The role of lignin for the δ13C signature in C4 grassland and C3 forest soils

C-13 contents of organic matter are changing during decomposition of plant material and stabilization as soil organic carbon (SOC). In this context, several studies showed C-13 enrichment in soil as compared to vegetation for C-3 forests, whereas depletion of C-13 was frequently reported for C-4 grassland soil as compared to C-4 vegetation. These changes were often attributed to selective preservation and/or stabilization of specific organic compounds. This study investigates if changes in the chemical composition of OC and specifically lignin may explain the observed shifts in delta C-13 values from plant material to SOC. We analyzed aboveground biomass, roots and heavy organo-mineral fractions from topsoils in both, long-term stable C-4 grasslands and C-3 Araucaria forest situated nearby in the southern Brazilian highlands on soils with andic properties. The stable carbon isotope (C-12/C-13) composition was analyzed for total organic carbon (OCtot) and lignin-derived phenols. The bulk chemical composition of OC was assessed by solid-state C-13 NMR spectroscopy while neutral sugar monomers were determined after acid hydrolysis. The shifts of the C-13/C-12 isotope signature during decomposition and stabilization (plant tissues versus soil heavy fractions) showed similar trends for VSC phenols and OCtot C-13 depletion in C-4 grassland soil and C-13 enrichment in C-3 forest soil compared to the corresponding vegetation). In this regard, the isotopic difference between roots and aboveground biomass was not relevant, but may become more important at greater soil depths. C-13 depletion of VSC lignins relative to Qat was higher in C-3-biomass and C-3-derived SOC compared to the C-4 counterparts. As lignin contents of heavy fractions were low, in particular for those with C-4 isotopic signature, the influence of lignin on OCtot delta C-13 values in grassland topsoils is presumably low. Rather, the presence of charred grass residues and the accumulation of alkyl C in heavy fractions as revealed by C-13 NMR spectroscopy contribute to decreasing delta C-13 values from grass biomass to C-4-derived heavy fractions. In forest topsoils, the accumulation of C-13 depleted VSC lignin residues in heavy fractions counteracts the prevailing C-13 enrichment of OCtot from plant biomass to heavy fractions. Nonetheless, non-lignin compounds with relatively high C-13 contents like microbial-derived OC have a stronger influence on delta C-13 values of OCtot in forest soils than lignins or aliphatic biopolymers. The mineral-associated SOC is in a late phase of decomposition with large contributions of microbial-derived carbohydrates, but distinct structural and isotopical alterations of lignin between C-4- and C-3-derived heavy fractions. This may indicate different processes and/or extent of lignin (and SOM) biodegradation between C-4 grassland and C-3 forest resulting from other kind of decomposer communities in association with distinct types and amounts of plant input as source of SOM and thus, carbon source for microbial transformation. Our results indicate that the importance of lignin for delta C-13 values of OCtot was overestimated in previous studies, at least in subtropical C-4 grassland and C-3 forest topsoils. (C) 2012 Elsevier Ltd. All rights reserved.