Significance levels: ***: p < 0.001; **: p< 0.01; *: p < 0.05.
Since forest age and soil depth also contributed to building microbial community composition, partial Mantel tests were performed to estimate the independent contribution of each environmental factor after removing the effects of forest age and soil depth. Partial Mantel test results showed that FI (r = 0.11, p < 0.01) and HIX (r = 0.23,p < 0.01) were environmental factors significantly associated with bacterial community composition (Table 2) and soil SOC (r = 0.11, p < 0.05) but their effects were apparently much smaller; FI was the key to influence the distribution of fungal communities (r = 0.14, p < 0.05) and other environmental factors were not significantly influenced.
Linear regression analyses showed that the diversity of bacteria and fungi all increased significantly as DOM quantity increased. Diversity of fungi all declined as FI increased (r = 0.32, p = 0.058; Fig. S3e) and bacteria increased as HIX increased (r = 0.71, p< 0.05; Fig. S3c). This indicated that fungal communities were more diverse when DOM was more plant-dominated in origin than those microbial sources dominated and increasing humification of DOM significantly enhanced bacteria diversity.
4. Discussion