Changes in Bulgarian society have strongly affected development of agriculture for the last decade. Mostly because of changes of land ownership fertilizers application has decreased dramatically for this period. Private farms and cooperatives of landowners replaced the cooperative centralized agriculture. The decapitalization of Bulgarian agriculture has been recognized as the major factor for drop in fertilizer use. Other factors are the increase of the number of landowners and the very low level of their agricultural education.

The low nutrients input in Bulgarian agriculture have resulted in a decrease of crop production amount. The total organic carbon stocks of Bulgarian soils (affecting soil fertility) has been estimated at 1.5 Gt. Changes in humus system stability and soil fertility depend on soil genetic features and land management. Soils play an important role in the earth’s carbon cycle because they contain most of the earth’s carbon pool. This is important not only to the global carbon balance, but also to the present and future potential of the soil to produce sufficient food and fiber to feed and clothe the world and to meet the demand for wood for fuel, building, and other domestic uses. Human conversion of natural ecosystems to agricultural use has a strong effect on the fate of this stored carbon. Agricultural soils show large changes in carbon concentrations, carbon stock, and associated properties as bulk density and soil structure (Schleisinger, 1986). Kobak (1988) estimated the global reservoir of terrestrial stored carbon at 5075 Pg yr-1, while Greenland (1995) reported estimates of soil carbon from to 700 to 3000 109 t. Globally the upper meter of mineral soils contains 1300-1600 Gt carbon (Eswaran et al., 1993; Neill Ch. et al., 1997), twice the carbon stored in terrestrial biomass (Deevy, 1979; Post et all., 1982; Schleisinger, 1986).

Investigations of Koutev et al. (1996, 1998, 2001) has shown that combined application of mineral and organic fertilizers is optimal for crop nutrition and soil fertility improvement. This way the nutrients and carbon reserve status of soils can be improved. Problems arise when some farmers do not use their own stock of farmyard manure, so it becomes a source of pollution instead of fertilizer. Reutilization of crop residues can be an important source of nutrients for agricultural soils. Instead, they are burned and the nutrients they contain are lost for the agriculture. Study of Koutev and Dinev (2003) shows that the soil stocks of available macronutrients are very poor and the optimal fertilization is the only way to improve crop yields and amount of production.

The ecological aspects of nutrient balance are associated with risk for enrichment of groundwater and surface water with nutrients and particularly with nitrate, the quality of plant production and the soil chemical degradation. These impacts depend on soil and aquifer vulnerability and land use practices.

References

Boyadgiev, T. 1994. Soil map of Bulgaria according to the Soil Taxonomy - explanatory notes. Soil Science, Agrochemistry and Ecology, v.29 (No 4-6): 43-51.

Boyadgiev, T., E. Filcheva, and L. Petrova. 1994. The organic carbon reserve of Bulgarian soils In: Soil Processes and Green House Effect.(Lal R., J. Kimble and E. Levine eds.). Advance in Soil Science. V.III, 19-23.

Deevy, E. S. 1979. Mineral Cycles. Sci. Am. 223, 148-158.

Eswaran, H., E. Van Denberg, and P. Reich. 1993. Organic carbon in soils of the World. Soil Sci. Soc. Am. J. vol. 57, 192-194.

FAO-UNESCO. 1990. Soil Map of the World. Revised Legend. FAO, Rome.

Filcheva E., S. Rousseva. 2004. Organic carbon stocks in Bulgarian soils groupped accrding to the revised legend of the FAO-Unesco soil map of the world. Modern physical and physicochemical methods and their applications in agroecological research, (Andrzej Bieganowski, Grzegorz Józefaciuk, Ryszard T. Walczak, Editors). Centre of Exellence for Applied Physics in Sustainable Agriculture AGROPHYSICS, N. Poushkarov Institute of Soil Science, Institute of Agrophysics, PAS, Lublin-Sofia, 36-42.

Filcheva E., Sv. Rousseva, D. Kulikov, S. Nedyalkov, Tz. Chernogorova. 2002. Organic carbon stocks in soils of Bulgaria. In: Agricultural Practices and Policies For Carbon Sequestration In Soil, (J. Kimble, R. Lal, R. Follet, eds.) Lewis Publishers, CRC Press LLC, chapter 45, 471-476.

Gitsova, S., Matev, B., and Stoichev, D., 1991. On some aspects of water quality management and water pollution problems in Bulgaria. Report in Water Quality Workshop, Poznan, Poland, 1-6.

Koleva V., Stoicheva D. 2005. Leaching of elements with the drainage flow in different vegetable crops growing. In Dilkova et al. (eds) Proceedings of National Conference with international participation “Management, use and protection of soil resources” 15-19 May 2005, 276-280.

Koutev V. and E. Ikonomova - Nitrogen Regime of Two Bulgarian Soils after 30 Years of Mineral and Organic-Mineral fertilisation (15N Study). 1998. 8th Conference of the FAO Network on Recycling of Agricultural, Municipal and Industrial Residues in Agriculture, RAMIRAN’98. Rennes, France, 26-29 May 1998.

Koutev V., P. Petkov and L. Raikova. 1996. Investigations on the Nitrogen Regime of Virgin Soils. Soil Science, Agrochemistry,and Ecology, No31, vol.3, p. 94 - 95 (Bulgarian).

Koutev V., R. Georgieva-Balabanova, E. Ikonomova. 2001. Nitrogen and carbon regime of two Bulgarian soils after 35 years of mineral and organic-mineral fertilisation. 11th Nitrogen Workshop. 9-12 September Reims, 2001, France pp. 311-312

Koutev V., N. Dinev. 2003. Spatial variability of inorganic nitrogen and available forms of phosphorus and potassium in heavy metals polluted area. 10th Lysimeter Conference, BAL Gumpenstein, Austria, 29 and 30 April 200


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