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.