Changes in soil microbial community structure after fertilisation with organic fertilisers and biological additives

Various forms of organic fertilisers are often used in agriculture. It is useful to have information on how microbial communities in arable land are affected by fertilisation with different types of fertilisers. One of the indicators of the stability and fertility of the soil ecosystem is the population structure and abundance of soil microorganisms. The aim of the study was to determine how fertilisation and biological additives affected the most active soil microorganisms of the studied functional groups in 2021 and 2022. It was found that organotrophic (113 ± 3 × 10 3 CFU g −1 ) and diazotrophic (161 ± 5 × 10 3 CFU g −1 ) bacteria were most abundant in the spring 2021. However, in 2022, the amount of organotrophs decreased, and their abundance was approximately the same throughout the growing season, probably due to a decrease in the amount of free organic matter in the soil. In 2022, the most abundant organotrophs (91 ± 2 × 10 3 CFU g −1 ) and diazotrophs (100 ± 1 × 10 3 CFU g −1 ) were also in the spring. Organic fertilisers and their different rates had a significant or very significant effect on the abundance of organotrophic, mineral nitrogen-assimilating, and non-symbiotic diazotrophic bacteria, except for diazotrophs in treatments with the biological additive Trichoderma spp. In 2021, nitrifiers (162 ± 5 × 10 3 CFU g −1 ) and fungi (2.29 ± 0.13 × 10 3 CFU g −1 ) were most abundant in the autumn. In 2022, nitrifiers were most abundant in the spring (128 ± 2 × 10 3 CFU g −1 ) but less abundant than in the autumn 2021. During the experimental period, the abundance of fungi and yeasts was highest in the autumn, and this trend was not affected by fertilisation. In the treatments with the biological additive Trichoderma spp., organic fertilisers significantly or very significantly affected the abundance of fungi and yeasts. However, no significant differences were found in the treatments with the biological additive Azotobacter spp.