| Summary: | The microbial biomass (MB) and light fraction (LF) of organic matter are often considered as active fraction of organic matter (AFOM) and as indices of soil fertility and microbial activity. This study was performed in order to assess the turnover of AFOM using long-term incubation (56 weeks) at25 °Cin 34 meadow soils with different physical and chemical properties such as soil texture, organic C and total N. The MB and LF were determined at 8 and 5 times during the incubation period using fumigation-extraction technique for MB and densimetric method for LF. The amount of MB-C and MB-N mineralized increased with time of incubation. At the beginning of incubation, the C and N content of soil MB represented respectively 0.76 to 3.7% of total organic C and 1.94 to 10.7% of total N. The C and N content of LF represented respectively 2.9 to 25.6% of total organic C and 1.7 to 17.5% of total N. At the end of incubation, the losses of MB-C and MB-N from soils reached respectively 71 and 82% of the initial amounts. The MB and LF dynamic were well described by a two-component first-order rate model. The amount of N in the labile MB and LF pools represented respectively 54% of total MB-N and 61% of total LF-N. The more stable MB and LF pools had higher half-life than labile pools. The results obtained indicated that the stable LF would be the precursor of soil humic compounds.
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