Rio das Mortes basin is an important area of coffee and potato production in Minas Gerais state (Brazil). In these crops, it is frequent the application of lime and phosphorus, as well as the use of practices of intensive revolving of soil, especially in potato croplands. Although these management practices are important for agricultural process, they affect soil organic matter (SOM) dynamics. In tropical regions, SOM alterations can influence productivity capacity of soils. For this reason, alternative management systems have been proposed in the region of Rio das Mortes. However, the effect of these management practices on SOM properties is still unclear. Many pools of SOM can be used to evaluate the effect of soil management on soil organic C. Total organic carbon (TOC) and humic and fulvic acids fractions are used as long-term indicators, while the light fraction of C is used as a short-term indicator of the SOM dynamics. In order to understand these processes in soils of Rio das Mortes Basin, a field study was carried out, in potato and coffee croplands, using as reference soils samples from undisturbed areas. Soils from areas under alternative management (rotation along two years with corn cropping, under no-till) for potato areas, and an organic management (only organic/green fertilization and no-use of pesticides) for coffee plantations were also sampled. Organic and conventional management systems decreased the TOC of the 0-20 cm soil layer, at coffee areas, in about 10% and 20%, respectively. For potato areas, alternative and conventional management systems decreased TOC in about 17% and 31%, respectively. Alternative management for potato cropping and organic practices for coffee cultivation improved the SOM by all the evaluated indicators. Areas under organic coffee plantation had the same HA-C/FA-C ratio, in comparison with the native areas, in all soil layers. For potato croplands, LF-C was not a good index to detect differences in the soil management systems. Differences were only observed between native and cropping areas. In potato areas, alternative management improved the soil quality, increasing the HA-C/FA-C. Either with or without liming, phosphate influences microbial activity, adsorption/desorption capacity of soil colloids, SOM mobilization, and the contents of C in soil solution – water soluble C (WSC). In order to study the effects of liming and P on WSC, a lisimeter study was done, using two Latosols (Oxisol) with medium and low iron oxide contents, sampled in the Rio das Mortes Basin. Leaching of WSC was mostly influenced by phosphorus, alone or with liming, especially in the low iron Latosol. In this soil, application of phosphate with liming accounted for 15 times more C in the leachates, as compared to the control. Besides, liming+P increased the molar mass of organic compounds in the leachates of both soils. In percolates collected at end of the experiment, it was observed the lost of molecules with similar to fulvic acids. Medium iron Latosol lost more C in the form of low molecular mass organic acids.
