Assessing biological soil quality with chloroform fumigation-incubation: Why subtract a control?

Microbial biomass, as part of the active pool of soil organic matter, is cr itical in decomposition of organic materials, nutrient cycling, and formati on of soil structure. We evaluated chloroform fumigation-incubation with su btraction of a control (CFI/F-C) and without subtraction of a control (CFI/ F) as methods to assess biological soil quality. Relationships between CFI/ F and:potential C mineralization, particulate organic C, and soil organic C were stronger (r(2) = 0.86 +/- 0.07, n = 232) than those between CFI/F-C a nd the same soil C pools (r(2) = 0.25 +/- 0.09) in soils from Georgia. From published data, relationships of CFI/F with potential C mineralization and soil organic C were stronger than those of chloroform fumigation-extractio n and substrate-induced respiration with these soil C pools. Effects of lan d management on biological soil quality using CFI/F were consistent with th ose determined using other soil C pools as response variables. However, lan d management effects on biological soil quality using CFI/F-C were either c ontrary to those using other soil C pools or not detectable because of grea ter inherent variability in CFI/F-C. Chloroform fumigation-incubation witho ut subtraction of a control is a robust and reliable method to assess biolo gical soil quality under a wide range of soil conditions.