Carbon input belowground is the major C flux contributing to leaf litter mass loss: Evidences from a C-13 labelled-leaf litter experiment

Partitioning of the quantities of C lost by leaf litter through decomposition into (i) CO2 efflux to the atmosphere and (ii) C input to soil organic matter (SOM) is essential in order to develop a deeper understanding of the litter-soil biogeochemical continuum. However, this is a challenging task due to the occurrence of many different processes contributing to litter biomass loss. With the aim of quantifying different fluxes of C lost by leaf litter decomposition, a field experiment was performed at a short rotation coppice poplar plantation in central Italy. Populus nigra leaf litter, enriched in C-13 (delta C-13 similar to +160 parts per thousand) was placed within collars to decompose in direct contact with the soil (delta C-13 similar to -26 parts per thousand) for 11 months. CO2 efflux from within the collars and its isotopic composition were determined at monthly intervals. After 11 months, remaining litter and soil profiles (0-20 cm) were sampled and analysed for their total C and C-13 content. Gas chromatography (GC), GC-mass spectrometry (MS) and GC-combustion-isotope ratio (GC/C/IRMS) were used to analyse phospholipid fatty acids (PLFA) extracted from soil samples to identify the groups of soil micro-organisms that had incorporated litter-derived C and to determine the quantity of C incorporated by the soil microbial biomass (SMB). By the end of the experiment, the litter had lost about 80% of its original weight. The fraction of litter c lost as an input into the soil (67 +/- 12% of the total C loss) was found to be twice as much as the fraction released as CO2 to the atmosphere (30 +/- 3%), thus demonstrating the importance of quantifying fitter-derived C input to soils, in litter decomposition studies. The mean delta C-13 values of PLFAs in soil (delta C-13 = -12.5 parts per thousand) showed sustained incorporation of litter-derived C after one year (7.8 +/- 1.6% of total PLFA-C). Thus, through the application of stable C-13 isotope analyses, we have quantified two major C fluxes contributing to litter decomposition, at macroscopic and microscopic levels