Effects of Warming and Precipitation on Soil CO 2 Flux and Its Stable Carbon Isotope Composition in the Temperate Desert Steppe

The stable carbon (C) isotope of soil CO 2 efflux (δ 13 CO 2e ) is closely associated with soil C dynamics, which have a complex feedback relationship with climate. Three levels of warming (T0: ambient temperature (15.7 °C); T1: T0 + 2 °C; T2: T0 + 4 °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 CO 2 flux and its δ 13 CO 2e 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 δ 13 CO 2e . The temperature (ST) and moisture (SM) of soil as well as soil organic carbon content (SOC) and δ 13 C values (δ 13 C soil ) were tested in order to interpret variations in soil CO 2 efflux and δ 13 CO 2e . 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. CO 2 flux ranged from 37 to 1103 mg m −2 ·h −1 , and emissions peaked in early August in the desert steppe. Warming of 2 °C to 4 °C stimulated a 14% to 30.9% increase in soil CO 2 efflux and a 0.4‰ to 1.8‰ enrichment in δ 13 CO 2e , respectively. Increased precipitation raised soil CO 2 efflux by 14% to 19.3%, and decreased δ 13 CO 2e by 0.5‰ to 0.9‰. There was a positive correlation between soil CO 2 efflux and ST and SOC indicating that ST affected soil CO 2 efflux by changing SOC content. Although the δ 13 CO 2e was positively correlated with ST, it was negatively correlated to SM. The decline of δ 13 CO 2e with soil moisture was predominantly due to intensified and increased diffusive fractionation. The mean δ 13 CO 2e value (−20.2‰) was higher than that of the soil carbon isotope signature at 0–20 cm (δ 13 C soil = −22.7‰). The difference between δ 13 CO 2e and δ 13 C soil (Δ e-s ) could be used to evaluate the likelihood of substrate utilization. 13 C enriched stable C pools were more likely to be utilized below 20 cm under warming of 2 °C in the desert steppe. Moreover, the interaction of T × W neither altered the CO 2 emitted by soil nor the δ 13 CO 2e or Δ e-s , indicating that warming combined with precipitation may alleviate the SOC oxidation of soil enriched in 13 C in the desert steppe.