Eucalypt plantation effects on organic carbon and aggregation of three different-textured soils in Brazil

Short-cycle (similar to 7 years) tropical eucalypt plantations can sequester much carbon (C) in biomass, but their impact on soil organic C (SOC) varies considerably according to soil properties. Three Oxisols (sandy, loamy, and clayey) under 14-year-old eucalypt (Eucalytus camaldulensis Dehnh.) coppice stands and native cerrado vegetation were sampled to a depth of 1m, in order to assess whether soil texture controls SOC response to afforestation in central Brazil. Both SOC concentration and stocks were proportional to contents of clay+silt to 1m depth, with no effect of eucalypt plantation. However, there was a 40% decrease in water-stable aggregates of 2-8mm size in all soils under eucalypt stands in the 0-0.05m depth, which probably helped to release occluded particulate organic matter (POM) into free POM. In the 0-0.05m depth, SOC partition throughout particle-size fractions (sand, silt, and clay) was unaltered in the loamy and clayey Oxisols, whereas in the sandy Oxisol, clay-bound SOC strongly decreased simultaneously with a strong increase in free POM. As free POM is often more susceptible to decay than clay-bound SOC, this change suggests a higher susceptibility to SOC change in sandy soils. Soil respiration in the 0-0.05m depth was more strongly correlated with the mean weight diameter and clay+silt contents than with SOC and nitrogen concentrations, with no effect of eucalypt plantation. In conclusion, soil texture affects total SOC retention and soil respiration, with little or no effect of eucalypt plantation. However, decreased aggregation and lower POM occlusion occurred in all soils planted with eucalyptus, suggesting that SOC levels in eucalypt plantations can be better conserved with less intensive soil preparation practices, especially in coarse-textured soils.