1.1.1. Reduction of soil organic matter (SOM)
There are no systematic observations of the state of soil organic matter (POM) in the soils of Bulgaria, but some research data, summarized recently in a separate thematic collection, indicate that the total stock of organic carbon in them amounts to 1.3 Gt (Filcheva, 2014). There is a clear tendency to reduce the content of SOM in arable land. The reduction of the content of SOM in the arable layer of the arable lands, in comparison with the uncultivated soils varies between 10 and 40%. (Filcheva, 2005; Artinova, 2014). This is due to the intensive cultivation, the burning of the post-harvest residues, the limited use of organic fertilizers. The removal of the surface soil layer as a result of water erosion also contributes to the reduction of soil organic matter. According to estimates, the area of soils in Bulgaria with humus content in the surface layer <1% occupies 490,000 ha, and the area of soils with SOM content between 1 and 2.5% occupies an area of about 3.7 million ha. For comparison, the area of arable land in the country is less than 5 million ha.