Carbon and ninhydrin-reactive nitrogen of the microbial biomass in rewetted compost samples

We studied the applicability of the so-called fumigation-extraction method for microbial biomass carbon and ninhydrin-reactive nitrogen to evaluate the stabilization of rewetted compost samples. Dried samples of composts of widely different composition, sampled at the initial, thermophilic, mesophilic and curing stages of composting, were remoistened to 50% of water content and incubated at 25 degrees C for 5 days before analysis. In rewetted raw materials, microbial biomass carbon (BC) ranged from 11,296 to 41,601 micrograms/g and significatively decreased in samples of increasing stabilization reaching only 2704-5837 micrograms/g in rewetted mature composts. The percentage of BC with respect to total organic carbon (C) varied from 2.5 to 9.5% in starting materials, but was lower than 1.7% in all the end products. Ninhydrin-reactive N of the microbial biomass (BNIN) showed the same trend that BC, ranging from 504 to 2044 micrograms/g in raw material samples, declining in samples of increasing composting time and achieving rather close values in the end products (from 90 to 303 micrograms/g). The percentage of BNIN respect to total N was higher in raw material samples (2.0-9.8%), with final values lower than 1.1%. The BC and BNIN showed a strong correlation (r=0.99, P<0.001) with a regression coefficient (20.1) in good agreement with those found for soils. Incubation at high temperature (53 degrees C) negatively influenced the content of BC and BNIN at all composting stages and had little, if any, effect on the chemical composition of microbial biomass. Results of this work show that the application of methods for the measure of microbial biomass C and ninhydrin-reactive N in soil to rewetted composts gives valuable information on the degree of stabilization achieved by the product.