Mineralisation dans le sol de materiaux microbiens marques au carbone 14 et a l'azote 15: Quantification de l'azote de la biomasse microbienne

The mineralization of microbial material of different C-to-N ratios (5.2, 7.9, 10.2, 12.7) was followed in fumigated soil. The microbial materials used were from Aspergillus flavus cultures, grown in liquid media and labelled with [¹⁴C]glucose and (¹⁵NN4)3804. Three contrasting soils were used and the microbial materials incubated with the fumigated soils for 28 days at 28°C.The evolution of the added organic microbial C was fast: 80% of the [¹⁴C]CO₂ produced during the whole 28 days incubation was evolved in the first week. Microbial C mineralization was mainly related to soil type; the C-to-N ratio had small effect on the ratio (mineralized microbial carbon-to-added microbial carbon). Calculation of the K_c- coefficient (the fraction of the added microbial C mineralized in 7 days) shows that K_c values lie between 0.38 and 0.43 in the 3 soils.Organic N in the added microbial material also breaks down quickly: between 60 and 100% of the organic nitrogen mineralized was evolved during the first week of incubation. Mineralization kinetics are related to soil type and to the C-to-N ratio of the microbial material.The proportion of N mineralized in 7 days was lower in an acid soil than in near neutral soils and lower with high C-to-N ratio material than with low C-to-N ratio material. The ratio (mineralized microbial N-to-added microbial N) depends on soil type and is negatively correlated with the C-to-N ratio of the microbial material. The K_N value (the fraction of the added microbial N mineralized in 7 days) lies between 0.22 and 0.47 for the three soils and four materials investigated. The added microbial material induced a priming effect on soil native N: materials with C-to-N ratios of 10.2 and 12.7 produced negative priming effects whereas materials with C-to-N ratios of 5.2 and 7.9 sometimes produced a positive priming action.From the relationship between the C-to-N ratio of the added material and the (mineralized microbial C-to-mineralized microbial N) ratio, the soil native microbial biomass was estimated using the fiush-C-to-flush-N ratio. Biomass nitrogen was then calculated from the formula biomass-N = biomassC/(biomass C-to-N ratio). Calculated in this way, 2–4% of the total nitrogen in the three soils was in microbial biomass.     

Decarboxylation des acides amines aromatiques-1-14C par les extraits de sol

Decarboxylation of the aromatic amino acids-1-¹⁴C by soilextracts: Neutral aseptic extracts of fresh soils (NAFS Extract) from forest, meadow and cultivated soils decarboxylate dl-DOPA-l-¹⁴C; dl-tyrosine-l-¹⁴C; dl-tryptophane-l-¹⁴C and dl-phenylalanine-l-¹⁴C with relative decarboxylation rates of 100; 30; 10; 0 respectively. The decarboxylation rate of dl-DOPA-l-¹⁴C has a highly significant correlation with the concentration of mineral fraction in the NAFS Extract. This result suggests that, in soil, the enzymatic system is protected by the easily extracted soil minerals. Considering the high value of E_act. (23 kcal/mole/degree), the catalytic power of the enzymatic system associated with the NAFS Extract is low. The decarboxylation rate of dl-DOPA-l-¹⁴C is optimum under aerobic conditions at pH 7 and at 37°C.The kinetic of the reaction follows the Michaelis-Menten law with K_m = 2·1 × 10^?3 M. The nature of the enzymatic system associated with the NAFS Extract is discussed.     

Effets de quelques pesticides sur la colonisation d'un substrat par le Trichoderma harzianum rifai en presence des autres champignons du sol

The effects of some pesticides upon substrate colonization by Trichoderma harzianum Rifai in the presence of other soil fungi.A herbicide (propyzamide) and 5 fungicides (benomyl, quintozene, vinclozolin. thiram, prothiocarb) were added at various concentrations to two Trichoderma harziunum-enriched garden soils. After thorough mixing, followed by solidifying with water agar, discs of these mixtures were cut with a cork-borer and transferred to a non specific nutrient medium (PDA). The development of T. harzianum on the agar, around each implant, was estimated after incubation for 1 week at 28°C.In the presence of other soil fungi, T. harzianum development was scarcely or not at all modified by prothiocarb, propyzamide and quintozene. It was highly depressed by benomyl. Vinclozolin was only slightly inhibitory. In our experimental conditions, T. harzianum development was strongly enhanced by thiram, even at a high dosage (300 μg ml^?1).The limits and the advantages of the method are estimated and the results are discussed in view of practical applications.