Quality control ofBradyrhizobiuminoculant strains: Detection ofnosZand correlation of symbiotic efficiency with soybean leaf chlorophyll levels

Abstract

AbstractGreenhouse 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 adaptedBradyrhizobiumstrains are recommended. In the frame of this goal, enhancing the quality control of these inoculants is required on two key aspects: determining the presence ofnosZ, 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 variousBradyrhizobiumfield 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. TwoB. diazoefficiensstrains stood out and possessednosZ. In contrast,B. elkaniiandB. japonicumisolates displayed lower chlorophyll and symbiotic performance, and lackednosZ. Our findings highlight the potential of measuring chlorophyll contents and testing for the presence ofnosZas two straightforward techniques that may enhance quality control, enabling selection of superior and safe locally isolated strains for soybean inoculants.