Changes in soil organic matter composition are associated with forest encroachment into grassland with long-term fire history

This study investigates if Araucaria forest (C₃ metabolism) expansion on frequently burnt grassland (C₄ metabolism) in the southern Brazilian highland is linked to the chemical composition of soil organic matter (SOM) in non-allophanic Andosols. We used the ¹³C/¹²C isotopic signature to group heavy organo-mineral fractions according to source vegetation and ¹³C NMR spectroscopy, lignin analyses (CuO oxidation) and measurement of soil colour lightness to characterize their chemical compositions. Large proportions of aromatic carbon (C) combined with small contents of lignin-derived phenols in the heavy fractions of grassland soils and grass-derived lower horizons of Araucaria forest soils indicate the presence of charred grass residues in SOM. The contribution of this material may have led to the unusual increase in C/N ratios with depth in burnt grassland soils and to the differentiation of C ₃- and C₄-derived SOM, because heavy fractions from unburnt Araucaria forest and shrubland soils have smaller proportions of aromatic C, smaller C/N ratios and are paler compared with those with C ₄ signatures. We found that lignins are not applicable as biomarkers for plant origin in these soils with small contents of strongly degraded and modified lignins as the plant-specific lignin patterns are absent in heavy fractions. In contrast, the characteristic contents of alkyl C and O/N-alkyl C of C₃ trees or shrubs and C₄ grasses are reflected in the heavy fractions. They show consistent changes of the (alkyl C)/(O/N-alkyl C) ratio and the ₁₃C/₁₂C isotopic signature with soil depth, indicating their association with C₄ and C₃ vegetation origin. This study demonstrates that soils may preserve organic matter components from earlier vegetation and land-use, indicating that the knowledge of past vegetation covers is necessary to interpret SOM composition.