1) Quantify varietal differences for nodulation and BNF (biological Nitrogen fixation) using 15N isotope dilution methods. Fixed N will be measured in seed and residue, including the amount of N available to the next crop. 2) Assess how rate of N applied at seeding maximizes both BNF and yield. 3) Determine relationship between nodule number, BNF, and grain protein. 4) Investigate if nodule numbers can be modified by applying different amounts of Bradyrhizobium inoculant (specific for soybean). 5) Assess impact of low temperatures on nodulation, BNF, and productivity.
In recent years soybean breeding programs have developed a number of short-season soybean varieties for Saskatchewan and Manitoba. While many of these varieties have adaptations that enable them to grow well in the western Canadian prairie provinces, resulting seed typically has lower seed protein levels that varieties adapted for growth in eastern Canada and elsewhere. Protein contents can be as much as 10% lower in the west compared to the east. It has been suggested that the varieties developed for the Prairies may not be as effective at biologically fixing nitrogen and that reduced nitrogen fixation might be responsible for the lower protein contents. This project evaluated if biological nitrogen fixation is linked/responsible for the lower protein contents through a series of controlled environment studies and field trials. None of the studies provided any evidence that restrictions on biological nitrogen fixation are responsible for the lower protein contents measured. While the varieties performed differently in the different environmental conditions, the biological N fixation was not correlated with seed protein content. When water was in adequate supply, there was a tendency for high yielding varieties to have the lowest seed protein. When cool spring temperatures were sustained over long periods of time, productivity was greatly reduced but biological N fixation was reduced to a lesser degree. In general the varieties are able to biologically fix N even under conditions where yield is compromised. While this study does not identify the reason for low seed protein contents in these short-season soybean varieties, producers can be confident that when seed is properly inoculated and receiving adequate soil water the varieties grown are actively and efficiently fixing N from the air.