Fire enhances recent carbon inputs without mobilizing ancient soil carbon in Andean páramos

Abstract. Tropical Andean páramos store large amounts of soil organic carbon, including long-term carbon reservoirs whose response to fire remains poorly understood. Understanding whether fire promotes the mobilization of old soil carbon or primarily affects recently fixed carbon is critical for predicting carbon–climate feedback in these ecosystems. We evaluated the effects of fire history on soil carbon dynamics in a Colombian páramo in across gradients of elevation, soil depth, and time since last fire. To do this, we measured total carbon concentrations, radiocarbon signatures (Δ¹⁴C) of bulk soil organic matter, and the radiocarbon signature of heterotrophically respired carbon dioxide (Δ¹⁴CO₂) from controlled soil incubations. Bulk soil Δ¹⁴C ranged from −474 to +65 ‰, with progressively older carbon pools in deeper soils and at higher elevations. Burned soils exhibited more modern radiocarbon signatures than unburned soils (−111 ± 96 ‰ vs. −158 ± 104 ‰, respectively) and higher carbon (11.1 ± 5.8 % vs. 9.1 ± 3.8 %) concentrations. In contrast, Δ14CO2 ranged from 18.8 to 55.3 ‰ across all fire-history categories with no significant differences among burned and unburned sites, remaining consistently enriched in 14C relative to bulk soil organic matter and indicating that microbial respiration was dominated by relatively young carbon sources despite the presence of older carbon pools. Our results indicate that fires modify the age of soil organic matter primarily through the incorporation and cycling of young carbon rather than through the accelerated mineralization of old, stabilized soil carbon. Specifically, long-term post-fire vegetation recovery visibly narrows the isotopic disequilibrium between stored and respired carbon by modernizing the bulk soil pool. Similar radiocarbon values in deeper and more elevated soils suggest that long-term carbon reservoirs remain relatively resistant to individual fire events. However, increasing fire frequency and climate warming may alter the balance between carbon inputs and outputs, potentially affecting the long-term stability of carbon-rich tropical alpine soils.