The primary objective of this paper was to estimate how the mass of Silver fir (Abies alba Mill.) and Scots pine (Pinus sylvestris L.) deadwood in two decay classes affected biochemical processes and the accumulation of soil organic matter, as well as the extent of this impact. We evaluated deadwood mass, as well as the biological activity and influence of the distance from deadwood on biological activity and carbon (C) storage. The investigation was carried out in Magurski National Park, southern Poland, in four randomly selected study plots. The organic C and nitrogen contents and soil pH and texture were analysed. The study also included the determination of enzyme activities. The results obtained demonstrated that soil C storage and the biological activity of the soil were influenced by deadwood form, tree species and decay class. Significant differences in the C content in the soil between the logs of fir and pine trees were noted. With the advancement of the deadwood decomposition process, the C concentration and enzyme activity increased slightly. Lower C accumulation occurred further away from the deadwood. At a 100-cm distance from the deadwood, the influence of the logs was not evident. Higher C storage was found in the soil around the stumps than in the decaying logs. This study confirmed an important role of deadwood in forest ecosystems, i.e., maintaining species biodiversity and increasing C accumulation in the soil environment with a simultaneous increase in the biological activity of the soil.