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

摘要

This study investigates if Araucaria forest (C metabolism) expansion on frequently burnt grassland (C metabolism) in the southern Brazilian highland is linked to the chemical composition of soil organic matter (SOM) in non-allophanic Andosols. We used the p#pdC/p#poC isotopic signature to group heavy organo-mineral fractions according to source vegetation and p#pdC 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- and C-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 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 C trees or shrubs and C grasses are reflected in the heavy fractions. They show consistent changes of the (alkyl C)/(O/N-alkyl C) ratio and the p#pdC/p#poC isotopic signature with soil depth, indicating their association with C and C 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.