## 4 Discussion
### Seasonal Variations in \(R_{s}\)
Because of the higher altitude of the study site, measurements could not be operated because of the snow cover and cold temperature from December to March. The spring in north China is short, and the air temperature rises rapidly in May. As a result, the \(R_{\rm s}\) in April was lower, while it obviously increased in May and June. In October, the temperature dropped sharply, resulting in the \(R_{\rm s}\) being significantly lower than in September. The soil moisture affected root growth, soil microbial activity, and soil metabolic activity, and therefore influenced \(R_{\rm s}\). Seasonal variations of \(M_{\rm s}\) at the study site were mainly affected by precipitation. Unlike the obvious seasonal variations in \(T_{\rm s}\), \(M_{\rm s}\) seasonal variations throughout the measuring period fluctuated between high and low levels. After the precipitation, \(M_{\rm s}\) increased rapidly, but then decreased until the next occurrence of precipitation because of water infiltration, soil evaporation, and plant transpiration. When compared to the relationship between \(R_{s}\) and temperature, that of \(R_{\rm s}\) and \(M_{\rm s}\) was relatively complex and uncertain because the response of \(R_{\rm s}\) to \(M_{\rm s}\) differed during different stages ([PERSON], 2013). Under dry soil conditions, the soil biochemistry metabolic activity became frequent as the soil water content improved. When \(M_{\rm s}\) reached 50%-80% of the soil saturated water content, the maximum soil microbial metabolic activity was reached and the \(R_{\rm s}\) rate achieved the largest value. As \(M_{\rm s}\) continued to increase, the lack of oxygen prevented aerobic respiration. Therefore, in a certain range of \(M_{\rm s}\), the relationship between the \(R_{\rm s}\) rate and \(M_{\rm s}\) was positively correlated, and when \(M_{\rm s}\) goes over a certain range, with the increase of \(M_{\rm s}\), \(R_{\rm s}\) decreased. Precipitation and drought could affect the relationship between \(R_{\rm s}\) and \(M_{\rm s}\), with rainfall increasing \(R_{\rm s}\) under drought conditions and reducing it under humid conditions. The \(M_{\rm s}\) in the study site was in the range of 35%-55%; therefore, there was virtually no stress of \(M_{\rm s}\) on \(R_{\rm s}\). The changes in \(M_{\rm s}\) in such a narrow range did not have a significant effect on \(R_{\rm s}\) rate during the measuring years. However, in July of 2009, \(R_{\rm s}\) was significantly lower than in the same period of other years. Specifically, the value of \(T_{\rm s}\), \(M_{\rm s}\), and \(R_{\rm s}\) during this period was 17.45 \({}^{\circ}\)C, 15.78%, and 4.45 \(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\), respectively, while in the same period of other measuring years, \(T_{\rm s}\) was around 18 \({}^{\circ}\)C, \(M_{\rm s}\) was around 45%, and \(R_{\rm s}\) was about 7 \(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\). It can be seen that \(T_{\rm s}\) value was close to each other, while the difference between \(M_{\rm s}\) in July of 2009 and during other years was about 30%, indicating that \(R_{\rm s}\) was inhibited by water stress in July of 2009.