Analysis and role of bitumens in acid sandy soils

In the acid sandy soils of the Medoc Landes (France), the podzols are observed to have a much higher bitumen content than the hydromorphic type. Study of the acid and neutral fractions of the bitumens extracted from surface vegetation and various horizons of these soils shows that the acid fractions and the hydrocarbons undergo less transformation and less degradation in the A1 horizons of the podzols than in the corresponding hydromorphic soil horizons. The neutral fractions are for the most part only slightly transformed, but are more rapidly degraded, in the soils. The incubation of soil samples in the presence of 14C-labelled maize straw indicates that the soil bitumens originate mainly from plants, but that microbial synthesis products are incorporated into this fraction, more especially in the podzols. The bitumens have a toxic effect on the soil microflora as a whole. The intensity of this effect varies with the concentration of the extracts, pH of the soil and origin of the microflora. The acid fraction and the hydrocarbons are the strongest inhibiting agents. The depressant action depends to some extent on the length of the chains and the nature of the functions. Acidity seems to play the main part in accumulation of bitumens in the podzols studied. Generally speaking, the bitumens appear to take part in the formation of mor type humus. (author)