Application of biochar and composts prepared from organic wastes as soil amendments has been recognized as a beneficial strategy to enhance soil fertility and crop production. However, the modification of manures with applied organic amendments such as biochar has not been well explained. Therefore, the preliminary study was designed to evaluate the impact of two doses of biochar (low 0.4 kg + 10 kg of manure and high 4 kg + 10 kg of manure) on the modification of resulting co-composted manure properties, and subsequently to evaluate the effect of matured manure amendment on the soil chemical and biological properties and plant yield in the pot experiment with barley (Hordeum vulgare L.). The following variants were tested: control, manure (M), manure + low biochar dose (M + LB), manure + high biochar dose (M + HB). Results revealed that, the M + HB significantly improved the co-composted manure properties as compared to control and M + LB, respectively. The most pronounced effects of M + HB treatment were observed on pH, NH4-N and humic acid to fulvic acid ratio (used as an index for manure maturity) relative to other treatments. Similarly, significant variations were observed between AOB (ammonium oxidizing bacteria) and nirs genes under M + HB which lowered the AOB and increased the nirs abundance as compared to other treatments. Moreover, when applied to soil, M + HB increased the observed soil chemical parameters with the exception of TN contents as compared to M and M + LB treatments. Similarly, plant biomass was significantly enhanced under the applied M + HB treatment. However, statistically insignificant differences were observed regarding soil enzyme activities and soil respiration values under the applied amendments. Thus, it was concluded that the co-composted manure with high biochar dose can have the potential to enhance the manure properties, soil fertilization value and plant biomass. However, its effects on soil microbiological and enzyme activities were intended be explored under long-term field experiments.