Ash-driven changes in topsoil characteristics following understorey wildfires in Eastern India’s deciduous forests

Understorey forest fires occur frequently within and beyond India. However, despite their ecological ramifications, such events are often overlooked, and research on fire impacts on soil properties and functions remain scarce. Therefore, this study explores the immediate depth-specific effects of understorey forest fires on soil properties in the dry deciduous forests of the Rarh region in the state of West Bengal in eastern India. 12 fire-affected sites were identified using Suomi National Polar-Orbiting Partnership near real-time fire alerts and were visited immediately after fire outbreaks. Direct fire behaviour monitoring at three sites revealed that understorey fire temperatures in this region range from ∼500 °C to ∼1100 °C, with spread rates between ∼8 m hr −1 and ∼13 m hr −1 . At each of the 12 field sites, soils were sampled from burnt patches and adjacent unburnt controls at three depths (0–5, 5–10 and 10–20 cm) during the 2024 fire season, which reigned from February to May. Following requisite laboratory and statistical analyses, significant ( p < 0.05) incremental changes were observed, particularly in the uppermost layer (0–5 cm), in pH (+16%), electrical conductivity (+82%), oxidisable organic carbon (+36%), organic matter (+36%), available N (+42%) and K 2 O (+74%), coupled with lower bulk density (−5%). The observed responses primarily reflect ash inputs derived from incomplete combustion of surface organic residues. This distinguishes such understorey fires from the more intense pyrogenic impacts observed in other regions dominated by highly flammable vegetation, such as coniferous forests and grasslands. Therefore, under low-intensity, short-residence fire regimes (∼60–100 s per 10 cm reference length), soils are only altered at shallow depths, indicating the need for soil sampling at minute intervals to properly capture the changes. Importantly however, despite the ash-driven soil fertility amelioration at burnt areas, the longer-term risks of recurrent fires and soil/nutrient loss through surficial erosion can render these landscapes susceptible to potential degradation, and in turn undermine forest sustainability.