Plant functional traits can affect biogeochemical processes through leaf litter by influencing community composition and the activity of soil microorganisms. Specifically, plant secondary chemicals and structural compounds are known to be recalcitrant to soil microbes. However, interactions between plant secondary compounds and soil properties at the community level across a wide range of environments and forest types remain poorly understood.
In this study, to examine interactions between plant traits and soil properties, including soil microbial communities, we analyzed relationships between the community weighted mean (CWM) of 11 plant functional traits (PFTs) and soil physico-chemical and microbial properties at 0-5 cm soil depth across 18 forest plots representing four forest types within the Monitoring Site 1000 network across the Japanese archipelago.
We measured leaf C, N, P, lignin, tannin, phenolics, thickness, toughness, leaf mass area (LMA), leaf dry matter content (LDMC) and leaf neutral detergent fiber (NDF) for all species occurring in each plot. PCA based on CWM of PFTs showed that PC1 (44.9%) represented a gradient in species dominance from higher N, P, and lignin to higher LMA, leaf toughness, and thickness across forest sites. Notably, dominant species in deciduous broadleaved forests showed both high leaf N and lignin content. In PCA of soil and microbial properties, PC1 (49.2%) represented a gradient in soil C, N and microbial biomass, while PC2 (28.9%) represented a soil pH gradient. Regarding correlations between CWM of PFTs and soil properties, CWM of LMA showed a negative correlation with soil pH and positive correlations with soil C:N ratio, fungal-to-bacteria ratio (F:B), saturated-to monounsaturated fatty acid ratio (Sat:Mono), and Gram-positive-to-Gram-negative bacterial ratio (GP:GN). On the other hand, CWMs of leaf N and lignin showed negative correlations with soil C:N ratio, F:B ratio and Sat:Mono ratio and a positive correlation with soil pH. Other secondary and structural compounds showed no significant correlations with soil properties.
Our findings suggest that structurally robust leaf traits support fungal-dominated microbial communities and conservative soil carbon dynamics, while the co-occurrence of high leaf N and lignin in deciduous broadleaved forests overrides classical decomposition trade-offs and shifts soil systems toward bacterial-dominated, low C:N conditions.