Soil microorganisms are one of the main indicators used for assessing the stability of the soil ecosystem, the metabolism in the soil, and its fertility. The most important are the active soil microorganisms and the influence of the fertilizer applied to the soil on the abundance of these microorganisms. We aimed to investigate how the applied organic fertilizers affect the most active soil microorganisms, which determine the soil fertility and stability. Fungi, yeast-like fungi abundance, and abundance of three physiological groups of bacteria were analyzed: non-symbiotic diazotrophic, organotrophic, and mineral nitrogen assimilating. This study is valuable because relatively few similar studies have been performed on infertile Lithuanian soils. The first results of a long-term study were obtained. The results show the effect of fertilizers on trends in the changes of microorganism community diversity; however, more analysis is needed to assess the impact of organic fertilizers on the most active soil microorganisms. Therefore, the investigation was continued. The results of the 2020 quantitative analysis of culturable soil microorganisms show that the highest abundance of organotrophic and non-symbiotic diazotrophic bacteria were recorded during the summer season. Meanwhile, the abundance of bacteria assimilating mineral nitrogen and fungi was higher in autumn. Agrochemical parameters were determined at the beginning of the experiment. The highest concentration of N_min in the soil was determined after fertilizing the plants with the combination of granulated poultry manure (N₁₇₀) + biological substance Azotobacter spp. The yield of barley was calculated. It was found that the highest yield of spring barley in 2020 was obtained by fertilizing the experimental field with organic in combination with mineral fertilizers.

A fertilisation experiment, with the aim to determine the effects of different potassium fertiliser rates and their interactions with nitrogen and phosphorus on field-rotation productivity, potassium balance, fertiliser utilization, and changes in the content of potassium in soil, was carried out in Lithuania between 1971 and 2020. The multi-factorial scheme with 45 treatment plots, where seven rates (including zero) of nitrogen, phosphorus, and potassium fertilisers were studied. The experimental treatments during the study period were carried out on winter wheat, spring wheat, spring barley, sugar beet, spring rapeseed, and annual and perennial grasses. It was found that potassium fertilisers were the most effective on agricultural crops when used in combination with other major plant nutrients—i.e., nitrogen and phosphorus. The required balance of potassium (K₂O) in the soil was measured, when nitrogen and phosphorus fertilisers were applied together to compensate for potassium removal; when applying low nitrogen (N) (72 kg ha⁻¹) and phosphorus (P₂O₅) (64 kg ha⁻¹) fertiliser rates, 128 kg ha⁻¹ of potassium fertilisers are required. When using high nitrogen (180 kg ha⁻¹) and phosphorus (160 kg ha⁻¹) fertiliser rates, 160 kg ha⁻¹ of potassium is needed. The highest potassium uptake, reaching 51.6%, was achieved when plants had been fertilised with nitrogen (108 kg ha⁻¹), phosphorus (96 kg ha⁻¹), and potassium (96 kg ha⁻¹). When fertilising with potassium fertilisers alone, the content of plant-available K₂O content in the soil increased, whereas with fertilisation with nitrogen and phosphorus combined K₂O content is decreased, except in the plots where the plants had been fertilised with potassium fertiliser at rates of 128 kg ha⁻¹ and higher. Due to the influence of fertilisers, the amount of non-exchangeable potassium in the soil also increased, but relatively little compared to the amount of available potassium content. Thus, one of the main conditions for the effective use of potassium fertilisers is ensuring optimal plant nutrition with other nutrition elements, especially nitrogen and phosphorus.

In this study, we aimed to compare the functional and taxonomic composition of soil microbial communities in different ecosystems, agricultural, natural grasslands, and old-growth forests, in the context of different environmental conditions. In this research, cultivable microbial quantification was performed by conventional plate-count techniques using different selective media. The taxonomic structure of microbe communities was evaluated using NGS metagenomic sequencing on the Illumina platform NovaSeq. The taxonomic analysis showed that individual land uses are characterized by the specific structure of communities; some taxonomic groups are specific only to agricultural, grassland, or forest ecosystems. After determining the abundance of functional groups of culturable microorganisms by the conventional plate-count method, statistically significant quantitative differences in physiological groups between the individual ecosystems were revealed. The metagenomic analysis revealed that different ecosystems are characterized by specific taxonomic groups of microorganisms and that general alpha diversity varies among individual land-use samples. Since the most unstable soil systems are agricultural, they are likely to suffer the most and will suffer more in the future from climate change than natural ones.

The application of organic fertilizers and biological inputs to soil inevitably affects its quality, agrochemical indicators, and microbiota. Sustainable agriculture is based on continuously learning about how to properly manage available soil, water, and biological resources. The aim of the study was to determine changes in microorganism communities and carbon stocks in infertile soils for fertilization using different organic fertilizers and their combinations with bio-inputs. Genetic analysis of microorganism populations was performed using the NGS approach. Our study showed that the application of organic fertilizers affects the soil microbiota and the taxonomic structure of its communities. Specific groups of bacteria, such as Bacillota, were promoted by organic fertilization, meanwhile the abundance of Pseudomonadota and Ascomycota decreased in most treatments after the application of poultry manure. Metagenomic analysis confirmed that the use of bio-inputs increased the relative abundance of Trichoderma spp. fungi; meanwhile, a significant change was not found in the representatives of Azotobacter compared to the treatments where the bio-inputs were not used. The positive influence of fertilization appeared on all the studied agrochemical indicators. Higher concentrations of C_org and N_min accumulated in the soil when we used granulated poultry manure, and pH_KCl when we used cattle manure.

A fertilisation experiment with aim to determine the effect of different potassium fertiliser rates and their interaction with nitrogen and phosphorus on field rotation productivity, potassium balance, fertiliser utilization and changes in content of potassium in soil was carried out in Lithuania between 1971 and 2020. The multi-factorial scheme with 45-treatment plots, where seven (including zero) rates of nitrogen, phosphorus and potassium fertilisers were studied. The experimental treatments during the study period included winter wheat, spring wheat, spring barley, sugar beet, spring rapeseed, annual and perennial grasses. It was found that potassium fertilisers was the most effective on agricultural crops when used in combination with other major plant nutrients – nitrogen and phosphorus. The balance of potassium (K2O) in soil, when nitrogen and phosphorus fertilisers were applied together, to compensate for potassium removal, when applying low nitrogen (N) 72 kg ha-1 and phosphorus (P2O5) 64 kg ha-1 fertiliser rates, 128 kg ha-1 of potassium fertilisers is required. When using high nitrogen 180 kg ha-1 and phosphorus 160 kg ha-1 fertiliser rates, 160 kg ha-1 of potassium is needed. The highest potassium uptake, reaching 51.6%, was achieved when plants had been fertilised with nitrogen 108 kg ha-1, phosphorus 96 kg ha-1, and potassium 96 kg ha-1. When fertilising with potassium fertilisers alone, the content of plant available K2O content in the soil increased, while fertilisation with nitrogen and phosphorus combined it’s decreased, except in the plots where the plants had been fertilised with potassium fertiliser at rates of 128 kg ha-1 and higher. Due to the influence of fertilizers, the amount of non-exchangeable potassium in the soil also increased, but relatively less compared to the amount of available potassium content.