We intend to use a marker-facilitated, fast-track, backcrossing scheme to create BNF- and BNF+ activity contrasts as two near-isogenic lines in five different high-yielding contemporary soybean backgrounds - one in each of five maturity groups 0, I, II, III, and IV. The lines can be used in NE and in the other 12 States of the NCSRP for research aimed at better understanding relationships between soybean yield potential and N-uptake by soybean plants. This project will provide genetic tools to better characterize the apparent decline in harvest index and in seed yield estimates at high-BNF activity. These tools will be made available to the research community upon completion of this project. Findings of future research using these genetic resources can have a profound impact on future soybean genetic improvement and field management practices.

There are no modern high-yielding varieties in maturity groups (MG) 0, I, II, III and IV zones of adaptation in the North Central US developed as isolines carrying the mutant non root-nodule phenotype that can be compared to their normal root-nodulating counterparts in order to measure the impact of their nitrogen (N) fixing capability. The only existing non-nodulating mutant isolines are two old varieties (Harosoy and Clark) that were developed in the 1960s. To facilitate ongoing research aimed at understanding the current contributions of Biological Nitrogen Fixation (BNF) and nitrate availability (soil residual or fertilizer) that is representative of production environments of MG II and III in Nebraska and the North Central growing region, it is imperative to develop and utilize modern isoline pairs that were bred for modern high-yielding environments. Thus, the key research objective in this proposal is to create a BNF contrasting pair of isolines that are representative of modern high-yielding genetic backgrounds in each MG. In addition, the genetic resources created will be available to soybean breeders, physiologists, and agronomists across the 12 states of the NCSRP region and will represent a valuable resource for research that is now pursuing optimization of both, BNF and soil Nitrate N uptake on high-yielding production environments.

This is a 3-year project from Oct 1, 2019 through Sep 30, 2022.
After the creation of the five high yield variety Rj1Rj1 and rj1rj1 NIL pairs, the PIs will work with breeders, physiologists, and agronomists in NE and the north central region to conduct multi- site tests to evaluate the impact of the paired NILs of BNF+ and BNF- in one or more of the five MG- varieties that can be grown in each state. Seed will be available after a seed increase of the NIL pairs during 2022. The PIs will work with researchers in the 12-state NCSRP to prepare a research proposal using the NILs in soybean-related N research for submission to the NCSRP in the spring of 2022.

Updated May 26, 2023:

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Producers need to know how to maximize the yield potential of modern soybean varieties and one approach to achieve this is to identify the optimal N-uptake route, whether it is via BNF, applied soil nitrate or a combination of both. In legumes, research has shown that the energy cost of a plant to acquire N from atmospheric N via BNF is 50 to 75% greater than the cost for the plant to obtain N from mineralized nitrate in the soil (Kaschuk et al., 2009). Because of the greater energy demand with BNF, plants are thought to favor N-uptake from nitrate in the soil (if present due to mineralization or fertilization) in order to satisfy the demand of N during vegetative and reproductive (seed development)
periods. Tamagno et al. (2018) demonstrated that soybean yields across 23 field sites of the North Central region declined, on average, by 13 kg/ha (i.e., 0.1933 bu/ac) per % unit increase in BNF, with BNF estimates ranging from 54 to 89% BNF. Hence, the cost of high-levels of BNF can have a significant impact on yield potential. The authors also reported that as levels of BNF increase, harvest index (HI) estimates decrease (where HI refers to the ratio of seed biomass produced to total aboveground biomass, including seed) suggesting that high-BNF levels favor the less costly vegetative biomass production at the expense of the more expensive reproductive biomass (seed). The availability of the genetic resources to better quantify this relationship and explore the optimal levels of N-uptake via different routes can have a profound impact on future soybean genetic improvement and field management practices.