Soil microbial biomass is triggered into activity by trace amounts of substrate

The microbial biomass in moist aerobic soils has an adenosine 5′-triphosphate (ATP) concentration of around 8–12 μmol ATP g−1 biomass C and an adenylate energy charge (AEC) of between about 0.8–0.95, both typical of micro-organisms undergoing exponential growth in vitro. In fact, only a very small fraction of the biomass can be in this condition at any time due to substrate limitations. Our hypothesis is that the microbial biomass expends energy to maintain a large ATP concentration and high AEC despite the expenditure of valuable energy reserves because this strategy offers an evolutionary advantage over one based on resting spores. Thus, by the time a spore becomes active in response to the presence of a suitable substrate, a more speculative organism may have captured it. We found that trace concentrations (i.e. μg g−1 quantities) of appropriate ‘trigger solutions’ of glucose, amino acids and root exudates can cause the biomass to evolve about 2- to 5-times more C as CO2 than was contained in the original ‘trigger solution’. The effect (essentially a “priming effect”) was mainly over with a single addition after 200 h, but the biomass could be reactivated with further additions. Addition of ‘trigger solutions’ to soils recently amended with cellulose produced an accelerated rate of mineralization of the cellulose until the experiment was terminated. We consider that our results describe a previously unreported response of the microbial biomass which equips it for survival in the generally substrate-poor soil environment.