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.

The Primary Objective to this Proposal is the creation of a contrasting pair of Rj1Rj1 (BNF+) and rj1rj1 (BNF-) NILs in
five different varieties – one in each MG 0, I, II, III, or IV - thus spanning the MG adaptation range in the 12-state
NCSRP region. After the creation of the five varietal NIL pairs, this genetic resource will be made available to soybean
research scientists (breeders/physiologists/agronomists/biologists) in NE and in the other 11 NCSRP States for use in
research aimed at understanding the relationship between soybean yield potential and the existent routes of Nuptake
by soybean plants. This proposed research project aligns with the NSB FY19-22 Strategic Plan of Supporting
Proactive and Deliberate Productive Research Projects, and falls into this year’s NSP RFP aimed at “improvement of
soybean yield potential” and at “the relationship between N derived from BNF and N derived from soil-applied urea”.

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 that can be used in NE and in the other 11 States of the NCSRP for research aimed
at better understanding the relationship between soybean yield potential and the existent routes of N-uptake by
soybean plants. This project will provide the 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 researchers in the NCSRP
states upon completion of this project and more importantly, the findings of future research using these genetic
resources can have a profound impact on future soybean genetic improvement and field management practices

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.