DEFINING A REALISTIC CONTROL FOR THE CHLOROFORM FUMIGATION-INCUBATIONMETHOD USING MICROSCOPIC COUNTING AND C-14 SUBSTRATES

Chloroform fumigation-incubation (CPI) has made possible the extensive characterization of soil microbial biomass carbon (C) (MBC). Defining the non-microbial C mineralized in soils following fumigation remains the major limitation of CFI. The mineralization of non-microbial C du ring CFI was examined by adding C-14-maize to soil before incubation. The decomposition of the C-14-maize during a 10-d incubation after fum igation was 22.5% that in non-fumigated control soils. Re-inoculation of the fumigated soil raised C-14-maize decomposition to 73% that of t he unfumigated control. A method was developed which varies the propor tion of mineralized C from the unfumigated soil (UFC) that is subtract ed in calculating CFI biomasss C. The proportion subtracted (P) varies according to a linear function of the ratio of C mineralized in the f umigated (F-C) and unfumigated samples (F-C/UFC) with two parameters K -1 and K-2 (P = K-1 F-C/UFC) + K-2). These parameters were estimated b y regression of CFI biomass C, calculated according to the equation MB C = (F-C - PUFC)/0.41, against that derived by direct microscopy in a series of California soils. Parameter values which gave the best estim ate of microscopic biomass from the fumigation data were K-1 = 0.29 an d K-2 = 0.23 (R(2) = 0.87). Substituting these parameter values, the e quation can be simplified to MBC = 1.73 F-C - 0.56UF(C). The equation was applied to other CFI data to determine its effect on the measureme nt of MBC. The use of this approach corrected data that were previousl y difficult to interpret and helped to reveal temporal trends and chan ges in MBC associated with soil depth.