The Effect of the Nitrogen-Fixing Bacteria and Companion Red Clover on the Total Protein Content and Yield of the Grain of Spring Barley Grown in a System of Organic Agriculture

Field research was conducted in Poland from 2019–2021 to determine the effect of the bacteria Azospirillum lipoferum Br17 and Azotobacter chroococcum, as well as companion red clover on the total protein content and yield in the grain of spring barley cultivated in a system of organic agriculture. Two factors were examined in the field experiment: I. bacterial formulations: 1—control, 2—nitrogen-fixing bacteria (Azospirillum lipoferum Br17, Azotobacter chroococcum), 3—nitrogen-fixing bacteria (Azospirillum lipoferum Br17, Azotobacter chroococcum) + phosphorus-releasing bacteria (Bacillus megaterium var. phosphaticum, Arthrobacter agilis), and 4—nitrogen-fixing bacteria (Azotobacter chroococcum) + plant growth-promoting rhizobacteria (PGPR) (Bacillus subtilis, Bacillus amyloliquefaciens, Pseudomonas fluorescens); II. companion crop: control without a companion crop, red clover, and red clover + Italian ryegrass. In spring barley grain, the total protein content was determined and the total protein yield was calculated. The obtained study results demonstrated that the growing season conditions significantly affected the total protein content and yield in the spring barley grain. The highest total protein content was recorded in the grain of spring barley following an application of nitrogen-fixing bacteria (Azotobacter chroococcum) combined with PGPR (Bacillus subtilis, Pseudomonas fluorescens) and grown with companion red clover.