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.

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.

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.

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 (co-PI Gunther) has led the deployment of 16 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.

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.

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.

3) The work from this project will be included in web sites that will be developed to understand the optimum nitrogen fertilizer rate to corn (e.g., the previous soybean residue input amount will be captured).

Update:
Performance metrics for the reporting period due April 1, 2023 included the following: 1) Compilation of a database that characterized corn yield response to nitrogen fertilizer and ancillary data; 2) Analysis of the database to determine the optimum nitrogen fertilizer input and the amount of corn residue and residual nitrate at the optimum nitrogen fertilizer input. Development of this database was critical to understand how the rotated soybean, to be planted in crop year 2023, responds to both the amount of corn residue and residual nitrogen fertilizer (i.e., the amount of nitrate remaining in the soil post-harvest). Our database of more than 60 unique trials demonstrates that the optimum nitrogen fertilizer input to corn ranged from 66 to 246 lbs N per acre and the yield at the optimum nitrogen fertilizer input ranged from 161 to 263 bushels per acre. Across the trials, residual soil nitrate levels at harvest ranged from zero to more than 400 lbs N/acre in 0-12” and zero to more than 200 lbs N/acre in 12-24”. Within trials, the ranges were narrower, but spanned more than 100 bushels per acre and 100 lbs of residual soil nitrate nitrogen per acre. These ranges of residue and residual nitrate levels, particularly within trials, will offer an unprecedented opportunity to identify how these two factors interact to affect soybean yields.

Our co-investigator, Theo Gunther, left ISA shortly after we received funding for this proposal. Since that time we have been working with Scott Nelson. We will identify approximately 10 of the fields that were planted to winter cereal rye cover and soybean (post-harvest crop year 2022 and crop year 2023, respectively). We anticipate accomplishing this activity this summer. Then, we will follow up with the growers to ensure we receive combine maps for the trials in the fall of 2023. By matching the combine maps of soybean yield with the previous corn yield (i.e., residue production) and residual nitrate we will determine these effects on soybean yield. This performance metric will allow us to accomplish our project goals to understand how corn residue levels and residual soil nitrate levels affect soybean yield.

Update:
The remaining project period was impacted by changes in investigators owing to staff turnover (Theo Gunther was original co-investigator and then Scott Nelson who also left). The lack of continuity made it difficult to identify growers with fields that qualified for the study. As an alternative plan, we assigned a postdoctoral researcher to work with the Iowa Nitrogen Initiative’s (INI) industry partner to collect soybean harvest data from fields that were part of the larger INI studies in 2022. These soybean yields are being paired with the previous corn yield and N fertilizer rate to that corn. The request to the partner was made within the project period and we have received approximately 15 trials; the data is still being prepared as of this report time. Because the data was not received in sufficient time to allow for preparation and submission, a peer reviewed or extension article has not been published.

As a broader goal of this work, the INI successfully conducted hundreds of on-farm nitrogen rate trials in crop year 2023, creating a larger pool of fields from which to identify corn fields that were planted to a winter cereal rye cover crop prior to the next year’s soybean planting. We are collaborating closely with ISA to recruit and manage these trials. Data will be used to perform 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.

Nitrogen rate trials conducted on corn fields in 2022 demonstrated that the optimum nitrogen fertilizer input to corn ranged from 66 to 246 lbs N per acre and the yield at the optimum nitrogen fertilizer input ranged from 161 to 263 bushels per acre. Across the trials, residual soil nitrate levels at harvest ranged from zero to more than 400 lbs N/acre in 0-12” and zero to more than 200 lbs N/acre in 12-24”. Within the nitrogen rate trials, the ranges were narrower, but spanned more than 100 bushels per acre and 100 lbs of residual soil nitrate nitrogen per acre.

We are now analyzing soybean yields in the following year (crop year 2023) to determine how soybeans yields are affected by the previous corn management including nitrogen fertilizer rate and residue production. In some of these trials, a winter cereal rye cover crop was planted following the corn prior to the soybeans. We will also determine if and how the rye cover crop affected soybean yields and soil nitrogen dynamics.

This project will help soybean farmers to better understand and manage crop residues including those from cover crops on soybean productivity and environmental nitrogen dynamics. Also, the project will help soybean farmers to understand how the management of soybean residues impact the optimum nitrogen rate to corn.