This study was carried out to evaluate the effect of cultivation systems on soil carbon and nitrogen contents and on the distribution of light and heavy fractions of soil organic matter (SOM). Density and particle-size fractionations were used to isolate free light fraction (FLF), intra-aggregate light fraction (ILF) and heavy fractions (HF) of a Red-Yellow Latosol from a field experiment at Embrapa Rice and Beans in Goiás, Brazil. Treatments consisted of a combination of soil tillage and crop rotation. Soil tillage systems utilized were no-tillage (NT) and conventional tillage (mouldboard ploughing followed by light disc harrowings - CT), under two crop rotations: fallow/rice - fallow/soybean and sunhemp/rice - millet/soybean. As a reference, soil samples were collected from a non-cultivated area nearby the field experiment. All samples were analyzed at Embrapa Soils between January and August 2000. Compared to the forest site, soil cultivation led to a decrease of 50 % in the C and N contents, under both tillage systems. Decreased C and N storages in the cultivated soils were less for ploughed soils than soils under no-tillage. The largest proportion (60-90 %) of C and N was associated with the finest soil particles, with ploughing increasing this tendency. The C content of light fractions decreased due to the replacement of the Cerrado vegetation by cropping systems. Light fraction of the soil organic matter was a sensitive indicator of changes in SOM content caused by cropping systems.
soil organic matter; physical fractionation; tillage; crop rotation; forest soil