4.3. Analysis on factors affecting soil CO2 flux
at different layers
Climate-driven losses of soil carbon are currently occurring across many
ecosystems, with a detectable and sustained trend emerging at the global
scale (Bond-Lamberty, Bailey, Chen, Gough, & Vargas, 2018). As the
major driving factors of soil CO2 production,
temperature and moisture vary vertically within the soil profile
(Davidson E A, 2006), soil temperature rises slowly from the surface to
the bottom in spring and begins to cool down from the surface (Subke et
al., 2003). Soil organic carbon (SOC) is one of the key indexes for
assessing soil quality. The content of soil organic carbon decreased
with soil depth increasing and is significantly different at vertical
profiles(P <0.01), and changes in SOC may have a huge
potential impact on global climate. (Wang, Fu et al. 2010). Furthermore,
related studies have shown that soil temperature and moisture explain
the temporal and spatial variation of soil CO2emissions. Similarly, changes in soil organic carbon significantly
increase soil CO2 flux, and there was a strong positive
correlation between CO2 flux and SOC (Mande et al.,
2015).
Some scholars believe that the average Q10 of forests in
the world is 1.5(Luo, Wan, Hui, & Wallace, 2001), and the
Q10 value of the forest in the north temperate zone was
between 0.9 and 2.2 (Gulledge & Schimel, 2000). In our study,the range
of Q10 in Robinia pseudoacacia of different stand
ages was 1.245±0.077 to 2.121±0.404, and Q10 value at
different layers was significantly different (P<0.01). With
the increase of soil depth, the Q10 at different layers
increased, and the Q10 (2.121±0.404) at deep layer (200 cm) was
significantly higher than that(1.245±0.077) at surface layer. A similar
phenomenon observed in Georgia, USA, was that the Q10increased with increasing the depth of the soil layer. and
the researchers believed that this phenomenon may be caused by a
decrease in soil temperature as the depth of the soil
increases(Pingintha, Leclerc, Beasley, Zhang, & Senthong, 2010), which
may reflect root growth and root input at oligorganic layer has a
relatively greater important effecct than that at O-horizon(Davidson,
Savage, et al., 2006). And higher Q10 at deep layers may
also be related to factors such as humidity limitation, nutrient
availability and so on(Davidson, Janssens, & Luo, 2006; Graf et al.,
2008). Some other scholars have found that, in a deep warming experiment
in mineral soil, all depths responded to warming with similar
temperature sensitivities, driven by decomposition of decadal-aged
carbon. Whole-soil warming revealed a larger soil respiration response
than many in situ experiments and models (Pries, Castanha, Porras, &
Torn, 2017). Our study further confirmed that: under the external
environment disturbance, the same temperature change occurs in deep
layers as shallow layers, deep layers revealed a larger soil
CO2 flux response than shallow layers. Therefore, the
disturbance occurred at deep layers should be close concerned in future
human activities and natural environment improvement.
The response mechanism of moisture to soil respiration is complicated,
and the change of moisture in soil surface was generally larger than
that in deep layers (Davidson E A, 2006). Therefore, soil
CO2 flux at different layers will change due to
fluctuations in moisture. In Kog.Ma experimental site of Thailand,
Hashimoto et al. (Hashimoto et al., 2007) found that the
CO2 flux at different layers varied with the change of
moisture, and the CO2 flux of surface soil was larger in
rainy seasons than that in dry seasons, while there presented the
opposite trend at deep layers. Davidson et al. (Davidson E A, 2006)also
found that the release rate of CO2 in the surface layer
and O-layer of mineral soil was significantly correlated with
moisture(P<0.001), but the correlation between the upper and
middle layers of A-layer was vary low. This indicates that the effect of
alternate drying-wetting on soil CO2 release only occurs
at the O-layer. In our study, the changes of soil moisture inRobinia pseudoacacia of stand ages or at different layers were
significant. There was certain quadratic function relation between soil
CO2 flux and moisture(Fig6.), due to temperature and
matrix and other factors at profile of (20cm- 80cm) lying between
surface layer and deep layer, the response of moisture to soil
CO2 flux was relatively slow compared to the surface and
deep layers. The reasons of above phenomenon may be that: destruction of
soil aggregates accelerated by near-surface water replenishment leads to
increase of matrix availability, and then which leads to enhance of
microbial activity, all this stimulates CO2 emissions.
At deep layes, soil CO2 flux, under the stimulation of
water replenishment, the dominant mechanism affecting
CO2 flux may shift from ”substrate supply” to ”microbial
stress”, and there presents a distinct feedback (J. Wang, Liu, Chen,
Liu, & Sainju, 2015).
In this study, soil CO2 flux increased with the increase
of soil organic carbon at the lower layers. while it decreased at
near-surface layers. Related studies suggested that soil organic carbon
concentration decreased with increasing of soil depth in arid and
semi-arid loess hilly area, and there was a significant difference in
vertical profiles (P <0.01) (Y. F. Wang, Fu, Lu, Song, &
Luan, 2010). Based on the theory of Michaelis equation (Moorhead &
Weintraub, 2018), the concentration of organic carbon substrate used
for mineralization reaction at the lower layer was not saturated, and it
still had a positive feedback on mineralization rate. At the
near-surface layers, in addition to the normal organic carbon
sequestration process, the forest floor litter had a great influence on
the physic-chemical environment, and the activities of microorganisms
and roots were greatly changed. The feedback mechanism of organic carbon
substrate on the CO2 flux became a far more complex, and
thus soil organic carbon substrate showeds a certain negative feedback
effect.