Application of the chloroform fumigation-incubation method to the estimation of soil microbial biomass in burned and unburned Japanese red pine forests

Abstract Estimation of soil microbial biomass in burned and unburned Japanese red pine forests was attempted using the chloroform fumigation-incubation method. As the amount of CO₂-C evolved from the fumigated soil for 10–20 days after fumigation (designated as F') was always lower than that from the unfumigated soil during the same period (UF'), the formula, microbial biomass-C(M) = the amount of CO₂-C evolved from the fumigated soil for 0–10 days after fumigation, F) − F'/ k _c, was proposed instead of Jenkinson's conventional formula, M = (F − UF')/ k _c. The k _c value was also determined as 0.30 using 3 fungal and 3 bacterial cultured species as internal standards. Microbial biomass-C calculated by (F − F')/0.30 decreased with soil depth at both the burned (Nenoura, 3.5 years after fire) and unburned (Ato) sites, showing the significant correlation with the decrease of soil respiration and organic C content along soil depth. Microbial biomass-C in the 0–2 cm soil layer at the burned site at Nenoura was 130 mg/100 g dry soil and those in the HF horizon and 0–2 cm soil layer at the unburned site at Ato were 686 and 146 mg/100 g dry soil, respectively.