In the developed economic and ecological con-ditions great interest causes studying of dynamics of easily mineralized organic substance (EMOS) in agrosoils. EMOS sources in the soils of agroecosystems are stocks of mortmass and dying-off roots of plants. It is the next reserve for decom-position due to which stocks of nutritious elements are formed and humus substances are updated. After harvesting in the top layer of the earth in corn agroecosystem there are about 13 t/hectare, in sunflower agroecosystem - 28 t/hectare of the vegetable remains consisting of roots and large mortmass (> 0.5 mm). The maximum stock of small mortmass in corn agroecosystem was in the se-cond half of July (1.75 t/hectare), in sunflower agroecosystem - in the middle of August (1.55 t/hectare). The general reserve of carbon of humus in the soil at cultivation of corn and sunflow-er made 67.2 t/hectare. In its structure the reserves of carbon of stable humus which in the agroecosystem of corn made 61 %, in sunflower agroecosystem - 72 % prevailed. Light mineralizable organic matter basically consists of set of water-soluble and alkali-hydrolyzable forms of carbon (mobile humus carbon), as well as car-bon of labile organic matter (small mortmass). Car-bon stock of labile organic matter in the maize agroecosystem was 4.9 t / hectare, in sunflower agroecosystem - 5.3 t / hectare, which was 7 % of Corg. The stocks of all organic matter of labile or-ganic substance were in agroecosystem of corn. The carbon stock of alkaline hydrolyzed was 22.8 t / hectare in maize agroecosystem of, newly formed fulvic acids prevailed in them, the ratio of Ch/Cfa was on average 0.8. The carbon stock of alkaline hydrolyzed amounted to 14.5 t / hectare, the ratio of newly formed humus and fulvic acids turned out to be 1.1 in sunflower agroecosystem. The stock of water-soluble forms of carbon in chernozyom during maize and sunflower cultivation was 1 % of Corg.