ABSTRACT Enrichment with beneficial microorganisms may increase the benefits of organic compost. The aim of this study was to evaluate the enrichment of a mature compost with plant growth-promoting bacteria and arbuscular mycorrhizal fungus (Rhizophagus clarus), using brachiaria (Urochloa brizantha) as host plant, totaling seven treatments: control (compost with no bacteria, brachiaria or mycorrhizal fungus); compost + brachiaria; compost + brachiaria + mycorrhizal fungus; compost + brachiaria + mycorrhizal fungus + Azorhizobium sp.; compost + brachiaria + mycorrhizal fungus + Azoarcus sp.; compost + brachiaria + mycorrhizal fungus + Bacillus subtilis; and compost + brachiaria + mycorrhizal fungus + Azotobacter sp., in a completely randomized design with three replicates. Brachiaria shoot biomass, N and P concentrations, mycorrhizal colonization, and chemical characteristics of the compost were assessed five times over 183 days. B. subtilis and Azotobacter increased ammonium-N concentration in the compost in two and three sampling dates, respectively. In contrast, Azotobacter and Azoarcus decreased the concentrations of nitrate-N in at least one sampling. Despite high P availability in the compost (951-2927 mg kg-1), mycorrhizal colonization reached up to 53%. In a field trial with the produced composts, in a randomized block design with six repetitions, the composts with brachiaria doubled the mycorrhizal colonization of corn (Zea mays) and wheat (Triticum aestivum), independent of the growth-promoting bacteria and, depending on the associated bacteria, increased grain yields.
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