3.2. Contribution of soil CO2 flux in deep profile to soil-atmosphere interface
In the growing season, the average proportion of CO2flux at deep layers (80-200 cm) to the total CO2 flux at the soil-atmosphere interface in Robinia pseudoacacia (CH10a, CH20a, CH30a, CH40a) and crop land was 11.14%、21.71%、27.43%、23.42% and 24.00%, respectively. while that was 8.50%、13.17%、23.83%、12.67% and 19.33% in non-growing seasons. The proportion in the growing season was discrete, but the proportion in non-growth season is relatively concentrated. This indicated that the proportion of CO2 flux at deep layers fluctuated greatly ( Fig. 3A).
It is a dynamic process for the CO2 emission from each section of the profils to the soil-atmosphere interface. Therefore,the static assessment method  which take the average proportion of CO2 fluxes at deep layers as index system may overestimate or underestimate contribution of CO2 flux in deep layers to soil-atmosphere interface. In order to accurately assessment contribution of CO2 flux in deep layers to soil-atmosphere interface, we took accumulated variance contribution of proportions as index system dynamic assessment. During the whole season,accumulated variance contribution of proportions at the deep layer (80-200cm) in Robinia pseudoacacia (CH10a, CH20a, CH30a, CH40a) and crop land was 9.74%, 23.61%, 21.81 %, 22.92% and 24.42%,respectively (Fig. 3B). The contribution of CO2flux at deep layers to the total CO2 flux at soil-atmosphere interface increased with with the increase of restoration time.