Soil CO2 efflux along an elevation gradient in Qinghai spruce forests in the upper reaches of the Heihe River, northwest China

This study was conducted in six plots along an elevation gradient in the Qinghai spruce (Picea crassifolia Kom.) forest ecosystem of the Qilian Mountains, northwest China. Soil CO2 efflux over bare soil (Rs) and moss covered soil (Rs+m) were investigated from June to September in 2010 and 2011 by means of an automated soil CO2 flux system (LI-8100). The results showed that Rs ranged from 1.51 to 3.96 (mean 2.64 ± 0.72) μmol m−2 s−1 for 2010, and from 1.41 to 4.09 (mean 2.55 ± 0.70) μmol m−2 s−1 for 2011. The daily change trend of Rs resembled that of air temperature (Ta), and there was a hysteresis between Rs and soil temperature (Ts). The seasonal variations of Rs at lowlands (i.e., Plot 1, Plot 2 and Plot 3) were driven by soil moisture and temperature (Ta and Ts), while that at highlands (i.e., Plot 4, Plot 5 and Plot 6) were obviously affected by temperature. There were higher values at Plot 2 and Plot 6, which were caused by the interaction between soil moisture and temperature. In addition, soil CO2 efflux over moss covered soil (Rs+m) was 8.83 % less than that over bare soil (Rs), indicating that moss was another factor affecting Rs. It was concluded that Rs had temporal and spatial variations and was mainly controlled by temperature and soil moisture; the main determinants differed at different elevations; moss could reduce Rs.