Characterization of Nitrogen Fixation in US Soybean Systems

PROJECT JUSTIFICATION AND RATIONALE
Soybean production heavily depends on the total nitrogen (N) uptake primarily supplied via biological N fixation (BNF) process. From a recent review summary performed by Dr. Ciampitti, a strong connection was documented between plant N demand and final yield for soybeans. For example, a 70 bu/ac soybean needed a total N uptake (aboveground only) of approximately 320 lbs/ac. Approximately 50% of the total N uptake is coming after full-pod stage (R4 stage), increasing the need of N for the crop during the later crop growth stages. From the end-season N uptake, on average 50-60% of this amount is potentially met by BNF (Salvagiotti et al. 2008).



The primary N sources for this nutrient are: 1) BNF and 2) inorganic N coming from the soil reservoir. The N fixed is assimilated and exported in the xylem as three N-sources: 1) ureides (allantoin and allantoic acids), 2) amino-N, and 3) nitrate-N. As nodulation activity increases, the ureide content on the xylem increases (Tajima and Yamamoto, 1975). Thus, the ureide concentration in the stem organ represents an indirect measurement of the N fixation activity for nodulated crops.
There are different methods to measure N fixation each one presenting different complexity and limitations. Among the most popular and well-documented ones are: 1) N difference method, 2) natural abundance, 3) isotopic dilution, and 4) ureides (Unkovich et al, 2008). The natural abundance method represents a direct measure of BNF but it is cost expensive and require of previous calibration in greenhouse and the presence of a non-fixing crop (check plant). The ureide method is an indirect and point-measurement of BNF, less labor intensive and cost expensive but needs calibration with isotopic labeled method (15N, for example natural abundance). Further details are presented in the below diagram.


Previous review analysis on soybean BNF quantified an overall fixation ranging from 40-to-70% with the rest of the N demand supplied by the inorganic N coming from the soil reservoir. New BNF procedures are under development (led by Dr. Ciampitti, K-State) in order to perform determination at the on-farm conditions, ureide methodology (“in-field testing”). This BNF methodology will be calibrated against the 15-N natural abundance technique.

PROPOSED RESEARCH
We are conducting a collaborative project with Winfield Solutions for providing a regional characterization of in-season soybean N fixation (ureide technique). A complementary study is to collect corn (unfertilized) and soybean samples from multiple locations (more than 20 total) across the US with the goal of evaluating isotopic labeled method (15N natural abundance). The primary goal of the proposed project is to characterize soybean BNF utilizing the 15N natural abundance method under contrasting yield environments (+sites across the US).

FOR ALL SITES – Year 1: Total N fixation will be characterized from 4 N treatments [(1) no N applied – only inoculation-, (2) fertilizer N applied at planting, (3) fertilizer N applied at early season –V4, four leaves stage- and (4) fertilizer N applied at late season- R2- application – when N was applied only 100 lbs/acre in each timing). For year 1, more than 20 locations across the US will be characterized for 15N (natural abundance technique- using unfertilized corn as a control plot). All data collected from all site-years will be summarized and presented in extension and scientific research articles.

Expected Outputs:

• New BNF information related to soybean farming systems. How much N is fixed and its contribution to total N uptake for the US “Corn-Belt” soybean region.
• At soybean events across the North Central region, this new BNF data will be introduced as a platform for improving understanding of soybean production systems.
• Presentations made at professional agronomy meetings and at soybean-related extension meetings.
• All data collected from all site-years will be summarized and presented in extension and scientific research articles. Results will also be published in industry forums such as Better Crops Magazine.

REFERENCES
Salvagiotti, F., K.G. Cassman, J.E. Specht, D.T. Walters, A. Weiss, and A. Dobermann. 2009. Nitrogen uptake, fixation and response to fertilizer N in soybeans: A review. Field Crops Res. 108:1-13.

Tajima, S. and Y. Yamamoto. 1975. Enzymes of purine catabolism in soybean plants. Plant Cell Physiol. 16:271-282.

Unkovich, M., D. Herridge, M. Peoples, G. Cadisch, R.M. Boddey, K.E. Giller, B.J.R. Alves, and P. Chalk. 2008. Measuring Plant-associated Nitrogen Fixation in
Agricultural Systems. ACIAR (Australian Council for International Agricultural Research) http://www.aciar.gov.au/publication/MN136, MonographNo. 136, 258 pp.