MICROBIAL BIOMASS AND BACTERIA IN 2 PASTURE SOILS - AN ASSESSMENT OF MEASUREMENT PROCEDURES, TEMPORAL VARIATIONS, AND THE INFLUENCE OF P FERTILITY STATUS

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.