Differential responses of soil CO2 dynamics along soil depth to rainfall patterns in the Chinese Loess Plateau

Abstract

Soil surface carbon dioxide (CO₂) efflux not only originates from topsoils, but also significantly involves contributions from deeper soil layers. Soil surface CO₂ efflux significantly fluctuated with rainfall patterns in arid and semiarid regions. However, how soil CO₂ dynamics respond at different soil depths to varying rainfall patterns remains largely unclear. To address this gap, we continuously monitored soil CO₂ concentrations, temperature, and moisture content at 10 cm, 50 cm, and 100 cm depths in situ under cropland and orchards located in the semiarid Loess Plateau over a full year. Rainfall events were meticulously recorded, categorizing them into light (<10 mm), moderate (10 mm–40 mm), and heavy (>40 mm) to discern their impact on soil CO₂ dynamics. Specifically, soil CO₂ flux was not affected during light rainfall. Moderate and heavy rainfall decreased soil CO₂ flux at 0–10 cm by an average of 70% and 83%, respectively. This decrease was associated with reduced gas diffusivity across rainfall patterns. For instance, heavy rainfall reduced gas diffusivity by an average of 83% and 53% at 10 cm and 50 cm soil depths, respectively. Furthermore, soil CO₂ concentrations slightly dropped as soil temperature decreased at 10 cm depth during light rainfall. Soil CO₂ concentrations at 10 cm and 50 cm depths initially decreased by up to 15% and subsequently increasing by up to 52% during moderate and heavy rainfall. This response was likely influenced by temperature reductions and subsequent rises in moisture content, with a hysteretic response of soil CO₂ concentrations to temperature. The rapid increase in soil CO₂ concentrations was mainly due to a substantial decrease in gas diffusivity. Notably, heavy rainfall induced a delayed increase in soil moisture content at 50 cm depth and a significant decrease in CO₂ concentration by 16% at 100 cm depth. A substantial decrease in soil CO₂ concentrations in deep soil layers was primarily related to decreased soil temperature. Additionally, the observed soil CO₂ dynamics were partly attributed to biotic factors (microbial biomass carbon and root density) mainly on cropland, but mainly abiotic factors (soil organic carbon and bulk density) under orchards. Overall, these results suggest that reduced gas diffusivity triggered by increased soil moisture content in topsoils and weakened biological processes caused by decreased soil temperature in deep soils typically drive the differential responses of soil CO₂ dynamics to rainfall patterns.