Biochar is not a uniform substance, especially when considering its effect on the hot-water extractable carbon (HWC) and nitrogen (HWN) contents, as well as on the content of available soil nitrogen. The high variability among biochars used as soil amendments has led to conflicting reports on the effects of biochar on different soil processes. The effect of biochar on the content and composition of HWC and HWN in soils has received little research attention. A laboratory experiment was conducted to study the effect of wood biochar with different crop residues on the change of HWC and HWN compounds and available nitrogen in sod-podzolic sandy loam soil. In the experiment, wood biochar from Karelian birch was used in combination with plant residues of clover and timothy. The experimental design included 6 treatments: (1) soil with biochar, (2) soil with clover plant residues, (3) soil with timothy plant residues, (4) soil with biochar and clover plant residues, (5) soil with biochar and timothy plant residues, in comparison with (6) control (without biochar or plant residues). Ammonium nitrogen, nitrates, and total nitrogen content in the soil were measured by conventional methods. The extraction of HWC and HWN was carried out by the Schulz-Kershens method. It was established that biochar application to the sod-podzolic sandy loam soil inhibited mineralization of organic soil compounds, which was accompanied by an increase in the content of HWC (by 10.9%) and HWN (by 13%) in the soil. The use of biochar stimulated the corn biomass formation, but did not significantly affect nitrogen intake of the plants. Biochar in combination with plant residues (especially clover) reduced the effect of the latter on the intensity of mineralization processes in the soil, nitrogen nutrition and the corn biomass formation.