ИСТИНА

Most of the research done in the recent years focus on the negative effect of tillage and agriculture in general on organic carbon sequestration in soils (Post & Mann, 1990; Lorenz & Lal, 2012). However, in the northern countries tillage, liming and the use of organic fertilizers lead to significant accumulation of humus in mineral soils (Jandl et al., 2011). The content of organic carbon in soils is strongly linked to the land use type (Hamelin et al., 2012). The organic soils in the northern ecosystems commonly suffer a severe loss of organic carbon after drainage and tillage, but its reforestation is believed to lead to secondary peatland formation. In this work we studied some mineral and organic soils in Southern Karelia under different land uses and at different stages of natural reforestation in order to develop a general regional scheme for soil development under different agricultural use and after abandonment. Mineral soils commonly lose a significant part of organic matter after deforestation; the major part of this lost organic matter is forest litter. Soil amendment with organic and mineral fertilizers and liming lead to the increase in total soil organic matter. When tillage is applied, the organic matter quickly mineralizes, and additional manure application is needed. Under perennial grasses and non-tillage conservation agriculture the organic matter content increases up to 4% of organic C in the topsoil. Total C, N, and P reserves are 2-3 times higher in these soils than in soils under natural forest vegetation. After reforestation the total reserve of biogenic elements increases in soils for several decades (up to 50 years in some sites), and then slowly decreases, because soil acidification favors organic carbon leaching, and organic matter mineralization rate is higher than the rate of humification. The rate of the loss of the reserves of organic carbon by peat soils after drainage and tillage is high, especially in the first few years. In places more than 1-2 m of peat can be mineralized within less than 50 years (annual loss of 2-5 cm). However, deep ploughing may lead to the formation of sustainable organic-mineral anthropogenic soils. After reforestation and flooding these soils either form secondary peatlands or transform to humus-enriched soils with a developed turf layer. The reserves of C increase in both cases.