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

Abstract

Summary 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 C3‐ 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 C4 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.

Deep soil organic matter—a key but poorly understood component of terrestrial C cycle

“…Mineral nitrogen was found to contribute to about 20% to the total nitrogen of deep soil horizons and even when subtracted, C/N ratios of most soils are decreasing with depth ). An increase in C/N with depth in some soils may be explained by the presence of charred material (Dümig et al 2009). …”

The Chemical Ecology of Soil Organic Matter Molecular Constituents

“…Similar methods can be used when labelled compounds, such as when 13 C or 15 N enriched substrates, are introduced to the soil (Kuzyakov and Bol, 2005;Providoli et al, 2006;Huygens et al, 2008;Dijkstra, 2009;Glaser et al, 2012;Hatton et al, 2012). The variation in 12 C vs. 13 C isotopes (expressed as δ 13 C) can also be used to identify variations in C3 vs. C4 plant-derived OM as well as to investigate OM turnover mechanisms (Blagodatskaya et al, 2011;Christensen et al, 2011;Dijkstra et al, 2008Dijkstra et al, , 2010Dümig et al, 2009;MendezMillan et al, 2012;Morgan et al, 2011;Torn et al, 2011). The use of radiocarbon to examine the fate of specific OM components in soil over decadal and millennial timescales can be examined by simultaneously using (Δ 14 C) to measure soil OM age and biomarkers to evaluate specific compounds.…”

Short-term changes in soil Munsell colour value, organic matter content and soil water repellency after a spring grassland fire in Lithuania

“…Darker soils correspond to low Munsell values (Viscarra Rosell et al, 2006;Shields et al, 1968;Conant et al, 2011), independently of the area being affected by fire or not. In burned areas, soil became darker with the increasing content of aromatic carbon, present in high amounts in the charred material produced by fires (Dümig et al, 2009). In soils affected by low-severity fires, the colour is darker due to the incomplete combustion of organic matter (Terefe et al, 2008).…”