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.

Bromopsis inermis and Phalaroides arundinacea are increasingly grown not only for feed but also for other purposes, such as obtaining energy. Composting sewage sludge and using such compost to fertilize various plants is also becoming more popular. Therefore, the aim of our study was to investigate how the fertilization of the mentioned plants with sewage sludge compost (SSC) affects the biomass yield and biomass quality of these plants. It was also important to determine whether fertilization with SSC is possible in low-yielding soils of light texture without harming these soils There are no similar studies in Lithuania, and there are very few in Europe and the world. It was found that investigated grasses formed a small biomass DM yield (6.6 t/ha within three years). The biomass of B. inermis and Ph. arundinacea had a very high ash content and concentrations of nitrogen and potassium. This greatly reduced the quality of the biomass of these grasses as a solid biofuel. SSC performed poorly as a fertilizer. Fertilization with 125 t/ha SSC significantly increased the total biomass yield of Ph. arundinacea only. At the same time, using SSC as a fertilizer significantly increased the concentration of Cu, Zn, and Cd in the soil.