FORMATION AND DESTRUCTION OF MICROBIAL BIOMASS DURING THE DECOMPOSITION OF GLUCOSE AND RYEGRASS IN SOIL

C-14-labelled glucose or ryegrass was incubated (aerobically, at 25-degrees-C; each substrate added at either 500 or 5000 mug C g-1 soil) with soil from permanent grassland. Soil microbial biomass was measured periodically during the incubation by the fumigation-extraction method. A 20 day incubation with the large addition of glucose (5000 mug C) caused the 'native' (i.e. unlabelled) microbial biomass to fall by half. The large addition of glucose caused a short-lived but marked priming effect and the size of this effect was consistent with its having been caused by a glucose-induced kill of native microbial biomass. Glucose added at 500 mug C did not cause a measurable kill of native microbial biomass, nor did it cause a priming effect. The proportion of glucose C retained in soil was greater throughout the incubation for the small (500 mug) than for the large (5000 mug) addition: likewise a greater proportion of the small addition was present in soil microbial biomass C (after 20 days, 21% of the added C) than of the large addition (13%). The labelled biomass declined over the 5 10 day period, with a half-life of 60 days. By contrast, labelled ryegrass added at 5000 mug C had little effect on the native microbial biomass, with the newly-synthesized labelled biomass merely adding to that already present. By 40 days, 12% of this ryegrass C was present as labelled microbial biomass, which then declined with a half-life of 66 days. After 30 days the proportion of labelled ryegrass C retained in soil was greater with the small than with the large addition. Again, a greater proportion of the small addition was present in soil microbial C (after 20 days 19% of the added C) than of the large addition (12%). Both the 500 and 5000 mug additions of ryegrass C caused positive priming effects over the 10 100 day period.