4.1. Seasonal and vertical variations of soil
CO2 flux
Because of the temporal and spatial heterogeneity in plant and animal
species, activities and organic carbon matrix, soil CO2fluxes exist strong distribution pattern of vertical heterogeneity and
seasonal variation.The CO2 flux in soil vertical profile
reflects the production and transport of CO2 in the soil
and also provides information on the dynamics of soil
CO2 (Fontaine et al., 2007; Franzluebbers et al., 2001).
The soil profile of this study extends from the surface to the depth of
200cm. With the deepening of the soil depth, the soil
CO2 flux of crop land and Robinia pseudoacacia of
different stand ages is decreasing, and the CO2 flux inRobinia pseudoacacia (80-200 cm) at deep layers only changed
slightly (Fig. 2A), which was affected by the interaction of land-use
types and profiles (P < 0.01) (Table 2). However, the
CO2 flux at shallow layers (5-80 cm) has a large change,
and the trend is similar to that in previous studies (Davidson, Savage,
et al., 2006; Fang C M, 2005; Pumpanen et al., 2008). The difference of
CO2 flux at different depth is mainly attributed to the
relatively stable soil environment, lower content of soil organic carbon
(Fig. 3), and small changes in soil temperature and moisture (Fig2B,
Fig2C) at deep layers than shallow layers.
Due to the difference in temperature and moisture caused by seasonal
changes, the characteristics of the CO2 soil flux at
different profiles also presents certain easonal. Pumpanen (Pumpanen et
al., 2008) and Hashimoto (Hashimoto et al., 2007) found that soil
CO2 fluxes at different depths represented significant
seasonal, high in summer ,low in winter, and soil CO2fluxes are relatively high in shallow layers than deep layers. However,
Risk et al (Risk, Kellman, & Beltrami, 2002) found the opposite result,
the CO2 flux in deep layers show stronger seasonality
than it in shallow layers, which may be caused by land-use types and
vegetation patterns in the experimental sites. In our study, there was a
two peaks trend in the soil CO2 flux. The trend in
shallow layers was more obvious than that in deep layer, and the peak
value always appeared in Dec.2012 and Jun.-Aug.,2013 (Fig. 2A). The
occurrence of double peaks indicates that the seasonal variation of
CO2 flux could not be explained simply by changes of
temperature and moisture. The double peaks appeared in the growing
season and non-growing season, respectively. All of which can further
indicate that restrictive factors affecting soil CO2flux by vegetation root activity may be converted, and further studies
on root and microbial activity are needed to explain double-peaks
changes. The slight change range of CO2 flux at deep
layers may be attributed to the relatively stable temperature, moisture,
and the lower content of soil organic carbon.