Quality control of Bradyrhizobium inoculant strains: Detection of nosZ and correlation of symbiotic efficiency with soybean leaf chlorophyll levels

Greenhouse gas emissions, such as N2O from excessive N-fertilizer use, are of concern. Symbiotic N2-fixation by pulses as soybean might mitigate this issue, for which inoculants carrying locally adapted Bradyrhizobium strains are recommended. In the frame of this goal, enhancing the quality control of these inoculants is required on two key aspects: determining the presence of nosZ, for the strains being able to reduce N2O, and assessing N2-fixation potential. Yet, simple and non- destructive methods to assess N2-fixation are lacking. Here we aimed to leverage the correlation between N and chlorophyll levels by cultivating soybeans in vermiculite with N-free nutrient solution, inoculating them with various Bradyrhizobium field isolates, and subsequently measuring chlorophyll with a portable chlorophyllometer, correlating it to symbiotic parameters. We observed significant correlations between chlorophyll and shoot nitrogen levels as well as with nodule dry mass. Two B. diazoefficiens strains stood out and possessed nosZ. In contrast, B. elkanii and B. japonicum isolates displayed lower chlorophyll and symbiotic performance, and lacked nosZ. Our findings highlight the potential of measuring chlorophyll contents and testing for the presence of nosZ as two straightforward techniques that may enhance quality control, enabling selection of superior and safe locally isolated strains for soybean inoculants.