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

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anglais. 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 [‘4C]glucose and (‘5NH,)2S0,. 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 [‘4C]C02 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 KC coefficient (the fraction of the added microbial C mineralized in 7 days) shows that KC 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 KN 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 flush-C-to-flush-N ratio. Biomass nitrogen was then calculated from the formula biomass-N = biomass- C/(biomass C-to-N ratio). Calculated in this way, 24% of the total nitrogen in the three soils was in microbial biomass.