen per acre. Our findings were later replicated in Nebraska and Kansas by University of Nebraska 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) As corn yields preceding soybeans increase, there will be less soil nitrogen for at least the following three 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 reduce soil 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.
Together, these results indicate that corn yield amount (residue production) and soybean yield potential should be positively associated with the probability of profitable soybean yield response to nitrogen fertilizer: high corn residue remaining in the soil and 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, residual soil ammonium and nitrate levels, and modeled soybean yield potential on soybean nitrogen uptake, response to nitrogen fertilizer, and yield.
We will use a coupled measurement and modeling approach to address our objective. We will leverage the Iowa State University long-term nitrogen rate trials. Note: these trials are not included in the ISU MRTN database but have long-term corn-soybean rotation systems (both phases present every year) with five nitrogen fertilizer inputs to corn only (0-250 lbs N/ac). The trials have been ongoing since 2002 and are located at four locations: Sutherland , Ames, Chariton, and Crawfordsville Iowa. Soybean yield data in these trials have not been analyzed as a function of previous corn yield and residual inorganic nitrogen levels. These trials are an ideal place to conduct this work because they contain gradients of crop residue and residual soil inorganic nitrogen levels within and across years. For every year from 2002-present, we have corn and soybean yield as functions of N input to corn and corn residue levels. We will complement these data with modeled soybean yield potential for each site-year and residual soil inorganic nitrogen level for each site year x corn residue level. In addition, in crop year 2022, we will measure soil inorganic nitrogen levels to validate modeled outputs. We will use the APSIM model, which is widely used by ISU and ISA scientists.
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 and nitrogen fertilizer input on soybean yield at the four long-term nitrogen rate trials from 2002-2022 (4 locations * 5 nitrogen rates * 20 years = 400 data). We will publish a second extension publication that addresses the effects of previous corn residue input and modeled soil inorganic nitrogen levels on 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 inform our developing ‘Iowa Nitrogen Initiative’ research (described below) that

Timeline and Milestones
• January-March: Compile a database with soybean yield response to nitrogen and the associated meta-data (weather, soils, management) from the long-term experiments described above. ISA stakeholders will see the data on soybean yield response to previous measured corn yield and nitrogen rate to corn from the long-term nitrogen database described above.
• March-September: Make new measurements of soil nitrate in the soybean phase of the long-term nitrogen experiments.
• April-August: Statistical data analysis and simulation modeling to calculate probabilities and develop a systems understanding of N fertilizer effects on soybean N fluxes and sustainability using existing data. Validate simulation models using 2022 measured data.
• July-September: Develop extension publications and disseminate results.

Update:
We have developed a database with soybean yields following different nitrogen fertilizer rates at seven long-term nitrogen fertilizer rate trials in the state of Iowa (Ames, Chariton, Crawfordsville, Kanawha, Lewis, Nashua, and Sutherland). We are currently working to present these data at a conference where ISA members will be attendance.
We are currently planting the long-term experiments and making nitrate measurements. We have also initiated an experiment to understand how soybean residue levels impact nitrogen fertility in the following corn crop. We have manipulated soybean residue levels at two locations to 0, 100% and 150% of ‘normal’ (normal was a 56 bushel harvest in crop year 2021). This year, we will measure the impact of soybean residue amount on the nitrogen fertility requirements of the following corn crop.
Furthermore, during the first reporting period we submitted and published a new paper on soybean N dynamics using simulation modeling and experimental datasets

View uploaded report

Update:
In 2022, we developed a database with soybean yields following different nitrogen fertilizer rates at seven long-term nitrogen fertilizer rate trials in the state of Iowa (Ames, Chariton, Crawfordsville, Kanawha, Lewis, Nashua, and Sutherland). We found no response of soybean yield to the previous corn yield or nitrogen fertilizer rate required to produce the corn yield.
As planned, we conducted an experiment to build on this analysis that manipulated soybean residue to 0, 100% and 150% of ‘normal’ (normal was a 56 bushel harvest in crop year 2021). We found a significant effect of the soybean residue level on corn yield and optimum N rate required to produce that corn yield:
Furthermore, during the first reporting period we submitted and published a new paper on soybean N dynamics using simulation modeling and experimental datasets. In one site-year in central Iowa, soybean residue harvest (baling) increased the following corn yield by 43% while decreasing the optimum nitrogen fertilizer rate required to produce that yield by 17%. As a result, the agronomic efficiency of corn production increased from 0.66 bushels per pound of nitrogen to 1.15 bushels per pound of nitrogen. We will repeat this experiment in 2023.

The amount of nitrogen fertilizer applied to corn and the amount of corn residue do not not appear to impact the following soybean yield. This result stems from more than 100 site-years of data spanning seven locations across the state of Iowa. However, in a single site-year preliminary experiment, we found that soybean residue amount has a significant effect on the following corn yield and optimum nitrogen fertilizer rate required to produce that yield. Although one site-year is insufficient data from which to draw conclusions, this is an important area to further research as it represents an opportunity to manage soybean for reduced nitrogen fertilizer needs and greater yield in the rotated corn crop. This could provide a significant economic and environmental benefit to the two-crop rotation.

This work will help Iowa soybean growers in the following ways:
1) We will determine how corn residue level impacts soybean yield and nitrogen dynamics. It is possible that corn residue 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.