Asymmetric and Symmetric Warming–Induced Stability of Organic Carbon in a Calcareous Soil

Seasonal asymmetric warming is a distinguishing feature of climate change that is not regularly considered when studying the effects of global change on terrestrial ecosystems. The responses of the stability of soil organic carbon (SOC) in calcareous soils to asymmetric and symmetric warming are not clear. A 5-yr warming experiment, using infrared radiators, was performed to evaluate the effects of symmetric and seasonal asymmetric warming on SOC stability in a calcareous soil in the Karst region, southwest China. The bulk soils were fractionated to density fractions (i.e., light and heavy fractions, separated by 1.7 g mL⁻¹ NaI solution) and then recalcitrant fractions with a combination of density fractionation and acid hydrolysis techniques. Soil organic C densities in the warming treatments (1.95–2.02 g kg⁻¹) were close to the control (nonwarming treatment, 1.94 g kg⁻¹). Warming negligibly affected organic C densities in the two density fractions and recalcitrant fractions, except for the recalcitrant C of light fraction (RC-LF). On average, the RC-LF density in the warming treatments was 18.18% higher than the control. Among the warming treatments, the RC-LF density in the symmetric warming treatment was significantly (P < 0.05) higher than the moderately, highly, and extremely asymmetric warming treatments. The findings reveal that short-term warming can enhance the recalcitrance of unprotected C in a calcareous soil. Symmetric warming potentially overestimates the recalcitrance of the unprotected C pool in the surface soil, compared with the projected asymmetric warming scenario. It may be related to the summer-autumn warming-induced increases in the turnover of SOC and biochemical recalcitrance of unprotected C.