Some researchers suggest the use of different SOM organic matter (SOM) fractions or compartiments like indicators of the agricultural systems sustainability. In order to evaluate the amount, composition and distribution in the profile of different SOM fractions from the soil on conservation agriculture systems, evaluations in an Ustoxic Quartzipsament soil located at Venezuelan savannahs, were carried out. Three cover crop treatments were evaluated, as improved land fallows for the establishing of maize no-tillage system and grazing with ovine cattle: Brachiaria dictyoneura (BD), Centrosema macrocarpum (CM), and spontaneous vegetation (SV), and its comparison with the natural savannah ecosystem (NS). Samples to three depths (0-5, 5-15 and 15-30 cm), and were taken at three times: initial, 286 days after the establishment (dae), and 1463 dae. At all depths and to 1463 dae were evaluated: total organic carbon (TOC), hidrosoluble carbon (HSC), and particulate organic matter carbon (POMC). At all times and in the the 0 to 5 cm layer physical and chemical fractions of the SOM, were evaluated. The results show statistical differences between cover crops types in some SOM fractions, mainly in the light fractions (HSC, and fulvic acid carbon), with the greater values in BD and the minors in SV. The cover crops introduction produced the diminution in some fractions; but after three continuous cycles under the proposed management, these fractions reached similar values or superior to those initials. The humification indexes, and the relations between the different fractions and the TOC, were the most sensible indicators.
