Widespread Higher Soil Respiration Rates at Nighttime Than Daytime Across Global Forest Ecosystems

Soil respiration (R_s) is the second largest terrestrial carbon flux and therefore its temporal dynamics exert a significant influence on the soil carbon budget. While the seasonal and annual dynamics of R_s and its temperature sensitivity have been well documented, the diel R_s dynamics remain poorly understood. Earth system models (ESMs) typically assume a constant temperature response of R_s over the diel cycle, thereby predicting lower R_s at night than during the warmer daytime. Here, by analyzing extensive in situ R_s datasets from 36 global forest sites, we reveal an unexpectedly widespread pattern of higher nighttime than daytime R_s, which is likely driven by the hourly temporal lag between recent photosynthetic assimilation and R_s associated with the transportation of recent photosynthates to the roots. Moreover, applying daytime R_s-temperature relationships systematically underestimates nighttime R_s by 2.5% to 28.7% across 31 sites, due to the significant diel difference in the temperature response of R_s. However, ESMs predict lower R_s at night than during the day, largely resulting from the significant underestimation of nighttime root respiration dynamics. Our findings demonstrate significant diel R_s patterns across forest ecosystems, suggesting that daytime and nighttime R_s may respond distinctly to future climatic changes. Incorporating these diel dynamics is essential for improving predictions of terrestrial carbon-climate feedbacks under global warming.