The significance of organic separates to carbon dynamics and its modelling in some cultivated soils

Soil organic matter (SOM) dynamics are usually described by compartmental models. We have sought SOM separates that might be related to SOM dynamic compartments, The turnover of C in various separates from long-term field experiments with maize was measured using the natural C-13 labelling technique. The Rothamsted carbon model gave a good prediction of the observed C turnover. Primary particle-size fractions coarser than 50 mu m had short lives, and could be associated with the plant structural compartment of models. Water-extractable components are enriched in young C but cannot be associated with labile compartments. None of the chemical separates obtained by acid hydrolysis, wet oxidation, thermic oxidation, pyrolysis or alkaline extraction, were enriched either in young or old C. The results showed neither a sequential relation between fulvic acids and humic acids nor a resistance of nonhydrolysable material. The range of lifetimes of soil C seems to be determined more by physical position and protection than by the chemical nature of SOM.