Changed by fire: linking carbon and energy fluxes by microbial decomposition of soil organic matter after frequent forest burning events

Frequent burning by wildfires and its induced dry–wet cycles pose increasing threats to soil organic matter (SOM) stability. Yet, their interactive effects on microbially-driven decomposition and priming effects remain unclear from the combined perspectives of CO2 emissions and energy (i.e., heat) release. The relationship between microbial substrate use efficiency (SUE) and the calorespirometric ratio (CR, heat-to-CO2) remains unclear. Here, we investigated how long-term prescribed burning over 46 years, applied at two- (B2) and four-year (B4) intervals, interacts with dry–wet cycles (defined as cycles of soil drying and rewetting that reflect fire-induced moisture fluctuations) and influences SOM decomposition. Using the addition of 14C-labeled glucose coupled with calorespirometry, we tracked SOM-derived CO2 and heat fluxes and quantified the priming effect during a 28-day microcosm experiment. B4 increased SOM-derived CO2 efflux and heat release vs. unburned soils (NB), while B2 suppressed both. Dry–wet cycles increased SOM-derived CO2 but reduced SUE, favoring respiration over biomass synthesis. B4 under wet conditions produced higher primed heat release than NB, which was linked to the use of a chemically complex substrates (indicated by elevated CR). The decoupled primed CO2-heat indicated distinct thermodynamic pathways for carbon (C) and energy release. A positive CR–SUE correlation revealed a metabolic coordination between energy release and organic matter assimilation, suggesting that microbes allocate additional energy to sustain biomass growth even under elevated energetic costs. These findings demonstrate that low-frequency burning accelerated C loss via energy-intensive decomposition of SOM, while dry–wet cycles increased soil C vulnerability by uncoupling microbial growth and respiration. Integrating C and energy flux metrics provides novel insights into C resilience in soil under compounding climate disturbances, urging balanced fire management and C conservation in vulnerable ecosystems.