The growing world population requires increased food and fodder quantity and quality production. Circularity in agriculture could support this demand, decreasing the use of chemical fertilizer by applying organic amendments. We hypothesize that green manuring of oat--vetch mixture will support fodder barley (Hordeum vulgare var. Zemela) growth and yield better under different fertilization schemes than those sowed after fallow. Vermicompost from agricultural wastes, oat biochar, their combination, mineral fertilizer, and an untreated control were applied. The results showed increased barley grain yield, with a mass of 1000 seeds, and hectoliter weight in the treatments after fallow compared to those after green manure. In combined and biochar treatments of this field, the highest CO₂ emission from soil was found, whilst the increased SPAD index showed improved chlorophyll content in the field after green manure.

Soil microbial activity was altered differentially depending on the barley antecessor, showing higher β-glucosidase and lower dehydrogenase activity in the amended treatments after green manure. At the same time, the expression of phosphatase was ambiguous. Additionally, rhizosphere communities expressed the highest metabolization rate of polymers, carboxylic acids, and amino acids in both fields and similar diversity indexes. Finally, we did not find significant differences among treatments concerning the organic matter content and composition.

Acknowledgements. This research was funded by the Bulgarian National "Science Fund", grant number KP-06-DO 02/5. The authors acknowledge the financial support of the partners of the Joint Call of the Cofund ERA-Nets SusCrop (Grant N° 771134), FACCE ERA-GAS (Grant N° 696356), ICT-AGRI-FOOD (Grant N° 862665), and SusAn (Grant N° 696231).