Isotopic evidences for microbiologically mediated and direct C input to soil compounds from three different leaf litters during their decomposition

We show the potentiality of coupling together different compound-specific isotopic analyses in a laboratory experiment, where (13)C-depleted leaf litter was incubated on a (13)C-enriched soil. The aim of our study was to identify the soil compounds where the C derived from three different litter species is retained. Three (13)C-depleted leaf litter (Liquidambar styraciflua L., Cercis canadensis L. and Pinus taeda L., δ(13)C(vsPDB) ≈ −43‰), differing in their degradability, were incubated on a C4 soil (δ(13)C(vsPDB) ≈ −18‰) under laboratory-controlled conditions for 8 months. At harvest, compound-specific isotope analyses were performed on different classes of soil compounds [i.e. phospholipids fatty acids (PLFAs), n-alkanes and soil pyrolysis products]. Linoleic acid (PLFA 18:2ω6,9) was found to be very depleted in (13)C (δ(13)C(vsPDB) ≈ from −38 to −42‰) compared to all other PLFAs (δ(13)C(vsPDB) ≈ from −14 to −35‰). Because of this, fungi were identified as the first among microbes to use the litter as source of C. Among n-alkanes, long-chain (C27–C31) n-alkanes were the only to have a depleted δ(13)C. This is an indication that not all of the C derived from litter in the soil was transformed by microbes. The depletion in (13)C was also found in different classes of pyrolysis products, suggesting that the litter-derived C is incorporated in less or more chemically stable compounds, even only after 8 months decomposition.