Priming effect after glucose amendment in two different soils evaluated by SIR- and PLFA-technique

This study investigated the metabolic and structural effects of adding glucose to the top soils of a contaminated sandy Eutric Cambisol and an uncontaminated silty Haplic Chernozem during substrate-induced respiration (SIR) measurement. We hypothesized that glucose amendment causes microbial community shifts. To indicate changes of the microbial structure during SIR measurement, we have evaluated the microbial community structure using phospholipid fatty acid (PLFA) analysis on soil samples immediately before they were enclosed in SIR apparatus (Start), after the equilibrium of basal respiration had been reached (Con-0), 8 h later (Con-8), and on the other hand immediately after adding glucose (Glu-0), and 8 h after that (Glu-8).The accumulated PLFA content of Start, Con-0 and Con-8 was of the same order of magnitude with no significant differences among them in the contaminated sandy Eutric Cambisol. In contrast, PLFA-biomass of the Glu-0 sample was only 52% of that measured in the Start. Furthermore, the PLFA-biomass was reduced even more drastically to 20% compared to the original Start value in Glu-8. The reduction of PLFA-microbial biomass after glucose amendment was accompanied by the inverse reaction of basal respiration. The PLFA profiles were dominated by the group of saturated fatty acids in the case of Start, Con-0 and Con-8, but by unsaturated fatty acids in the Glu-0 and Glu-8. In contrast to these results, the uncontaminated silty Haplic Chernozem showed no significant differences between Start, Con-0 and Glu-0 but a 243% and a 274% higher PLFA content of Con-8 and Glu-8 compared to the Start, respectively.The findings of triggered metabolic activities indicate that the microflora of these soils is affected and that PLFA analysis reflects a shift in the soil microbial community after adding glucose. We hypothesized that this shift from slow-growing microbial oligotrophs with low substrate needs to fast-growing copiotrophs with high substrate demands might be caused by the glucose added. Structural differences of the microbial community before and after glucose amendment should be taken into consideration when interpreting the metabolic SIR results in future.

Research highlights▶ PLFA profiles proved to be sensitive indicators of changes in microbial community structure due to glucose amendment. ▶ The ratios of fatty acid fractions were considerable altered after glucose addition in both soil samples. PLFA profiles were dominated by the fraction of saturated fatty acids. ▶ The contaminated and sand-dominated Eutric Cambisol show after glucose amendment: (i) a reduction of microbial biomass and a inverse reaction of basal respiration rates; (ii) a decreased amount of saturated fatty acids of bacteria, Gram-positive bacteria and actinomycetes might be because these members of the microbial community show less resilience to substrate richness than others; (iii) no fungal biomarker (18:2ω6,9c). ▶ The uncontaminated silty Haplic Chernozem shows after glucose amendment: (i) an increased PLFA content after 8 h (Con-8). This might be due to the shift of oligotrophic to copiotrophic members of the microbial community because the nutrients might have been become more available based on higher soil moisture. ▶ Structural differences of the microbial community before and after glucose amendment should be taken into consideration in future when interpreting the metabolic SIR results.