3.1. Soil carbon dioxide flux
Two-way analysis of variance showed that there was no significant difference in soil CO2 fluxes of land-use types, but significant difference at profiles (P<0.01), and there were some interaction effects between land-use types and profiles (P<0.01) (Table 2). At the soil profile of 0-200cm, with the depth increases, the annual average soil CO2 flux in crop land (CK) and Robinia pseudoacacia of different stand ages (experimental group) showed the downtrend. The soil CO2flux in shallow layers (0- 20cm) of Robinia pseudoacaciadecreased significantly with the increase of soil depth, the range of change was 0.12-3.91μmol m-2 s-1; while the soil CO2 flux of the deep layers (80-200cm) only changed slightly from 0.02 to 0.50 μmol m-2s-1 (Fig. 2A). From sep.2012 to sep.2013, there was a two peaks trend in the soil CO2 flux. From the diagram, greater changes have taken place in shallow layers (5-20cm) than that in deep layers, and the peak value always appeared in Dec.2012 and Jun.2013, while the peak value in Robinia pseudoacacia of CH10a appeared in Aug.2013 (Fig. 2A).