Diurnal Freeze-Thaw Cycles Modify Winter Soil Respiration in a Desert Shrub-Land Ecosystem

Abstract

Winter soil respiration (R_s) is becoming a significant component of annual carbon budgets with more warming in winter than summer. However, little is known about the controlling mechanisms of winter R_s in dryland. We made continuous measurements of R_s in four microsites (non-crust (BS), lichen (LC), moss (MC), and a mixture of moss and lichen (ML)) in a desert shrub-land ecosystem northern China, to investigate the causes of R_s dynamics in winter. The mean winter R_s ranged from 0.10 to 0.17 µmol CO₂ m⁻²·s⁻¹ across microsites, with the highest value in BS. Winter Q₁₀ (known as the increase in respiration rate per 10 °C increase in temperature) values (2.8–19) were much higher than those from the growing season (1.5). R_s and Q₁₀ were greatly enhanced in freeze-thaw cycles compared to frozen days. Diurnal patterns of R_s between freeze-thaw and frozen days differed. Although the freeze-thaw period was relatively short, its cumulative R_s contributed significantly to winter R_s. The presence of biocrust might induce lower temperature, thus having fewer freeze-thaw cycles relative to bare soil, leading to the lower R_s for microsites with biocrusts. In conclusion, winter R_s in drylands was sensitive to soil temperature (T_s) and T_s-induced freeze-thaw cycles. The temperature impact on R_s varied among soil cover types. Winter R_s in drylands may become more important as the climate is continuously getting warmer.