Effect of experimental warming on sources of soil respiration in a tropical forest

Soil respiration (Rs) is the largest terrestrial flux of carbon dioxide (CO2) to the atmosphere, with tropical forests contributing disproportionately to global Rs. Rising temperatures associated with climate change are expected to substantially alter tropical soil carbon (C) cycling by stimulating microbial activity and accelerating organic matter decomposition and influencing plant and root respiration. However, the extent to which warming alters the relative contributions of autotrophic (Ra) and heterotrophic (Rh) respiration remains poorly constrained. Distinguishing these sources is necessary to understand the mechanisms underlying their responses. Radiocarbon (14C) analyses provide a powerful approach for resolving respiration sources and assessing the age of respired C. Here, we used 14C measurements to examine how experimental warming alters source contributions at the Tropical Responses to Altered Climate Experiment (TRACE) in Puerto Rico, where infrared heaters increase understory and surface soil temperatures by 4°C above ambient conditions. Surface soil gas samples were collected for 14C analysis of Rs, soil incubations were used to constrain the Rh end-member, and atmospheric samples represented the Rₐ end-member. All gas samples were purified for CO2, graphitized, and analyzed by accelerator mass spectrometry. Soil respiration in control plots exhibited a modern radiocarbon signature, whereas warmed plots showed significantly higher Δ14C values, indicating increased contributions from decades-old C (“bomb C”). The Rₕ end-member also became significantly older under warming. Isotope mixing models revealed a pronounced shift in source contributions, with Ra decreasing and Rh approximately doubling under warming. These results indicate that increased temperatures enhanced microbial decomposition of older soil C, altering the balance between autotrophic and heterotrophic respiration. Such warming-induced shifts in respiration sources are not detectable from measurements of total CO2 fluxes alone and highlight the importance of source partitioning for assessing the vulnerability of tropical soil C under sustained warming. These findings also provide critical constraints for improving Earth system model representations of tropical soil C-climate feedbacks.