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

Abstract

13C contents of organic matter are changing during decomposition of plant material and stabilization as soil organic carbon (SOC). In this context, several studies showed 13C enrichment in soil as compared to vegetation for C3 forests, whereas depletion of 13C was frequently reported for C4 grassland soil as compared to C4 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 δ13C values from plant material to SOC. We analyzed aboveground biomass, roots and heavy organo-mineral fractions from topsoils in both, long-term stable C4 grasslands and C3 Araucaria forest situated nearby in the southern Brazilian highlands on soils with andic properties. The stable carbon isotope (12C/13C) composition was analyzed for total organic carbon (OCtot) and lignin-derived phenols. The bulk chemical composition of OC was assessed by solid-state 13C NMR spectroscopy while neutral sugar monomers were determined after acid hydrolysis. The shifts of the 13C/12C isotope signature during decomposition and stabilization (plant tissues versus soil heavy fractions) showed similar trends for VSC phenols and OCtot (13C depletion in C4 grassland soil and 13C enrichment in C3 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. 13C depletion of VSC lignins relative to OCtot was higher in C3-biomass and C3-derived SOC compared to the C4 counterparts. As lignin contents of heavy fractions were low, in particular for those with C4 isotopic signature, the influence of lignin on OCtot δ13C 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 13C NMR spectroscopy contribute to decreasing δ13C values from grass biomass to C4-derived heavy fractions. In forest topsoils, the accumulation of 13C depleted VSC lignin residues in heavy fractions counteracts the prevailing 13C enrichment of OCtot from plant biomass to heavy fractions. Nonetheless, non-lignin compounds with relatively high 13C contents like microbial-derived OC have a stronger influence on δ13C 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 C4- and C3-derived heavy fractions. This may indicate different processes and/or extent of lignin (and SOM) biodegradation between C4 grassland and C3 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 δ13C values of OCtot was overestimated in previous studies, at least in subtropical C4 grassland and C3 forest topsoils.