Effects of Warming and Precipitation on Soil CO₂ Flux and Its Stable Carbon Isotope Composition in the Temperate Desert Steppe

The stable carbon (C) isotope of soil CO2 efflux (delta(CO2e)-C-13) is closely associated with soil C dynamics, which have a complex feedback relationship with climate. Three levels of warming (T0: ambient temperature (15.7 degrees C); T1: T0 + 2 degrees C; T2: T0 + 4 degrees C) were combined with three levels of increased precipitation (W0: ambient precipitation (245.2 mm); W1: W0 + 25%; W2: W0 + 50%) in order to quantify soil CO2 flux and its delta(CO2e)-C-13 values under nine treatment conditions (T0W0, T0W1, T0W2, T1W0, T1W1, T1W2, T2W0, T2W1, and T2W2) in desert steppe in an experimental beginning in 2015. A non-steady state chamber system relying on Keeling plots was used to estimate delta(CO2e)-C-13. The temperature (ST) and moisture (SM) of soil as well as soil organic carbon content (SOC) and delta C-13 values (delta C-13(soil)) were tested in order to interpret variations in soil CO2 efflux and delta(CO2e)-C-13. Sampling was carried out during the growing season in 2018 and 2019. During the experiment, the ST and SM correspondingly increased due to warming and increased precipitation. CO2 flux ranged from 37 to 1103 mg m(-2)center dot h(-1), and emissions peaked in early August in the desert steppe. Warming of 2 degrees C to 4 degrees C stimulated a 14% to 30.9% increase in soil CO2 efflux and a 0.4 parts per thousand to 1.8 parts per thousand enrichment in delta(CO2e)-C-13, respectively. Increased precipitation raised soil CO2 efflux by 14% to 19.3%, and decreased delta(CO2e)-C-13 by 0.5 parts per thousand to 0.9 parts per thousand. There was a positive correlation between soil CO2 efflux and ST and SOC indicating that ST affected soil CO2 efflux by changing SOC content. Although the delta(CO2e)-C-13 was positively correlated with ST, it was negatively correlated to SM. The decline of delta(CO2e)-C-13 with soil moisture was predominantly due to intensified and increased diffusive fractionation. The mean delta(CO2e)-C-13 value (-20.2 parts per thousand) was higher than that of the soil carbon isotope signature at 0-20 cm (delta C-13(soil) = -22.7 parts per thousand). The difference between delta(CO2e)-C-13 and delta C-13(soil) (Delta(e-s)) could be used to evaluate the likelihood of substrate utilization. C-13 enriched stable C pools were more likely to be utilized below 20 cm under warming of 2 degrees C in the desert steppe. Moreover, the interaction of T x W neither altered the CO2 emitted by soil nor the delta(CO2e)-C-13 or Delta(e-s), indicating that warming combined with precipitation may alleviate the SOC oxidation of soil enriched in C-13 in the desert steppe.