Relationships of chloroform fumigation–incubation to soil organic matter pools

Microbial biomass is part of the active pool of soil organic matter that plays focal roles in decomposition of organic materials, nutrient cycling and biophysical manipulation of soil structure. We compared two commonly used variants of the chloroform fumigation–incubation method in their relationships with other active, passive and total soil C and N pools in soils from Texas, Georgia, Alberta and British Columbia. The relationship of potential C mineralization with chloroform fumigation–incubation without subtraction of a control was much stronger (r2=0.81±0.10 among five data sets with a total of 844 observations) than with subtraction of a control (r2=0.30±0.22). Similarly, the relationship of soil organic C with chloroform fumigation–incubation without subtraction of a control was better (r2=0.80±0.13) than with subtraction of a control (r2=0.38±0.32). Relationships of net N mineralization, flush of N following fumigation–incubation, flush of CO2-C during the first day following rewetting of dried soil, particulate organic C and N, mean weight diameter of water-stable aggregation and total porosity with chloroform fumigation–incubation were also better without subtraction of a control than with subtraction of a control. In analyses of data taken from published reports, chloroform fumigation–incubation without subtraction of a control was better related with active soil C pools than with subtraction of a control. Chloroform fumigation–incubation without subtraction of a control, unlike that with subtraction of a control, should be considered a more robust method to determine microbial biomass under a wide range of environmental conditions.