Tracing Soil CO₂ Fluxes under Drying-Rewetting Cycles: Isotopic Insights from an Automatic Soil Incubation System

Climate change is expected to increase the intensity and frequency of droughts and heavy rainfall events globally, with significant consequences on the terrestrial carbon cycle. One of the most critical yet highly variable components of the carbon cycle is the soil CO₂ pulse triggered by precipitation events in dryland ecosystems. To examine the processes underlying the soil CO₂ pulse under changing precipitation patterns, we developed a unique Online Automatic Soil Incubation System (OASIS) that allows (1) accurate manipulation of drying and rewetting regimes; (2) continuous monitoring of soil CO₂ fluxes; and (3) identification of their isotopic sources (¹³C and ¹⁴C). Using OASIS, we investigated how normal and extreme drying-rewetting cycles (NDWC vs EDWC) influence CO₂ pulse emissions and their isotopic signatures from soils of the Loess Plateau, while controlling for total water input. Our results showed that EDWC induced a rapid peak in the CO₂ release rate within minutes, but this was offset by reduced emissions during the dry phase compared to NDWC. In addition, total CO₂ release was strongly influenced by CO₂ influx through dissolution, which was limited during the prolonged dry phase under EDWC. Isotopic data indicated that the CO₂ pulse that originated from substrates was derived from recent plant carbon input within minutes of rewetting and was potentially influenced by exchange with the inorganic carbon pool, followed by contributions from bulk SOC that may have persisted for hundreds to thousands of years. These findings underscore the importance of accounting for CO₂ pulses driven by substrate availability and different carbon sources, which is crucial for improving model predictions of soil CO₂ flux and carbon storage in drylands under changing precipitation patterns. We highlight the applications of OASIS in revealing how interacting climatic, biological, and physicochemical factors drive soil greenhouse gas emissions.