3.3. Relationships between soil CO2 flux, temperature, moisture and SOC
The soil temperature was not significantly different in land-use types and profiles (Table 2). From Sep.2012 to Sep.2013, soil temperature reduced and then climbed up. where the change range of the temperature in shallow layers (5-20cm) was larger than it in deep layers (80-200cm), but this fluctuation of the deep layers presents an obvious hysteretic nature compared with shallow layers (Fig.2B). We used the exponential model to fit the relationship between soil CO2 flux (F) and soil temperature (T), and calculate the temperature sensitivity coefficient (Q10), significant correlation was found between CO2 flux and temperature(P < 0.01) (Fig.5). Sensitivity of soil CO2 flux to temperature in deep layers was generally higher than that in shallow layers.
Soil moisture was significantly different in land-use types and profiles (P<0.01), and and interaction effect existed between them (Table 2). The change trend of moisture in crop land was more complicated. On the contrary, certain difference appeared in moisture among Robinia pseudoacacia of stand ages. The moisture of shallow soil layers (5-20cm) decreased and then increased with the seasons, while a slow increase trend took place in deep soil layers (80-200cm). There was a quadratic function relation between soil CO2flux (F) and soil moisture (M) (Fig.6). On the basis of the coefficient of determination(R2), correlation between soil CO2 flux and soil moisture in shallow layer (5-20cm) was stronger than that in deep layer (80-200cm).
The soil organic carbon was significantly different in different soil profile depths (P<0.01). The organic carbon content in the deep layers (80-200 cm) was significantly lower than that in the shallow layer (5-20 cm). As the main substrate in the process of soil carbon emission, soil organic carbon (SOC) has an important impact on soil CO2 flux. Correlation analysis was carried out between soil CO2 flux and concentration of soil organic carbon in different land-use types and profiles (Table 3). The results showed that correlation at depth of 20cm and 140cm were significant (P<0.05), and it was extremely significant at depth of 200cm(P<0.01), but there was no significant correlation at depth of 5cm and 10cm.