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The reliability of three methods (microbial C and mineral-N flush by fumigation-incubation, and ATP) for measuring soil microbial biomass was assessed on two silt-loam soils of different P fertility status under grazed perennial pastures. The mineral-N flush and ATP methods provided a reasonably reliable index of microbial biomass, but the fumigation-incubation procedure for CO2-C flush, using preincubated samples and an unfumigated 0-10 day control, was inappropriate for these soils. The numbers of bacteria (direct microscopy) and the percentage metabolically active were also measured.Generally, in both soils, total microbial biomass and the numbers, mass and metabolic activity of bacteria were influenced more by temporal factors in samples taken monthly than by the fertility status. Temporal fluctuations were greater in the high-fertility (Waikanae) soil, but no consistent seasonal trends in mineral-N flush and ATP values were apparent. In both soils, numbers and biomass of bacteria were at a minimum in spring.Values of two biomass indices (mineral-N flush and ATP contents) were similar in the high-and low-fertility (Pomare) soil, and comprised similar percentages of organic-matter contents. The percentages of metabolically active bacteria, however, tended to be higher in Pomare than in Waikanae soil, and, therefore, did not reflect soil fertility status.Methodological and field aspects of these results are discussed.

S U M M A R YAutoradiographic methods have been developed to determine the proportion of metabolizing bacteria on leaves of Elodea canadensis. Detection with a light microscope of tritium labelling of pure cultures of bacteria was optimal when samples were incubated with 20 pCi of [3H]glucose or [3H]thymidine/ml for 1-75 to 2 h and the bacteria exposed to K2 emulsion for 14 days. Two fluorescent pseudomonads did not form autoradiograms when labelled with [3H]thymidine.For autoradiograms of bacteria from E. canadensis, L4 emulsion was more suitable than K2 emulsion.High-resolution autoradiograms were prepared of a pure culture of a pseudomonad and of bacteria from moribund leaves of Elodea canadensis. Counts of labelled and unlabelled cells in light-microscopic and high-resolution autoradiograms suggested that valid counts of labelled bacteria could be obtained from light-microscopic autoradiograms. Direct counts of bacteria from E. canadensis leaves of different ages revealed greater increases as leaves matured and died than did indirect methods. Autoradiographic data from young leaves indicated that the percentage of bacteria labelled with [3H]glucose was similar to the percentage of the total population which was estimated by the plate method. On the mature and moribund leaves larger proportions of the populations were labelled with [3H]glucose than were included in the plate counts. I N T R O D U C T I O NEcological studies of micro-organisms are frequently limited by the difficulty of relating numbers of organisms to the activity of the population (Jannasch, 1965 Quantitative data can be obtained from autoradiograms by counting the proportions of labelled and unlabelled cells . However, if such a count is to be a direct measure of the metabolic activity of the population, certain conditions must be satisfied. The labelled substrate must be a metabolite which can be assimilated by the majority of bacteria. The sample must be labelled with a sufficiently high concentration of isotope and for long enough for all metabolizing bacteria to accumulate enough isotope to form an autoradiogram, but the sample must not be incubated with the label for so long that inactive bacteria begin to grow.In the present study autoradiography was used to determine the proportions of metabolizing bacteria in populations from the pond weed Elodea canadensis sampled from a lake. Preliminary experiments were carried out with pure cultures of bacteria to determine the optimum conditions for autoradiography of tritium-labelled bacteria, such that all bacteria metabolizing at the time of sampling would be detected. Most of the autoradiograms were prepared for the light microscope (LM), but some methods of preparing autoradiograms for an electron microscope (EM) were investigated. Counts of labelled bacteria from LM and EM autoradiograms of similar material were compared. Relationships between the proportions of metabolizing bacteria on leaves of Elodea canadensis, determined by lightmicroscopic autoradiography, and direct and indirect co...