Data of two-year observation over seasonal dynamics of the content and reserves of the total and labile carbon of humus substances of an arable variant of podzolic soils of a southern taiga are presented. It is revealed that arable soils contain insignificant amount of humus carbon; its content and reserves essentially vary not only on depth of a soil profile but also during a growth season. The content and reserves of humus organic carbon essentially vary in a profile of arable soils during all term of observation within 0.58-1.11% (of soil weight) and 31.49-45.58 t/ha accordingly. In transitive eluvial horizon AB the content and reserves of the total carbon is much less (from 0.17 to 0.66%). The horizon B contains insignificant amount of carbon - from 0.15 to 0.30%. The maximum content of labile carbon is characteristic for arable horizon (from 0.010 to 0.222%). In horizon AB the content fluctuates from 0.016 to 0.160%. Character of dynamics of labile carbon in horizon AB is close to that in arable horizon. The content maximum is marked in the beginning of a season, a minimum - in the end. In horizon B it is noted sharp fluctuations of labile carbon content. As a whole the carbon content in this horizon is minimal (from 0.030 to 0.094%) and varies rather smoothly during a season. The top layers of soil are characterized by rather constant reserves of carbon with small amplitudes of fluctuation during a season. The thicker is considered layer, the higher amplitude of fluctuation of the content of carbon during a season. The general tendency of change of reserves of labile compounds of carbon in both years of research is decrease in reserves from the beginning of the growth period up to the end with different in amplitude fluctuations of the content. In horizon of a forest ground litter (0-5 cm) of forest biogeocenosis reserves stocks of organic matter have made 42.94-50.25 t/ha on the average for a season. During the various periods of a growth season significant wavy fluctuations of carbon reserves are observed. In the beginning of a growth season, carbon reserves of a ground litter made 38.81 t/ha. To end of a season reserves have increased by 20.1% and have made 45.76-48.68 t/ha. The maximum value of reserves of organic matter in a ground litter is noted in the second 10-day period of June (55.64 t/ha). The minimum has been noted in the end of May and has made 26.47 t/ha. The system of organic matter in a forest ground litter is stable and possesses dynamic balance. In the beginning of the growth period the minimum reserves of carbon in horizon Oi of forest soil - 6.52 t/ha are noted. The maximum value of reserves of organic carbon in the given horizon (21.86-25.15 t/ha) has been noted in the second 10-day period of June. In the end of the period of observation reserves of organic carbon have made 12.57-15.35 t/ha that significantly exceeds values in the season beginning. Horizon Ahe differs by the greatest amplitude of fluctuation of carbon reserves. In the beginning of a season it was equal to 13.15-23.03 t/ha, in the end - 18.24-29.41 t/ha. The maximum value of reserves is noted in the middle of September, it has made 52.21 t/ha whereas the minimum reserve is noted in the third 10-day period of May and has made 16.64 t/ha. In the heaviest horizon B the greatest reserves of organic carbon of humus substances are contained. In the beginning of a season of researches they made 36.27-43.71 t/ha, in the end - 31.62-33.48 t/ha. Reserves of the total carbon in this horizon in the end of a season were significantly reduced by 23.4%. The minimum values of reserves of organic carbon of humus are characteristic as a rule to beginning of a growth season, maximum - for second half of growth season. Despite considerable fluctuations of carbon reserves during the growth periods, in the beginning and the end of seasons of research its values did not differ statistically that can be a sign of stability of all system of soil organic substance. Stability of system remains in annual dynamics too.