Soil respiration provides the main carbon flux from terrestrial ecosystems to the atmosphere. Most soil respiration measurements are conducted during the plant-growing season. Yet small but continuous rates of ecosystem soil respiration during the winter can, in some cases, completely determine annual rates of carbon sequestration. Mid-latitude ecosystems are considered to be major terrestrial carbon sinks in the northern hemisphere. However, little is known about the winter soil respiration and its contribution to annual soil CO₂ flux in different mid-latitude ecosystems, which may yield inaccurate regional and global C budget predictions.

In this study, the contribution of winter soil respiration to annual soil respiration was measured for three sites (grassland: dominated by Artemisia sacrorum, Bothriochloa ischaemum and Themeda japonica; shrubland: dominated by Vitex negundo var. heterophylla; plantation: dominated by Populus tomatosa) in a mountainous area of north China.

Mean soil respiration rates ranged from 0.26 to 0.45 μmol m^-2 s^-1 in the winter (December to February), and between 2.38 to 3.16 μmol m^-2 s^-1 during the growing season (May-September). The winter soil carbon flux was 24.6 to 42.8 g C m^-2, which contributed 4.8 to 7.1% of the annual soil carbon flux. Based on exponential functions, soil temperature explained 73.8 to 91.8% of the within year variability in soil respiration rates. The Q₁₀values of SR against ST at 10 cm ranged from 3.60 to 4.90 among different sites.

In addition, the equation between soil respiration and soil temperature for the growing season was used to calculate the “modeled” annual soil carbon flux based on the actual measured soil temperature. The “measured” annual value was significantly higher than the “modeled” annual value. Our results suggest that winter soil respiration plays a significant role in annual soil carbon balance, and should not be neglected when soil ecosystems are assessed as either sinks or sources of atmospheric CO₂.

This work is financially supported by the “Strategic Priority Research Program” ofChinese Academy of Sciences (XDA05060600), the Twelfth Five-Year Plan of National Science and Technology Project in Rural Areas (2011BAD31B02), and CFERN & GENE Awards Funds on Ecological Paper. This paper has been published online in Plos ONE on 10 March, 2014.