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:
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.