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

The microbial biomass in moist aerobic soils has an adenosine 5 ' -triphosp hate (ATP) concentration of around 8-12 mu mol ATP g(-1) biomass C and an a denylate energy charge (AEC) of between about 0.8-0.95, both typical of mic ro-organisms undergoing exponential growth in vitro. In fact, only a very s mall fraction of the biomass can be in this condition at any time due to su bstrate limitations. Our hypothesis is that the microbial biomass expends e nergy to maintain a large ATP concentration and high AEC despite the expend iture of valuable energy reserves because this strategy offers an evolution ary advantage over one based on resting spores. Thus, by the time a spore b ecomes active in response to the presence of a suitable substrate, a more s peculative organism may have captured it. We found that trace concentration s (i.e. mug 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 singl e addition after 200 h, but the biomass could be reactivated with further a dditions. Addition of 'trigger solutions' to soils recently amended with ce llulose produced an accelerated rate of mineralization of the cellulose unt il the experiment was terminated. We consider that our results describe a p reviously unreported response of the microbial biomass which equips it for survival in the generally substrate-poor soil environment. (C) 2001 Elsevie r Science Ltd. All rights reserved.