A new method for measuring microbial biomass nitrogen in soils: Direct extraction after toluene treatment

Attempts were made to develop a new method for measuring the microbial biomass N in upland soils by direct extraction after toluene treatment. This method which involves the use of ninhydrin reagent for the estimation of nitrogenous compounds released by autolysis after incubation of the soil with buffered (pH 7.0) toluene at 30 degrees C for 24 h, is rapid, sensitive, and precise. The factors responsible for the difference in the value of the K-N nin. factor between volcanic and non-volcanic ash upland soils were also examined. The results obtained can be summarized as follows: 1) This method can be used to estimate the size of microbial biomass N in volcanic or non-volcanic ash upland soils. For 20 microorganisms (10 bacterial strains and 10 fungal strains) the mean value of the K-N nin. factor was 0.255 for volcanic ash soils and 0.343 for the non-volcanic ash soils. 2) The newly proposed toluene treatment method enabled to extract a much larger quantity of ninhydrin-reactive nitrogen than the chloroform fumigation method. 3) The adsorption of an added amino acid by the older volcanic ash soil samples was the largest, followed by the samples of younger volcanic ash soil which that by the alluvial soil samples was the smallest. These results appeared to reveal a positive correlation with the K-N nin. factor of the respective soil samples.