Chemical changes during composting of plant residues reduce their mineralisation in soil and cancel the priming effect

Applying composts is useful for increasing soil carbon (C) stocks and improving agricultural productivity. In order to understand the effects of composts on soil organic matter (SOM) formation and mineralisation, C-13-labelled plant residues, previously composted or not, were incubated in an arable soil. The amount of C-13 was quantified in the CO2 evolved, the dissolved organic carbon (DOC) and in the microbial biomass (MB). Composting decreased (from 29 to 9%) the proportion of the plant residue labile pool and increased the residence time of both labile and more stable pools from 21 to 34 days and 1.5-5.5 years, respectively. At the beginning of the incubation, the amounts of C-13 in the DOC and MB were significantly higher when adding fresh residues than composted ones to the soil. A priming effect on SOM mineralisation (+ 21% over 3 years) was only observed for non-composted residues. These differences were attributed to changes in the chemical composition of plant materials during composting (less sugars and lipids, more lignin). In terms of C budget, the total loss of CO2 (including the composting process and the SOM priming by fresh residues) was comparable for both treatments after 600 days of incubation.