Reponse de la biomasse microbienne a l'adjonction au sol de materiel vegetal marque au 14C: Role des racines vivantes

Abstract

14C and 15N-labelled immature wheat straw was incubated in the laboratory for 450 days in either a sandy soil or a clay soil, under controlled conditions of temperature and humidity. One-half of the treatments were cropped 4 times in succession with spring wheat. After each harvest, the roots and shoots were removed from the soil. The remaining treatments were kept bare, without plants. After 277 days, 1% unlabelled wheat straw was again mixed with the soils. Microbial biomass was measured after 0, 25, 53, 80, 185, 318 and 430 days, using the fumigation technique. This paper presents the 14C-data. The half-life of the labelled compounds in soil was from 60 to 70 days. After 430 days about 10% more labelled C remained in bare soil than in cropped soil. Labelled biomass carbon reached its maximum before day 25. By then 50% of the biomass-C was labelled and the biomass represented 20% of the total labelled C remaining in the soils. This percentage decreased slowly to 15% after 430 days in bare sandy soil and to 17% in bare clay soil. A second incorporation of plant material, this time unlabelled, did not appreciably alter the shape of the curve representing the decrease of labelled C in biomass, expressed as % of the total remaining labelled C. Total biomass-C (labelled + unlabelled) in cropped soil was sometimes higher and sometimes lower than in bare soil. However, the labelled C/total C ratio in biomass was always lower; in cropped soils than in soils without plants, clearly showing the effect of rhizodeposition. From days 25 to 430 an increasing difference appeared between the ratio labelled C/total and C in CO 2 and the corresponding ratio labelled C/total C in biomass. In CO 2-C the ratio diminished rapidly, in biomass-C it remained at a high level, most probably indicating a lower turnover of C in resting but living microorganisms. Other explanations are also discussed. The amount of CO 2-C released mg −1 of biomass-C was higher in cropped than in bare soil, presumably because the microorganisms were activated by the living (or dying) root system.