Soil Biology and Biochemistry, February 2013, Vol.57, pp.1-13
C 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 enrichment in soil as compared to vegetation for C forests, whereas depletion of C was frequently reported for C grassland soil as compared to C 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 δ C values from plant material to SOC. We analyzed aboveground biomass, roots and heavy organo-mineral fractions from topsoils in both, long-term stable C grasslands and C Araucaria forest situated nearby in the southern Brazilian highlands on soils with andic properties. The stable carbon isotope ( C/ C) composition was analyzed for total organic carbon (OC ) and lignin-derived phenols. The bulk chemical composition of OC was assessed by solid-state C NMR spectroscopy while neutral sugar monomers were determined after acid hydrolysis. The shifts of the C/ C isotope signature during decomposition and stabilization (plant tissues versus soil heavy fractions) showed similar trends for VSC phenols and OC ( C depletion in C grassland soil and C enrichment in C 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 depletion of VSC lignins relative to OC was higher in C -biomass and C -derived SOC compared to the C counterparts. As lignin contents of heavy fractions were low, in particular for those with C isotopic signature, the influence of lignin on OC δ C 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 NMR spectroscopy contribute to decreasing δ C values from grass biomass to C -derived heavy fractions. In forest topsoils, the accumulation of C depleted VSC lignin residues in heavy fractions counteracts the prevailing C enrichment of OC from plant biomass to heavy fractions. Nonetheless, non-lignin compounds with relatively high C contents like microbial-derived OC have a stronger influence on δ C values of OC 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 - and C -derived heavy fractions. This may indicate different processes and/or extent of lignin (and SOM) biodegradation between C grassland and C 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 δ C values of OC was overestimated in previous studies, at least in subtropical C grassland and C forest topsoils. ► Lignin and SOC showed C enrichment in C forest and C depletion in C grassland. ► Low influence of lignin on δ C values of SOC in C grassland soils. ► C depleted lignin counteracts the prevailing C enrichment of SOC in forest soils. ► C enriched non-lignin compounds strongly influence δ C values in C forest soils. ► Structural and isotopical degradation of lignin differs between grassland and forest.
13c/12c ; Soil Organic Matter ; 13c NMR Spectroscopy ; Cuo Oxidation ; Gc/C-Irms ; Carbohydrates ; Brazil ; Agriculture ; Chemistry
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