Fig. 4: Indicators of changing microbial carbon processing in organic and mineral soils.

Individual circular points represent June or July soil measurements from two watersheds, four landscape positions, and up to five replicate cores per landscape position each year (n = 40 points per year) within one horizon type (Oi/Oe, Oa/A, or surface mineral soil). Triangle points are the mean (± standard error) of all 40 measurements within a year. Individual circular point color shows the soil respiration period (pre- or post-breakpoint) for a given landscape position; point color overlaps in some cases because four landscape positions and watersheds are grouped within a year. Best-fit lines (for visualization only) show simple linear regression for each pre- and post-breakpoint time segment. Stars indicate linear mixed model ANOVA p-value for change in each parameter over time in the post-breakpoint phase only, where NS = p > 0.10, * = p < 0.10, ** =  p < 0.05, and *** = p < 0.001 (Supplementary Tables 2 and 3). a Microbial biomass carbon (MBC) (chloroform-fumigation 10-d incubation); b Microbial biomass C:N ratio; c Microbial heterotrophic mineralization rate (aerobic 10-d incubation); d Biomass specific respiration rate (mineralized CO₂ normalized to MBC).