Fig. 3: Soil-derived (i.e., pre-existing) carbon losses in the microcosm experiment.

a Path analysis showing the direct and indirect effects of the litter quality index (Litter quality) on soil-derived mineral-associated SOC (MA-SOC). Indirect effects of litter quality are mediated through the microbial physiological trait index. Numbers above and below paths represent standardized coefficients during early- and intermediate-stage decomposition, respectively, with significance levels indicated (*p < 0.1, **p < 0.05, and ***p < 0.01). Thickness and color of lines correspond to coefficient magnitude and direction, respectively. Total and indirect effects of litter quality on soil C formation are also summarized with standardized coefficients. b Linear relationship (±SE; n = 16) between the litter nitrogen index—i.e., the second axis of the litter quality PCA which correlated negatively with litter C:N and AUR:N (Fig. S1A)—and cumulative soil-derived respiration (CO₂-C) after 30 days (Early: R² = 0.59; P < 0.01) and 185 days of decomposition (Intermediate: R² = 0.32; P = 0.02).