4.2. Contribution of CO2 flux in deep layers to
soil-atmosphere interface
The contribution of soil CO2 flux at different layers to
the total CO2 flux at the soil-atmosphere interface
exist some differences. The contribution is always evaluated with
average proportion of soil CO2 flux at certain layer to
soil-atmosphere interface. Nevertheless, with temporal and spatial
change, the factors affecting the CO2 flux change, and
there are dynamic changes in CO2 flux of each layers. In
this study, variance contribution of proportions was introduced for
dynamic evaluation,and this method can further reveal the dynamic
process in the contribution of CO2 flux at profiles.
Davidson et al. (Davidson & Trumbore, 1995) took study in forests and
cultivated land of Amazon and found that soil CO2emissions at depth of 1 m accounted for 70%-80% of total soil
emissions at soil-atmosphere interface; Gaudinski’ studies showed that
Soil CO2 fluxes within shallow layers of 15 cm accounted
for 63% of total soil respiration in temperate forests(Gaudinski,
Trumbore, Davidson, & Zheng, 2000); Davidson et al. (Davidson E A,
2006) found that the contribution of CO2 flux at in the
O-horizon soils to total soil respiration was about 40% -48% in the
broadleaved mixed forest of Massachusetts. Compared with the previous
research results, our study focuses on the contribution of soil
CO2 flux at deep layers (< 80 cm) to the total
CO2 flux. The static evaluation results show that The
contribution rate of CO2 flux in Robinia
pseudoacacia of stand ages (CH10a, CH20a, CH30a, CH40a) at deep layers
to soil-atmosphere interface is generally larger in growing seasons, and
the difference among different stand ages is not obvious. The soil
CO2 flux of at deep layers accounts for 17.76%-21.86%
of the total CO2 flux at soil-atmosphere interface. To
sum up, that phenomenon may be due to the rich soil organic carbon
content at deep layers, combined with the results of soil microbes and
roots. Therefore, in the study of carbon emissions at soil-atmosphere
interface,we cannot follow the traditional understanding and ignore the
impact of CO2 flux deep layers just owning to its low
content. According to the results of dynamic evaluation,the variation
of contribution in growing season was lager than that in non-growing
season, and the contribution rate in Robinia pseudoacacia of
CH10a was 27.46% in the growing season, which was larger than that in
other stand ages, showing a certain heterogeneity. Similar results have
been reported that, Pumpanen (Pumpanen, Ilvesniemi, & Hari, 2003)
believeed that the contribution of CO2 flux at bottom
soils in winter to the total flux is greater than that in summer, while
Risk (Risk, Kellman, & Beltrami, 2008) proved that the contribution
rate at deep soil was larger in the later growing season, all of which
corroborated the results of our study to a certain extent, but there is
also a certain heterogeneity. The reasons for this phenomenon may be
that: thick soil layer was formed due to its unique soil development
process in the loess hilly region, the hysteresis of the seasonal
temperature change, the dry soil layer and temporary dry layer formed by
water deficiency and provisional water deficiency have a great influence
on the soil CO2 flux at the deep soil layer. The
heterogeneity of the contribution in Robinia pseudoacacia of
CH10a in the just confirms the fact that intensification of organic
carbon mineralization induced by the rhizosphere stimuli in agricultural
land and abandoned arable land (Shahzad et al., 2018).