Carbon uptake by a microbial community during 30-day treatment with 13C-glucose of a sandy loam soil fertilized for 20 years with NPK or compost as determined by a GC–C–IRMS analysis of phospholipid fatty acids

Abstract

To investigate the uptake by the microbial community of easily decomposable exogenous organic C and the proportion of this organic C remaining in soils under long-term fertilization schemes, 13C-glucose was supplied to arable soils (aquic inceptisol) following a 20-year (1989–2009) application of compost (CM) or inorganic NPK (NPK), along with a control (no fertilizer). Phospholipid fatty acids (PLFAs) were used as biomarkers for actinobacteria, bacteria and fungi. Gas chromatography–combustion–stable isotope ratio mass spectrometry (GC–C–IRMS) was used to determine the incorporation of 13C into individual PLFAs. The concentrations of soil microbial PLFAs significantly (P < 0.05) increased in all three soils after the addition of 13C-glucose. Over a 30-day incubation period, the highest PLFA concentrations were on day 7 (control) or day 15 (NPK and CM) for bacteria, and on day 30 for both fungi and actinobacteria. The added 13C-glucose was incorporated into bacterial PLFAs first, whilst an increase of 13C in fungal and actinobacterial PLFAs was measured on day 7 and 15, respectively. The mean amounts of 13C in bacterial, actinobacterial and fungal PLFAs in CM-treated soil during the 30-day incubation period were 0.589, 0.030 and 0.056 μg g−1 soil, respectively, which were significantly (P < 0.05) higher than levels measured in the NPK and control soils. Among the bacterial groups, the amount of 13C in Gram-positive (G+) bacteria over the entire incubation ranged from 0.326 to 0.440 μg g−1 soil in the CM scheme, which was significantly (P < 0.05) higher than levels detected in the NPK and control regimes. In contrast, 13C concentrations in monounsaturated PLFAs (aerobic microorganisms) in the CM-treated soil were 0.030–0.045 μg g−1 soil, which was significantly (P < 0.05) lower than in the NPK schemes. The proportion of glucose-derived 13C remaining in soils was ranked as follows: CM (53%) > NPK (41%) > control (28%) after 30 days of incubation. Easily decomposable exogenous organic C was thus more effectively maintained under the CM regime, primarily because, after 20 years, CM had altered the microbial community by reducing the ratio of aerobic to anaerobic microorganisms whilst increasing levels of G+ bacteria in soil compared to the control and NPK soils. This study aids our understanding of the transformation and maintenance of easily decomposable organic C in soil over long-term fertilization regimes.