Indirect Measures of Biomass

Chemical Methods

Jenkinson (1966) cited an earlier suggestion of Stormer (1908) that a flush of CO2, evolved after fumigation, was due to the decomposition of organisms killed during fumigation by the surviving microorganisms remaining after fumigation. This relates to the extensive work done on "partial sterilization" of soils, in Great Britain and elsewhere (Russell and Hutchinson, 1909; Powlson, 1975), under the misguided assumption that most soil microorganisms were somehow deleterious to subsequent plant growth, particularly in agricultural fields. Many of the soil heterotrophs are now considered generally beneficial, particularly when viewed in a whole-system nutrient cycling context.

The Chloroform Fumigation and Incubation (CFI) Technique

We now consider ways to use fumigants to measure microbial biomass. Using a fumigant such as chloroform, and incubating the soil for 10 or 20 days, the size of the flush of CO2 output can be related to the size of the microbial biomass by the expression B = F/kc; where B = soil biomass C (in |g C • g-1 soil); F = carbon dioxide carbon (CO2—C) evolved by fumigated soil minus CO2 evolved by unfumigated soil over the same time period; and kc = fraction of biomass mineralized to CO2 during the incubation (Jenkinson and Powlson, 1976). The kc value, calculated from a range of microorganisms in controlled experiments, is assigned a general value of 0.45 (Jenkinson, 1988).

Jenkinson and Powlson (1976) relied on laboratory measurements of microbial cells added to soil. Voroney and Paul (1984) extended this work to include labile nitrogen, and measured both kc and kn (fraction of biomass nitrogen mineralized to inorganic nitrogen). A review of usage of 14C to measure microbial biomass and turnover is given by Voroney et al. (1991), with step-by-step procedures for this research. They introduced carbon by labeling plants via photosynthetic pathways, and then followed the carbon into the microbial biomass via root exudates and turnover, and in turn into the soil organic matter.

A wide range of soils has been compared for biomass carbon calculated from biovolume (the measured volume of the cell), using the CFI method, and a ratio of biomass carbon from biovolume to biomass carbon, also from CFI, has been determined (Powlson, 1994) (Table 3.1). These ratios range from 0.86 to 1.25 from soils in arable lands and up to 6.47 from soils in deciduous woodland. Forest soils, including those with low pH, have proven more difficult to analyze for microbial biomass, and are considered next.

TABLE 3.1. Comparison of Biomass Carbon as Calculated from Direct Microscopy and the Fumigation Incubation (FI) Method
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