Tillage system and crop rotation effects on soil organic matter fractions

Cultivation practices such as tillage and crop rotation can modify soil organic matter (MOS). A crop rotation-tillage study was conducted for seven years in order to evaluate *the effects on the different pools of C and N. The experiment was located in Turin, Portuguesa State, Venezuela, with two tillage systems: conventional tillage (CT), and no tillage (NT) and two crop rotations, corn-cotton (CC), and corn-bean (CB). Soil samples were collected at three depths and analyzed for total organic C (COT) and N (TON), microbial biomass C (MMC) and N (MMN), mineralized C and N (C.j. and N.J, and potentially mineralized labile N (N,) and its decay rate (k(l)). The changes observed in COT and NOT in response to tillage and rotation were related to the quantity and quality of plant residues returned to the soil, Put not to their distribution in the soil profile, with exception of MO mineralitation. This study shows the importance of the MMC, MMN, C and N mineralization as indexes of MOS dynamics in a tropical region. On the other hand, N, can be used as an indicator of the relative soil capacity to release N from MOS,