Eucalypt plantation areas in Brazil have increased continuously over the years. The practice of removing more lignified material, such as bark and branches, from the ground after wood harvesting in eucalypt forests may prevent the stabilization of carbon (C) in recalcitrant fractions of soil organic matter (SOM). The recalcitrance of such organic fractions is due to the presence of chemical compounds which are toxic to soil microorganisms, thus delaying the decomposition of stable SOM. It is well known that the residues of eucalypt bark and branches is effective in maintaining a protective mulch against soil erosion and the compaction of soils by harvesting machines. Little is known about the effectiveness of this more resistant material in increasing SOM levels, as well as about the factors which may influence its stabilization. The chemical quality of the available substrates markedly influences SOM decomposition and, possibly, its stabilization, and decomposition rates are a consequence of the type and relative concentration of some of the chemical compounds in residues. Nitrogen (N) status in soil may also influence C stabilization in SOM fractions. This work aimed at determining C levels in different eucalypt plant parts and evaluating the effect of higher N availability in C conversion efficiency from those plat parts to SOM labile and stable fractions based on the natural abundance of 13C. The experiment was set up in an incubation room (25°C) with soil collected in na area under pasture (Brachiaria brizantha) from the town of Paula Cândido, Minas Gerais State, and harvest residues of the eucalypt hybrid Eucalyptus grandis x Eucalyptus urophylla from Aracruz, Espírito Santo State. The experiment was designed as randomized blocks, in a 6 x 2 x 6 factorial, which consisted of five residue treatments (no residues or N, leaves, branches, bark, roots, and a mixture of residues), two N treatments (no N, and the application of 50mg dm-3 N), and six incubation periods (0, 15, 30, 60, 120, and 240 days). The quantity of residues applied to soil was equivalent to the application of 20g kg-1 of C to soil. The N source was dissolved in deionized water so to increase soil water content to 80% HE. The application of eucalypt residues to soil favored a fast increase in soil microbial biomass (SMB) C, and nitrogen fertilization favored fast microbial growth, especially when leaves residues are added. Residue application, especially bark, branches and the mixture of residues, alters the allocation pattern of C derived from the residues on soil, favoring its stabilization in the humin fraction, apparently reallocated from humic acids. Nitrogen application did not favor the increase of C levels in soil humic substances but favored the cycling of C from the light fraction when labile residues (leaves) were applied and decreased its decomposition when more recalcitrant residues (bark, branches, and roots) were applied. Nitrogen application helps to stabilize C from eucalypt residues, especially in treatments with more recalcitrant residues. Eucalypt bark residue application was more effective in stabilizing C in SOM heavy fraction. Most of the residue derived C, mainly from bark and branches, was stabilized in soil humin fraction.
