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 C-13-depleted leaf litter was incubated on a C-13-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 C-13-depleted leaf litter (Liquidambar styraciflua L., Cercis canadensis L. and Pinus taeda L., delta C-13(vsPDB) a parts per thousand -43aEuro degrees), differing in their degradability, were incubated on a C₄ soil (delta C-13(vsPDB) a parts per thousand -18aEuro degrees) 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 18:2 omega 6,9) was found to be very depleted in C-13 (delta C-13(vsPDB) a parts per thousand from -38 to -42aEuro degrees) compared to all other PLFAs (delta C-13(vsPDB) a parts per thousand from -14 to -35aEuro degrees). 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 delta C-13. This is an indication that not all of the C derived from litter in the soil was transformed by microbes. The depletion in C-13 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. [References: 50