Fig. 3
13C from the added 13C-cellulose was used to synthetize new cell membrane lipids in both unfrozen and frozen soil samples. Incorporation of the 13C label into cell membrane lipids as determined by 1D 1H analysis and a 1D variant of a 1H, 13C Heteronuclear Single Quantum Coherence NMR spectroscopy, in incubations at 4 °C (panel a) and −4 °C (panel b). The bars show the median contents and max and min ranges of 13C-enrichments (relative to natural abundance) in acyl chains at the start (day 1), mid-point (days 14 and 58 at 4 and −4 °C, respectively) and end (days 28 and 195 at 4 and −4 °C, respectively) of the incubations. We obtained signals for spectral regions assigned to the following hydrogen and corresponding C atoms (illustrated in the model phospholipid fatty acid chain): 2.28 p.p.m.-H-2 (blue circle and bars); 1.62 p.p.m.-H-3 (brown circle and bars); 1.28 p.p.m.-H-4 to H-(Ω-1) (gray elipse and bars) and 0.9 p.p.m.-H-Ω (green circle and bars). Differences in levels of 13C-labeling between the start and the end of the incubation were examined with a Kruskal–Wallis test, which does not assume normality of data
