The impact of exotic forest plantations on the chemical composition of soil organic matter in Southern Brazil as assessed by Py–GC/MS and lipid extracts study

This study evaluated the effects of Acacia and Eucalyptus plantations on the soil chemical properties and molecular composition of soil organic matter (SOM) in Cambisol profiles in Southern Brazil over 7 years. Control soil profiles were collected from neighboring areas that corresponded to the initial soil conditions predating the forest plantations. Soil exchangeable cations, total N, available P and K, soil pH, and the molecular composition of SOM were determined. The molecular composition of SOM was assessed by performing pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) analysis of the soil samples and GC/MS analysis of the Soxhlet-extracted free lipid composition. The Acacia plantation was associated with reductions of available P and K+, exchangeable Ca2 + and Mg2 + as well as soil pH down to depths of 40 cm, whereas only the available K+ content was reduced in the Eucalyptus plantation as compared with their respective control areas. The nitrogen (N) content was depleted in the superficial soil (5 cm) in both plantations, whereas no change was found in the soil carbon (C) content for the entire soil profile (0–80 cm). Alterations of the SOM composition occurred in both plantations at a molecular level. Analytical pyrolysis revealed a relative enrichment of polysaccharide-derived compounds in the soil under the Acacia plantation. Together with the increase in high molecular weight n-alkanes from lipid extracts, this result suggests the input of biomass from the new forest vegetation. Increase in the proportions of degraded compounds from lignin was observed in the SOM from Eucalyptus plantation, suggesting the degradation of the indigenous organic matter which promoted the change of the SOM composition.