differences in the chemical composition of unburned and burned
SOM using 13C-NMR (Supplementary Fig. S1). Approximately 35%
of unburned and burned SOM was comprised of O-alkyl-C
(Supplementary Fig. S1). Alkyl-C was the second most abundant
chemical functional group, accounting for approximately 20% of
SOM (Supplementary Fig. S1). Despite having no detectable differences
in initial chemical composition, burned SOM (i.e., burned
SOM and underlying unburned SOM collected from the burned
site) overall had slightly, but significantly, lower moisture
than unburned SOM (P ¼ 0.048, overall ANOVA; Table 1;
Supplementary Fig. S2).
Although we did not detect differences in the initial chemical
composition of SOM using 13C-NMR spectroscopy, we found some
evidence that post-fire changes in SOM had a negative effect on
microbial decomposition. For example, after 24 months of
decomposition, burned SOM lost 18.2% less mass than unburned
SOM (P ¼ 0.009, post hoc ANOVA; Supplementary Table S2; Fig. 1b).
In addition, the overall ANOVA indicated that there was a signifi-
cant abiotic environment by SOM origin interaction (P ¼ 0.049,
overall ANOVA; Table 1). Specifically, we observed that burned SOM
lost less mass than unburned SOM when both were decomposing at
the unburned site (Fig. 1C). In addition, there was a strong effect of
SOM origin on soil microbial biomass. Burned SOM had signifi-
cantly lower microbial biomass than unburned SOM, and this
pattern was consistent across each sampling date (P < 0.001, overall
ANOVA; Table 1; Fig. 4B). However, we found no main effects or
interactions of SOM origin on microbial respiration (Table 1).
3.3. Microbial community effects
There were no significant effects of microbial community origin
on mass loss, respiration, moisture content, or soil microbial
biomass (Table 1).