Defining a realistic control for the chloroform fumigation-incubation method 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 during 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 fumigation was 22.5% that in non-fumigated control soils. Re-inoculation of the fumigated soil raised C-14-maize decomposition to 73% that of the unfumigated control. A method was developed which varies the proportion of mineralized C from the unfumigated soil (UFC) that is subtracted in calculating CFI biomasss C. The proportion subtracted (P) varies according to a linear function of the ratio of C mineralized in the fumigated (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 by regression of CFI biomass C, calculated according to the equation MBC = (F-C - PUFC)/0.41, against that derived by direct microscopy in a series of California soils. Parameter values which gave the best estimate of microscopic biomass from the fumigation data were K-1 = 0.29 and K-2 = 0.23 (R(2) = 0.87). Substituting these parameter values, the equation 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 measurement of MBC. The use of this approach corrected data that were previously difficult to interpret and helped to reveal temporal trends and changes in MBC associated with soil depth.