Soil organic carbon as affected by afforestation with Eucalyptus and Pinus in the Cerrado region of Brazil

In the Brazilian tropical savanna (Cerrado), soil organic carbon (SOC) is an important factor for crop productivity of the highly weathered, low-fertility soils, and can be easily depleted by soil management or disturbance, affecting sustainability of land use. In order to assess the impact of the first cycle of fast-growing exotic tree afforestation on SOC contents and quality, one site under Pinus caribaea var. hondurensis and two sites under Eucalyptus camaldulensis were studied. The soil under Pinus is a clayey Oxisol with no significant slope. Eucalyptus forests were planted along a toposequence of a sandy Entisol to a loamy Oxisol, which permitted to compare the afforestation effects on SOC contents and properties in two different soils under the same climatic conditions and submitted to the same management practices. Site preparation consisted of mechanical clearing of native vegetation followed by use of heavy disk plow. Litter and soil up to 60 cm were sampled, and the same soil under native Cerrado-type vegetation was sampled as control for each forest site. Analysis of total organic C, soil particle-size fractionation by physical procedures, cellulosic (CPS) and non-cellulosic (NCPS) carbohydrates, lignin phenolic derivatives and C-13 nuclear magnetic resonance (NMR) were run to characterize SOC. The results indicate different patterns of SOC dynamics in each site. Pinus forest accumulated a large amount of litter (37.7 Mg ha(-1)), while litter under Eucalypt was less (7.6 Mg ha(-1)) than under Cerrado control (10.9 Mg ha(-1)) in the loamy Oxisol. On the sandy Entisol there was no significant difference in litter quantities (13.9 and 13.5 Mg ha(-1) for Eucalyptus and Cerrado, respectively). Organic C in the surface layer (0-5 cm) was significantly lower under planted forests than under the controls. The SOC losses in topsoil were found to be more significant in the soils with the lower clay content, varying from 19% in the clayey Oxisol to 48% in the sandy Entisol, and in the coarser particle-size fractions, considered more labile. When the total carbon in the 0-60 cm profile is considered, the C losses were 11 Mg ha(-1) (-9%) under Pinus and 9 Mg ha-1 (-17%) in the sandy Entisol under Eucalyptus, and no net losses occurred in the Oxisol under Eucalyptus. Chemical properties of SOC were affected by afforestation, but not in the sandy Entisol; analyses of carbohydrates and lignin derivatives showed a more advanced decomposition status in the Oxisols, while C-13 NMR spectra of humic substances showed a reduction of O-alkyl, and an increase of paraffinic C in the Oxisols. This absence of chemical alterations in the organic carbon of the sandy Entisol, despite the high C loss in topsoil, may indicate a weak adsorption of compounds issued during decomposition, once the clay content is low. On the toposequence, NMR analyses showed the influence of soil type in humic substances, which contained more paraffinic groups and less aromaticity in the sandy soil. In general, although forest plantations in the tropics are expected to conserve SOC, significant losses occurred in the surface layer, which may be reduced through less intensive practices of soil preparation such as reduced tillage, especially in coarse-textured soils. (C) 2002 Elsevier Science B.V. All rights reserved.