Evaluating the ‘triggering response’ in soils, using 13C-glucose, and effects on dynamics of microbial biomass

Very small inputs of labile substrates can cause several more times CO2–C to be rapidly evolved from soil than was contained in the added substrates. This effect, termed the triggering response, does not currently have a conclusive mechanistic explanation. In this work 13C-labeled glucose was added (10 μg glucose C g−1 soil) to two Chinese soils, one from a mulberry plantation and another from a grassland soil. An immediate and significant increase in CO2–C evolution occurred from 0 to 72 h after glucose addition in both soils and, after 7 days of incubation. It was equivalent to 149.2% and 195.4% respectively of the C added as glucose. The dynamics of CO2–C, glucose mineralization, ATP and microbial biomass C (biomass C) were monitored in the amended and nonamended control soils after glucose addition (0–3, 3–5, 5–8, 8–24, 24–72 and 72–168 h). The most rapid increase in cumulative triggered CO2–C, glucose mineralization and ATP concentrations occurred within 3–5 h and the largest changes in concentrations occurred within 72 h. During this period, biomass C concentrations decreased by about 10% in both amended soils. The dominant source of triggered CO2–C in the first 24 h was glucose, whereas from 24 to 72 h, the mineralization of biomass C and soil organic C (SOC) dominated in both soils. All microbial indicators recovered by the end of the incubation (168 h). The overall taxonomic composition of the soil bacteria was little influenced, but co-occurrence patterns of bacterial communities, analysed using network analysis based on statistically significant correlations, showed that co-occurrence associations in bacterial networks increased during the triggering response.