Shallow soil depth and low soil moisture constrain soil pCO2 on hillslopes: Insights from field experiments and model comparisons

Abstract

The partial pressure of CO₂ (pCO₂) in soil is a key parameter that affects both soil CO₂ efflux and rock weathering processes. However, there is a lack of systematic studies of soil pCO₂ and its driving factors in hillslope areas with a thin soil cover. We compared the results of a nearly three-year (2017–2019) monitoring study of hillslope soil pCO₂ in four experimental plots (cultivated cabbage, natural grass, broadleaved forest, bamboo forest) in a subtropical karst trough valley in Southwest China. We found that high soil pCO₂ occurred in the plots dominated by woody vegetation, while a higher soil temperature and soil water content (SWC) occurred in the two herbaceous plots. In addition, the deepest soil depth (∼40 cm) had higher pCO₂ than the shallower soil depth (∼20 cm). We also found that SWC was the most important factor driving soil pCO₂ changes (62.25 %, 54.94 %) in the two experimental plots under woody vegetation, whereas soil temperature was the main driver (91.46 %, 98.13 %) of soil pCO₂ changes in the two plots with herbaceous vegetation. We applied several different models to simulate changes in soil pCO₂ in field plots close to the experimental plots and found that these previous models need to incorporate soil depth as a controlling factor to generate accurate pCO₂ results. The application of a modified soil pCO₂ model showed that previous soil pCO₂ simulations at the rock-soil interface on global hillslope and karst hillslope areas may be overestimated by nearly 79.0 % and 65.3 % respectively. These findings have potentially significant implications for future soil pCO₂ simulations at regional and global scales.