Soil functional carbon fraction accrual in temperate forests is linked to understory herbs, soil nutrients and...

Enhancing stable soil organic carbon (SOC) storage is vital for climate change mitigation. This study challenges the tree-centric paradigm in forest carbon management by investigating the relative roles of trees and understory herbs in driving SOC sequestration in temperate forests of Northeast China. Analyzing 720 soil samples from a 7.2 ha experimental forest, we measured oxidizable SOC fractions and tested the hypothesis that the understory herb layer is a primary driver of SOC accrual, mediated by soil properties and microbial communities. Results strongly supported our hypotheses. Plots with dense, tall herbs exhibited significantly higher levels of active and passive carbon fractions (increases of 7%-16%, amplified to 21%-45% when accounting for soil nutrients, physiochemistry, and water-holding capacity), whereas tree size showed no significant effect. Soil nitrogen was the strongest predictor of SOC variation. Herbs intensified the positive SOC-nitrogen relationship and were positively associated with beneficial soil conditions (e.g., near-neutral pH), contrary to the weak or negative correlations observed for trees. Structural equation modeling revealed that herbs exerted significant direct and indirect positive effects on carbon fractions, while the effects of trees were nonsignificant. Metagenomic analysis identified two contrasting microbial phyla groups: "positive-SOC" phyla (e.g., Thaumarchaeota, Planctomycetes) associated with herbs and high SOC, and "negative-SOC" phyla (e.g., Chloroflexi, Gemmatimonadetes). These findings underscore the critical, underappreciated role of the understory herb layer in SOC sequestration, mediated through soil nutrient enhancement, soil acidity, water retention, and shifts in microbial communities. Forest management strategies aiming to maximize carbon storage should prioritize herb layer conservation alongside tree layer considerations.