To help solve the actual problem of global climate warming, it is important to comprehensively study soil organic carbon (SOC), soil fungi, and other parameters at different depths in the soil. This study was aimed at investigating the chemical and microbiological parameters and their interactions at various soil depths (0–5 to 195–200 cm) in an Arenosol in a Scots pine stand in southwestern Lithuania, with a focus on the main groups of fungi and their influence on SOC. The highest diversity of soil fungi species was found at a depth of 50–55 cm. Saprotrophs were dominant at all investigated soil depths. Ectomycorrhizal fungi were mostly abundant at depths of up to 50–55 cm. The C:N ratio gradually decreased down to 50–55 cm, then increased in deeper soil layers (from 50–55 to 195–200 cm). This means that the most active mineralization processes occur at depths of between 0 and 55 cm. Carbon stabilization processes occur at depths of 100–105 to 195–200 cm, and most of this carbon does not enter the atmosphere nor contribute to the process of climate change.

Due to intensified use of fertilizers and inconsiderable organic matter return, the intensive cropping system is evidently changing soil properties. Even though the changes are hardly predictable spontaneously, it could appear with imbalanced soil mineral nitrogen transformation and decreased biological nitrogen immobilization. To address this uncertainty, we investigated the linkage of soil nitrogen transformation and soil microbial community distribution with the mineral nitrogen fertilization in long-term intensive cropping system during 2019-2022. In this study a three-factor (Factor A: rate of nitrogen (100, 150, 180 and 230 kg N ha⁻¹); Factor B: organic fertilizers (0 and 300 kg ha⁻¹); Factor C: liquid biological activator (0 and 0.1 L ha⁻¹)) experiment carried out on a loam soil (Calcaric Luvisol) in intensive cropping system (in rotation: winter wheat, winter wheat, winter rape and winter wheat). At the study site, soil organic carbon was significantly higher at higher rates of nitrogen application combined jointly with application of organic matter and biological activator. Although the rate of nitrogen fertilization was increasing, either in combination with organic matter or biological activator, induced no significant changes in the accumulation of total nitrogen. Thus, with higher rates of nitrogen fertilization, the content of biologically transformed nitrogen significantly increased. As nitrogen is released from organic matter, it was evident that organic matter inputs affected the biological nitrogen transformation. Organic matter inputs also affected the increase soil fungal community, however, with higher nitrogen inputs soil fungal and bacteria ratio was decreasing. This study highlights the significance of sustainably maintaining of nitrogen and organic matter inputs in intensive cropping systems.

Intensive cropping systems often disbalance soil nitrogen (N) cycling due to high mineral fertilization and low organic matter return. We investigated the linkage between soil N transformation and microorganism responses to mineral N fertilization in long-term intensive cropping systems during 2019–2022. A three-factor experiment was established on loam soil, including varying mineral N rates (100, 150, 180, 230 kg N ha⁻¹), organic fertilizer (0 or 300 kg ha⁻¹), and a biological activator (0 or 0.1 L ha⁻¹). Higher mineral N rates combined with organic fertilizer and biological activator significantly increased soil organic carbon and enhanced biologically transformed N (NH₄⁺ + NO₃⁻), with the strongest effects observed at N180. Despite increased mineral N inputs, total soil N did not rise, indicating limited long-term N retention. Increasing mineral N inputs reduced total microbial abundance and shifted the community toward bacterial dominance, lowering soil fungi and bacteria ratio and decreasing biological N immobilization. Organic fertilizer inputs counteracted these effects by increasing fungal abundance and improving biological N transformation efficiency. Overall, the study demonstrates that maintaining balanced mineral N inputs together with organic and biological amendments is essential for stable soil N transformation in intensive cropping systems. For sustainable N management, mineral N should not exceed 150 kg N ha⁻¹ or 180 kg N ha⁻¹, when supplemented with organic fertilizer to avoid soil microbial functioning disturbance and to improve N-use efficiency.