Molecular dynamics of organic matter in a cultivated soil

The dynamics of soil organic carbon are included in global carbon (C) cycle scenarios using different, but generally arbitrary defined, kinetic pools. To improve global C models, better relationships between the chemical structure of soil organic matter (SOM) and its kinetic pools are needed. To assess the molecular residence time of SOM and the relation with plant inputs, pyrolysis-GC/MS-C-IRMS was performed on maize plants and on two samples from the same soil that had undergone a vegetation change from the C3 plant wheat to the C4 plant maize. This vegetation change has added naturally C-13-enriched material to the soil. Most pyrolysis products from the maize were derived from polysaccharides and lignins, and were not detected in soils. However, polysaccharide-derived products were also major pyrolysis products in soils, N-containing or unspecific pyrolysis products were also detected. The residence times (based on C-13 natural labelling) revealed a continuum of values, that was independent of chemical structure, with only two pyrolysis products presenting a relatively long residence time (ca. 100 years). An unexpected long life-time for N-containing (similar to49 years) and polysaccharide-derived (similar to54 years) pyrolysis products was found. Our results suggest that mainly recycling of carbon in carbohydrates and N-containing materials in addition to physical and chemical protection is responsible for SOM stabilization in the slow carbon pool. (C) 2002 Elsevier Science Ltd. All rights reserved.