Variations and controlling factors of soil CO2 release at daytime and nighttime scales in the loess hilly regions of China

Soil carbon dioxide (CO₂) release is a critical ecosystem process affecting regional and global carbon cycles. Currently, one of the key uncertainties in projecting carbon-climate feedback is the ongoing poor representation of the deep and nighttime soil CO₂ release. Using CO₂ probes at hourly intervals in the Robinia. pseudoacacia plantation in the loess hilly regions of China, this study explored the relationship of soil respiration between daytime and nighttime and the discrepancy in the influence of climate, vegetation, and soil properties on soil respiration at the 0–10, 10–50, and 50–100 cm soil depth. We estimated that the cumulative CO₂ release at 0–100 cm soil depth reached 688.6 g·m⁻²·year⁻¹, including a 29.1 % relative contribution from the 10–100 cm soil depths. This outcome showed the necessity for accurate quantification of deep soil CO₂ release. We also revealed that the cumulative CO₂ release was similar between daytime and nighttime throughout four seasons at the 0–100 cm soil depths. This result demonstrated that soil CO₂ release can be predicted based on daytime measurements. Soil temperature < 0℃ was not identified as a primary driver, which only explained 1 %–4% of the variation in soil respiration. Meanwhile, the temperature sensitivity of soil respiration decreased by 1.3–1.8 times when soil temperatures were < 0°C compared to when soil temperatures were > 0°C. Thus, using the correlation model based on soil temperature to predict soil respiration might introduce slight inaccuracies in outcomes when soil temperatures are < 0°C. Soil respiration is intimately associated with soil temperature, soil organic carbon content, root biomass, and leaf carbon content; the cumulative contributions of climate, vegetation, and soil properties to soil respiration were 12 %–18 %, 18 %–30 %, and 41 %–50 % during daytime and 12 %–25 % 24 %–28 %, and 40 %–46 % during nighttime at soil depths of 0–10, 10–50, 50–100 cm. Additionally, Structural Equation Modelling implied that soil moisture and temperature directly affected soil respiration during the daytime, and air temperature and relative humidity acted as indirect factors during the nighttime. Clarifying the cumulative soil CO₂ release relationship between the daytime and nighttime could help predict the soil C cycle with high precision within various climates in forest ecosystems.