THE DYNAMICS OF CARBON IN PARTICLE-SIZE FRACTIONS OF SOIL IN A FOREST-CULTIVATION SEQUENCE

Cultivation of forest and grassland soils induces heavy changes in soi l organic matter (SOM) dynamics. To better predict the effect of culti vation, there is a need to describe which organic pools are affected a nd to which extent. We used a chronosequence of thick humic forest soi ls converted to maize cultivation for 40 yr in southwest France. The d ynamics of soil carbon was investigated through particle-size fraction ation and the use of C-13 allowed to distinguish forest-derived organi c matter and new crop-derived organic matter. This partitioning of soi l carbon by size on one hand and by age on the other provided a precis e description of carbon turnover. The level towards which tend the org anic pools under cultivation showed that the decay rates of soil carbo n were one order of magnitude higher under cultivation than under fore st. SOM can thus be considered as deprotected under cultivation. All s ize fractions appeared to be deprotected to the same extent. A progres sive transfer of silt-sized C to clay-sized C was nevertheless suspect ed and attributed to the decreasing stability of fine silt-sized micro aggregates with cultivation. SOM furthermore contained some very stabl e C present as silt-sized and possibly clay-sized particles. The turno ver times of maize-derived organic matter was the same as that observe d in similar soils cultivated for centuries. This indicated that the n ew conditions induced by cultivation were reached in the very first ye ars after forest clearing and that the high initial SOM content and hi gh mineralization rate of initial organic matter did not affect the dy namics of newly incorporated carbon.