Green Manuring Reduces Agronomic Indicators of Fodder Winter Barley Regardless of Fertilization Type

Due to the intensive cultivation of various crops, the surface soil layer is depleted. This leads to a decrease in fertility, losses of organic matter and nutrients, and an overall decrease in soil health. We aimed to investigate the role of green manure application and organic fertilization on winter fodder barley (Hordeum vulgare L., Zemela cult.) in terms of agronomic and soil parameters. The cultivation was carried out in two fields, the predecessors of which were oats–vetch green manure (field 1) or fallow (field 2). In each field, five treatments were prepared: a control without fertilization, mineral fertilization, vermicompost, mineral fertilizer + vermicompost, and biochar. The green manure incorporation led to a decrease in grain yield of barley by 10.8–20.0% depending on the treatment. A similar tendency was observed for the rest of the studied agronomic parameters (thousand-grain mass, hectolitre weight, ear number, plants per hectare). Additionally, the vermicompost application had the most substantial effect, accounting for a 20.1% increase compared to the control, while the smallest was expressed by biochar—1.6%. Nevertheless, the photosynthesis intensity was higher in treatments after green manure. The microbiome’s activity was boosted in the vermicompost treatments, while amino acids, carboxylic acids, and polymers were the most fully metabolised compounds by the soil communities. In conclusion, the type of predecessor influenced mainly grain protein, carotenoids, and chlorophyll contents, as well as microbial activities, respiration, and dehydrogenase, while the fertilization impacted primarily on soil water and organic content, total soil N, and photosynthetic pigments of barley plants.