M. Nankova1*, P. Yankov2

1Dobrudzha Agricultural Institute, 9520 General Toshevo, Bulgaria
2Department of Plant Production, Faculty of Marine Sciences and Ecology, Technical University, 1 Studentska, 9010 Varna, Bulgaria

Abstract. The effect of the different soil tillage systems on the mineralization ability of the slightly leached chernozem soil (Haplic Chernozems) has been investigated in a stationary field trial carried out at Dobrudzha Agricultural Institute, General Toshevo since 1987. The trial was designed by the non-standard method, with size of the trial plots 72 m2. The investigation was performed under the following annually applied soil tillage systems: plowing, disking, cutting and nil tillage, as well as under three alternated soil tillage systems plowing-disking, plowing-nil and disking-nil. The potential nitrogen-supplying ability of soil (NO3- N mg/1000 g soil) was estimated in soil samples taken from depth 0 –10, 10 – 20 and 20 – 30 cm after harvesting of wheat. Soil was filtered through a series of sieves with apertures 10, 5, 3, 1 and 0.25 mm, respectively. Thus the following soil fractions were obtained: >10 mm, 5 –10 mm, 3 – 5 mm, 1 – 3 mm, 0.25 –1 mm and <0.25 mm. The analyses were made in air-dry soil. Composting was done in a thermostat under controlled temperature conditions 30oС and 60% of marginal soil moisture absorbtion of soil in dynamics after 14, 28 and 56 days of incubation. The potential nitrogen-supplying ability of soil was strongly influenced by the type of the tillage system. Maximum amounts of nitrified nitrogen under all soil tillage systems were determined after 56-day incubation. The rate of pure mineralization was highest between the 14th and the 28th day. Under optimal conditions for occurrence of nitrification, the systems which involved plowing were able after 14-day incubation to mineralize greater amounts of nitrogen in comparison to the reduced types of soil tillage and the tillage without turning of the soil layer. The potential of nitrogen mineralization under constant disking, direct sowing and tillage without turning of the soil layer at long-term favorable conditions for the occurrence of nitrification (28 and 56 days) was higher than the potential under the systems involving plowing. In the surface layers, the small-sized structural soil units (0.25-1 mm and <0.25 mm) contributed significantly to the higher mineralization ability of soil. At depth 20–30 cm the tendency remained the same under the systems involving plowing. The correlation of the mineralization ability with ammonium nitrogen content was better expressed than the correlation with nitrate nitrogen content. Mineralized nitrogen showed high and permanent correlations till the 28th and the 56th day from the incubation period.