Impact of fire frequency and elevation on soil carbon fractions, nitrogen dynamics and physicochemical properties in Himalayan Chir Pine forests

Given the rising vulnerability of forest carbon sinks to climate-intensified disturbances, this study evaluates how varying fire frequencies viz. High (HFZ), medium (MFZ) low (LFZ) and negligible fire zone (NFZ), and elevation gradients viz. Lower Shiwalik Chir pine forest (E 1 ) and upper Himalayan Chir pine forest (E 2 ) shape soil organic carbon fractions, nitrogen pools and physicochemical properties. Soil carbon fractions varied significantly with fire frequency, where very labile carbon (VLC) increased as fire frequency decreased and non-labile carbon (NLC) dominated high fire zones. Labile carbon in the NFZ and recalcitrant carbon in the HFZ significantly exceeded over other fire zones. LFZ exhibited higher carbon management index (CMI) and lability index (LI), whereas HFZ showed higher carbon pool index (CPI) and recalcitrance, underscoring the influence of fire regimes and elevation on soil carbon dynamics and forest productivity. Total nitrogen, ammoniacal nitrogen and nitrate nitrogen were significantly higher in upper Himalayan Chir pine forest ecosystems than in Shiwalik Chir pine forests, with the highest nitrogen concentrations generally recorded in mid-frequency fire zones (MFZ). Upper Himalayan Chir pine forests and low fire-frequency zones exhibited superior soil quality, characterized by higher nutrient concentrations (N, P, K, Mn) and organic carbon alongside lower bulk density, while fire-affected zones showed increased pH, K and electrical conductivity. These findings highlight that while frequent fires can promote carbon stabilization through recalcitrant carbon fractions, low-to-negligible fire regimes are essential for enhancing the lability index, soil fertility, active carbon pools and overall ecosystem productivity in Himalayan Chir pine forests. • Fire frequency and elevation strongly influenced soil carbon and nitrogen pools. • Very labile carbon increased under low-fire conditions. • Passive carbon pools dominated medium and high fire frequencies. • Upper Himalayan Chir pine forests exhibited better soil quality than Shiwalik Chir pine forests. • Low fire frequency improved CMI and lability, while high fire frequency increased recalcitrance.