Contrasting vertical patterns of lignin phenols in karst and non-karst forest soils

Plant-derived carbon is a major source of soil organic carbon (SOC), but how its content, composition, and controls vary with bedrock lithology and soil depth remains unclear. We quantified SOC and lignin phenols in surface (0-10 cm) and subsurface (10-20 cm) soils from karst and non-karst forests across southwestern China and examined their relationships with climatic, plant, soil, and geochemical attributes. SOC content did not differ between lithologies at either depth. However, total lignin phenol content was higher in non-karst surface soils but similar between lithologies in subsurface soils. Cinnamyl-type phenols were higher in non-karst soils at both depths, whereas syringyl- and vanillyl-type phenols were higher in karst subsurface soils. Total lignin phenol content increased with depth only in karst soils, driven by subsurface syringyl and vanillyl enrichment. Lignin phenol variation was more strongly related to soil and geochemical properties than to plant attributes. In karst soils, lignin phenols were negatively associated with temperature and precipitation at the surface but positively associated with total nitrogen and exchangeable calcium and magnesium in the subsurface. In non-karst soils, lignin phenols were negatively associated with precipitation only at the surface but positively associated with total nitrogen and reactive aluminum (hydr)oxides at both depths. These findings show that similar SOC contents can mask lithology-dependent differences in the composition and vertical distribution of plant-derived carbon and highlight the value of combining molecular biomarkers with soil geochemistry.