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