CARBON AND NITROGEN POOLS IN SOIL ORGANIC MATTER UNDER EUCALYPT, PASTURE AND SAVANNA VEGETATION IN BRAZIL

The substitution of the native Cerrado (Brazilian savanna) vegetation by planted eucalyptus forests can influence the C and N pools in the different soil organic matter (SOM) fractions. However, the intensity and direction of this change depend on the soil type, climate, and management techniques in the planted area. The objective of this study was to determine the effect of eucalypt plantations on the C and N pool of labile and stable SOM fractions in an area previously occupied with savanna-like vegetation in the Vale do Jequitinhonha, MG, Brazil. Soil samples were collected from Eucalyptus urophylla plantations and adjacent areas with natural savanna vegetation, and managed pasture. The eucalypt plantation was 20-year old, and had succeeded the savanna vegetation, which was cut, the firewood removed and the residues burned. The pasture had been planted about 15 years earlier using the same site preparation procedures. There were no differences in total organic carbon (TOC) among soil samples of the three vegetation types. In the humic acid (HA) and fulvic acid (FA) fractions, the C contents were about 16.6 and 17.5 % higher for the eucalypt soil in comparison with the soils from the pasture and, 17.5 and 36.9 % higher than in soils from the savanna, respectively. Replacing the savanna vegetation by eucalypt did not reduce the C and N pools of the soil microbial biomass of the soil, but increased the amounts of C and N in the light organic matter (LOM), contributing to an increase in SOM. The estimated contribution of C derived from eucalypt to SOM based on C-13 natural abundance was 5 %. This indicates a low substitution rate of native C by C derived from eucalypt, with an mean annual rate of 0.25 %. It can be concluded that substituting the native savanna vegetation by eucalypt did not reduce the C pools in the different SOM fractions.