Background and Aims <p>Plants release a variety of small molecules into soils through litters and root exudates, playing an important role in dynamic fraction of soil organic matter. However, these molecules’ specific roles in regulating microbial processes and carbon cycling remain poorly understood. This study aimed to investigate the degradation dynamics of the plant-derived pentacyclic triterpenoid friedelin and its effects on soil microbial activity and community composition.</p> Methods <p>A 480-day laboratory incubation experiment was conducted with an abandoned cropland soil amended with friedelin at 0, 5 and 25&#xa0;μg&#xa0;g⁻¹. The degradation kinetics was examined during the entire incubation period. Soil respiration rates, microbial biomass, metabolic quotient (qCO₂), and phospholipid fatty acid (PLFA) profiles were determined at the early stage.</p> Results <p>Friedelin followed exponential degradation kinetics, with half-lives of 68.7 and 189.1&#xa0;days for the low and high doses, respectively. Friedelin addition significantly stimulated soil respiration and increased the microbial qCO₂. The PLFA profiles revealed that friedelin enriched fast-growing bacterial groups and actinomycetes while increasing the ratio of cyclopropyl fatty acids to their monoenoic precursors, an indicator of physiological stress. Unexpectedly, the abundance of arbuscular mycorrhizal fungi biomarker increased, whereas that of saprophytic fungi was unaffected. The observed respiratory pulse was strongly correlated with the shifts in bacterial community and stress status, rather than increased general carbon availability.</p> Conclusion <p>The results indicated that friedelin, at environmentally relevant concentrations, may directly alter microbial community composition, induce physiological stress, and accelerate respiratory carbon loss. This highlights the importance of specific plant metabolites in mediating soil ecological processes and plant-soil feedbacks.</p>