CARBON AND NINHYDRIN-REACTIVE NITROGEN OF THE MICROBIAL BIOMASS IN REWETTED COMPOST SAMPLES

We studied the applicability of the so-called fumigation-extraction me thod for microbial biomass carbon and ninhydrin-reactive nitrogen to e valuate the stabilization of rewetted compost samples. Dried samples o f composts of widely different composition, sampled at the initial, th ermophilic, mesophilic and curing stages of composting, were remoisten ed to 50% of water content and incubated at 25 degrees C for 5 days be fore analysis. ln rewetted raw materials, microbial biomass carbon (B- C) ranged from 11,296 to 41,601 mu g/g and significatively decreased i n samples of increasing stabilization reaching only 2704-5837 mu g/g i n rewetted mature composts. The percentage of B-C with respect to tota l organic carbon (C) varied from 2.5 to 9.5% in starting materials, bu t was lower than 1.7% in all the end products. Ninhydrin-reactive N of the microbial biomass (B-NIN) showed the same trend that B-C, ranging from 504 to 2044 mu g/g in raw material samples, declining in samples of increasing composting time and achieving rather close values in th e end products (from 90 to 303 mu g/g). The percentage of B-NIN respec t to total N was higher in raw material samples (2.0-9.8%), with final values lower than 1.1%. The B-C and B-NIN showed a strong correlation (r=0.99, P<0.001) with a regression coefficient (20.1) in good agreem ent with those found for soils. Incubation at high temperature (53 deg rees C) negatively influenced the content of B-C and B-NIN at all comp osting stages and had little, if any, effect on the chemical compositi on of microbial biomass. Results of this work show that the applicatio n of methods for the measure of microbial biomass C and ninhydrin-reac tive N in soil to rewetted composts gives valuable information on the degree of stabilization achieved by the product.