Reversible transition between active and dormant microbial states in soil.
The rate of respiration obtained in the substrate-induced respiration (SIR) method can be divided into the respiration rate of growing (r) and non-growing (K) microorganisms. The fraction of r is generally small (5-20%) in soils with no recent addition of substrates, but can be 100% in soils with high substrate availability. This suggests that substrate availability determines the proportion of biomass between these groups, and implies that transitions between them can take place reversibly. These hypotheses were tested by adding three different amounts of glucose which induced first-order, zero-order, and growth-associated respiration kinetics to three soils at four pre-incubation times (4, 12, 27, and 46 days) before the SIR measurement. An abiotic flush of CO(2) in the SIR measurement was detected and corrected for before data analysis. Accumulated CO(2)-C over 4 days after glucose addition, corrected for the respiration in unamended controls, corresponded to 41-50% mineralization of the glucose-C, and the relative amount mineralized by each soil was independent of the glucose amount added. The high glucose concentration gave an increased SIR, which reverted to the initial value within 27-46 days. In a specific sample, the maximum respiration rate induced during the pre-incubation, and the amount of organisms transformed from the K to the r state, as quantified in respiration rate units in the SIR measurement, were identical to each other, and these parameters were also highly correlated to the initial glucose concentration. The K-->r transition was very fast, probably concurrent with the instantaneous increase in the respiration rate obtained by the glucose amendment. Thereafter, a slow first-order back-transition from the r to the K state ensued, with half-lives of 12, 23, and 70 days for the three soils. The results suggest the existence of community-level controls by which growth within or of the whole biomass is inhibited until it has been completely transformed into the r state. The data also suggest that the microbial specific activity is not related to the availability of exogenous substrate in a continuous fashion, rather it responds as a sharp transition between dormant and fully active. Furthermore, the inherent physiological state of the microbial biomass is strongly related to its history. It is proposed that the normal dynamics of the soil microbial biomass is an oscillation between active and dormant physiological states, while significant growth occurs only at substantial substrate amendment.