Relationship between microbial biomass and extractable organic carbon content in volcanic and non-volcanic ash soil

A significant portion of plant-available nutrients is derived from the transformations of soil organic matter through microbial biomass. Therefore, it is important to elucidate the factors determining the amount of soil microbial biomass. Using five volcanic ash and five non-volcanic ash soils from Japan, we investigated the relationships between the amount of microbial biomass and several fractions of soil organic C. A portion of each soil sample was amended with plant debris. After 3-6 months incubation, the microbial biomass C was measured by chloroform fumigation-extraction and soil organic C was extracted with 0.5 M K2SO4, 1 M KNO3, or cold water. When all soil samples were taken into consideration, microbial biomass C was correlated with the amount of 1 M KNO3- (r = 0.577**) or cold water- (r = 0.561**) extractable organic C. When the volcanic and non-volcanic ash soils were extracted with 0.5 M K2SO4, it was possible to discriminate between the organic C of the two groups of soils. The amounts of microbial biomass (r = 0.683**) and 1 M KNO3-extractable organic C (r = 0.863**) correlated well with hot water-extractable organic C. The results may indicate that the 1 M KNO3-extractable fraction is the major fraction of soil organic C that controls the amount of microbial biomass irrespective of the soil type. (C) 2001 Elsevier Science B.V. All rights reserved.