Rapid recovery of soil respiration during tropical forest secondary succession on former pastures

Tropical forests are responsible for the highest rates of soil respiration globally. As the natural regeneration of secondary forest becomes increasingly widespread in the tropics, there is a need to understand how pasture-to-forest conversion and secondary forest succession impact patterns and drivers of soil carbon dioxide (CO₂) fluxes. This study examined changes in soil respiration rates and associated drivers (soil temperature, gravimetric moisture content, soil organic carbon (C), and forest floor mass) in replicate age classes across a tropical secondary forest chronosequence including pasture and forests ranging in age from 7 to over 80 years since establishment. Soil CO₂ fluxes were highest in the pasture plots (10.0 ± 0.85 µmol m⁻² s⁻¹) and declined by nearly two-fold during the early pasture-to-forest transition. Soil CO₂ fluxes remained fairly consistent across the forest successional sere. Soil respiration was primarily influenced by soil temperature across the chronosequence, while soil moisture and substrate availability mediated the impact of temperature on soil CO₂ fluxes. The rapid decline of soil CO₂ fluxes during pasture-to-forest conversion was surprising and has not been reported in previous studies, most of which have focused on fluxes in different stand ages after forest establishment. Our results suggest that soil temperature may be a key driver of soil CO₂ fluxes during secondary tropical forest regrowth on pasture lands, potentially due to significant shifts in microclimate conditions during the early stages of forest establishment.