Evaluating C sources and microbial biomass dynamics involved in the triggering response with soil depth

The triggering response has attracted much research interest. However, the main C source of triggered CO₂–C and the dynamics of soil microbial biomass C (biomass C) and adenosine 5′-triphosphate (ATP) concentrations during the triggering response still remain largely unknown. We labeled the soil microbial biomass of three soil layers (0–10 cm, 10–30 cm and 30–60 cm) of a forest soil with dried aboveground maize (22 atom%, 2.5 mg g⁻¹ soil) and glucose (22 atom%, 2.5 mg g⁻¹ soil) to determine the contribution of the four potential C sources involved in the triggering response: ¹³C-labeled maize and glucose, biomass C, soil organic carbon (SOC), and triggering glucose. After 56 days of soil pre-incubation, the ¹³C abundances of biomass C and SOC were 5.27–6.71 and 1.94–2.82 atom%, respectively. Addition of 10 μg glucose-C g⁻¹ soil caused a strong and rapid triggering response resulting in 2.3–6 times more triggered CO₂–C evolved than was contained in the triggering glucose, which increased with soil depth. The atom% ¹³C–CO₂ values of model calculations were closest to measured values when assumed to derive completely from biomass C. Biomass C did not significantly change at day 1 but significantly decreased by 7.1–27.6% at day 5. ATP concentrations (nmol ATP g⁻¹ soil) increased from 6.2-10.8% by day 1 to 9.2- 28% by day 5 (p < 0.05). Soil biomass C and ATP contents reached values similar to those of the control after 10 days. These results indicate that the CO₂–C from the triggering response mainly derived from mineralization of biomass C during the early triggering stage and from mineralization of residual substrates thereafter. Biomass ATP concentrations (μmol ATP g⁻¹ biomass C) remained constant during the later stages. We conclude that the triggering response is mainly due to the rapid activation of soil microorganisms through mineralization of biomass C. Microorganisms that adopt this survival strategy, may have a rapid reaction in the utilization of fresh substrates.