Post-fire δ¹⁵N enrichment and C:N shifts reveal decoupled trajectories of litter and soil following a prescribed burn

Prescribed fire is used to reduce fuel loads in forestry, but its impact on litter and soil properties requires further understanding. We measured pH, carbon and nitrogen concentrations, C:N ratio, and stable isotope composition (δ¹³C, δ¹⁵N) in litter and surface soil before, one day after, and one year after a low-severity prescribed burn in a Mexican montane forest. Because prescribed burns primarily combust surface litter while limiting soil heating, we expected rapid litter responses and delayed soil responses. One day after the burn, litter pH increased from 4.9 to 8.4, δ¹⁵N increased from -3.7‰ to -0.3‰, and the C:N ratio decreased from 44 to 21. One year after the burn, litter δ¹⁵N decreased toward pre-burn values (-1.5‰), while the C:N ratio increased to 84, nearly double the initial value. In soil, pH did not differ significantly one day after the burn (5.1 to 5.2). Soil δ¹⁵N increased from -0.2‰ to 1.6‰ one day after the burn and remained enriched one year after (2.0‰). Litter and soil δ¹³C showed no significant changes across the burn chronology.δ¹⁵N and C:N revealed decoupled trajectories of litter and soil and can be used as sensitive indicators of fire effects in managed forests.