Variable responses of the soil microbial biomass to trace concentrations of 13C-labelled glucose, using 13C-PLFA analysis

13C-labelled glucose was applied to arable (Broadbalk NPK) and permanent grassland (Woburn Grassland) soils to investigate the response of the soil microbial biomass (SMB) to carbon (C) applied at trace concentrations. Phospholipid fatty acids (PLFA) were used as biomarkers for G+ve (odd-chained and iso/anteiso FA) and G-ve bacteria (mono-unsaturated and cyclic FA), actinobacteria (10-methyl-branched FA), fungi (octadecadienoic acid) and general membrane lipids [16:0,18:0]. Gas chromatography-combustion-stable isotope ratio mass spectrometry (GC-C-IRMS) was used to determine the incorporation of 13C into individual PLFA in two experiments: first, after application of a single concentration (15 mu g C g-1) of 13C-glucose over a time sequence (0, 8, 24, 48, 120 and 240 hours), and second after application at three concentrations (25, 83, 416 mu g C g-1 soil after 120 hours). 13C incorporation into PLFA over time was similar in both soils. However, in the permanent grassland soil, 13C incorporation was increased in actinobacteria PLFA at 120 hours when [16:0,18:0] was reduced. At 240 hours, 13C incorporation increased in [16:0,18:0] concurrently with a reduction in G+ve bacteria PLFA. Increasing glucose concentration caused different responses in the SMB of both soils. In the arable soil, all biomarker PLFA concentrations increased at all rates of application. In contrast, in the permanent grassland soil PLFA concentrations were similar to the control in all SMB groups, except G+ve bacteria after the greatest rate of application. However, the delta 13C values of the same PLFA indicated that uptake of applied 13C-glucose was proportional to the applied concentration in all groups of soil bacteria in both soils.