Organic matter in a soil under conventional tillage and no tillage: comparative analysis of distribution profile and physical fractioning.

The abundance of diverse organic matter fractions differing in its evolution degree may be assesed by granulometric fractions studies. In the current work the method of organic matter fractioning by shakening and later wet sieving was employed with soil samples proceeding from wheat plots farmed under two different systems (conventional tillage and no till) at two different depths (0-5 and 10-15 cm.). The proportion of aggregates corresponding to large fraction (2000-100 µm) oscilated between 38,45 % and 27,38 %. The highest values were obtained under no till at 10-15 cm layer and indicate the best structural stability conditions. The lowest records were obtained in conventional tillage at 0-5 cm. Highest whole organic carbon percentages were recorded in no till at the surface (13,62 Mg.ha-1) while lowest values were obtained under the same farming system but in deep samples (8,00 Mg.ha-1). Differences were due to preferencial accumulation of the least evolved and most labile fraction (young organic carbon). Under our assay conditions we may state that the evaluated tillage systems does not determine differences in the whole organic matter budget but different shapes in the accumulation profiles.

Key words: Organic matter, Physical fractioning, Conventional tillage, No-till.