Isotopic evidences for microbiologically mediated and directC input to soil compounds from three different leaf littersduring their decomposition

We show the potentiality of coupling together

different compound-specific isotopic analyses in a laboratory

experiment, where 13C-depleted leaf litter was

incubated on a 13C-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 13C-depleted

leaf litter (Liquidambar styraciflua L., Cercis canadensis L.

and Pinus taeda L., d13CvsPDB & -43%), differing in their

degradability, were incubated on a C4 soil (d13CvsPDB &

-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 18:2x6,9) was

found to be very depleted in 13C (d13CvsPDB & from -38 to

-42%) compared to all other PLFAs (d13CvsPDB & 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 d13C. This is an indication that not all of

the C derived from litter in the soil was transformed by

microbes. The depletion in 13C was also found in different

classes of pyrolysis products, suggesting that the litterderived

C is incorporated in less or more chemically stable

compounds, even only after 8 months decomposition