Distribuição vertical de carbono orgânico em Latossolo sob diferentes usos

Terrestrial ecosystems play an important role in the global carbon cycle, and the soil is one of the largest C reservoirs in the planet. Thus, knowledge of the vertical distribution pattern in the soil profile of organic matter and its fractions, in different land use systems, contribute to increase the knowledge about its dynamics; the precision in accounting the organic carbon (OC) stored in the subsoil is also increased. Decisions on the best soil use and management system to be adopted may be sounder and easier when OC and its fractions are well known. This study aimed to quantify the levels and stocks of CO and its fractions, and total nitrogen (TN), stored in the profile (0-100 cm) of a Hapludox (according to Soil Taxonomy), in areas under different land use systems, in Lavras-Minas Gerais State, Brazil. In a homogeneous soil strip, four areas were selected according to the land use system, which were classified as forest (NF), eucalyptus (EUC), pine (PIN) and pasture (PAS). Samples were taken at the soil depths of 0-10, 10-20, 20-40, 40-60, 60-80, 80-100 cm, in order to assess levels of OC, and TN and the contents of labile carbon (LC) and water soluble carbon (WSC). Significative correlations were observed for the levels of OC, WSC, LC, C-light fraction (LF) and microbial biomass (MB). The impacts of the land use system on the stock of carbon (OC) and total nitrogen (TN) varied according to the soil depth evaluated, but to most soil layers, the highest OC stock were observed for the forest area. The results of this study confirm the hypothesis that, in an undisturbed systems, the carbon content is higher than in the cultivated system counterpart. Despite the decreasing levels of OC in the deeper soil layers, the C stored below 0-40 cm soil layer represents 37 to 53% of TOC in the whole soil profile (0-100 cm). It was also found that the land use system influences the labile organic carbon levels in both surface and subsoil layers. The LC levels are higher in the NF in all soil layers, except for the 10-20 cm, where there is more LC in the eucalyptus area. CL levels decrease as soil depth increases. The labile compartment was more sensitive to land use change than OC and thus has a good potential to assess the impact of different land use sistems on the organic matter dynamics and pools. In general, all organic C indicators showed negative impact of NF conversion to other land use and on the levels of organic matter and soil quality. The levels of WSC were influenced by the land use system, in the following order in the 0-10 cm layer: MTN> PAS> EUC> PIN. In the 0-40 cm soil depth, the land use systems changed significantly the WSC levels; however, below 40 cm, land use had no effect on WSC. The results also showed a decrease in WSC levels as soil depth increases. Positive correlations between WSC and OC, LC, MB and LF were observed, supporting the hypothesis that WSC is directly related to the OC levels and it is probably also part and residue of the soil reactions involved in the decomposition of labile carbon fractions.