Geochemical Interactions and Stability of Organic Carbon in the Teak‐Planted Humid Tropical Soils of Southern Western Ghats, India

Soil organic carbon (SOC) stabilization is influenced by the chemical composition of humic substances as well as their geochemical interactions. Surface soil samples (20 cm) from the major teak‐planted zones of the Southern Western Ghats—Northern Foot Hills (NFH) (11° 52′18″ N 75° 52′39″ E; Inceptisol), North Central Laterites (NCL) (10° 52′57″ N 76° 38′40″ E; Ultisol), Southern Foot Hills (SFH) (9° 19′34″ N 77° 01′50″ E; Inceptisol), and Southern High Hills (SHH) (10° 07′20″ N 76° 37′89″ E; Inceptisol)—were used in this study to analyze the carbon fractions of SOC and chemical composition of humic acids (the dominant SOC fraction in soils). Carbon pool analysis indicates that a major portion of the SOC is stored in the relatively stable slow and passive organic carbon pools (> 60%) in all the teak‐growing zones, except NCL. The stability of the SOC was found to be partially attributed to the chemical recalcitrance of humic substances in these soils. FTIR spectra of the humic substances in these soils indicated a highly polymerized mix of organic molecules, predominantly aromatic, linked with phenolic or amine or long aliphatic chains. Besides chemical structure, geochemical interactions by way of physical protection in soil aggregates > 20 μm (43%–48% of the total organic carbon content) were found to be a predominant factor regulating SOC stability in these soils. Soils rich in micas and mixed‐layer minerals (SFH and SHH) were also found to retain a substantial amount of SOC through mineral linkages. Thermal sensitivity assessments by way of incubation studies at 4 different temperatures, viz., 25°C, 30°C, 35°C, and 40°C, for 150 days showed that activation energy (Ea) of carbon decomposition in these soils varied from 9.75 to 30.40 kJ mol −1 . Pearson’s correlation analysis indicated that Ea was positively correlated with passive carbon (0.630 ∗ ), pH (0.956 ∗∗ ), ΔpH (0.891 ∗∗ ), Ca content (0.676 ∗ ), and net ion equilibrium (0.685 ∗∗ ) and negatively with extractable Fe (−0.899 ∗∗ ) and Al (−0.843 ∗∗ ) contents in soil. The results present potential management strategies wherein the carbon storage and its stability could be improved by managing the factors regulating soil particle aggregation and organo‐mineral associations in these forested ecosystems.