Seasonal Evolution of Soil Respiration and Sources of Respirable Carbon in Three Forest Stands on the Loess Plateau of China

The litter and root systems of forest stands can influence soil organic matter content and are subject to the effects of soil temperature and moisture. Through the decomposition activity of soil microorganisms, a portion of the carbon stored in aboveground vegetation is transferred to the underground carbon pool, thereby impacting soil respiration. When we investigated the effects of various components and environmental factors on soil respiration, ¹³C served as an effective tool for analyzing their contributions. In this study, conducted in a region of the Loess Plateau, three forest stands' soil respiration (Quercus acutissima forest—QAF, Pinus tabuliformis forest—PTF, and mixed forests—MF) was examined. Both soil respiration rate (Rs) and ¹³C exhibited seasonal fluctuations linked to changes in surface soil temperature and moisture. The soil respiration rate of all forest stands decreased to below 1 μmol m⁻² s⁻¹ during winter. The average range of δ¹³C fell between −22‰ and −17‰. Over the course of four seasons, we monitored soil respiration and identified hydrothermal factors. The correlation between hydrothermal factors and CO₂ releases from soil respiration varied significantly across seasons among different forest structures (p < 0.001). Additionally, the contribution of litter to soil respiration was the main source and it was higher in autumn and winter, with a maximum of over 75%. This study holds significant importance for understanding the processes underlying the carbon sources of soil respiration.