Effects of elevated root zone CO 2 on xerophytic shrubs in re-vegetated sandy dunes at smaller spatial and temporal scales
Actual CO
2
concentrations in soil
A total of 48 soil gas samples were taken every month during the 3-year study period.
In the control group, the average CO
2
concentrations in the root zones of C. korshinskii and A. ordosica were 659.504 ± 246.257 and 637.271 ± 334.098 ?mol/mol, respectively. And as shown
in Table 1, the elevated CO
2
concentrations in the CACE system of 700, 1,000, 2,000, 5,000, 10,000, 20,000, and
50,000Â ?mol/mol resulted in 0.069, 0.086, 0.152, 0.317, 0.554, 0.626, and 0.797% increases
in the soil profile. The actural CO
2
concentration in the soil profile increased sharply with the increasing CO
2
gradient from 700 to 10,000Â ?mol/mol, but when the CO
2
concentration in the cylinders was large than 10,000Â ?mol/mol, the increasing rate
of actural CO
2
concentration in the soil profile was not obvious, and finally the soil CO
2
concentrations would saturated. In addition, the magnitude of fluctuation decreased
with the increasing soil CO
2
concentrations, which also refer to the fluctuant soil CO
2
concentrations in the soil profile.
Table 1. CO
2
concentration in the steel cylinder and the actual CO
2
concentration in the soil (?mol/mol)
Pn, Tr, Gs, and WUE under different CO
2
concentrations
The relief valves of the CACE system were closed in 2010, as shown in Figures 1 and 2, the average Pn, Gs and Tr of the C. korshinskii and A. ordosica during the growing season (March 2010 to October 2010) had distinct seasonal variations
with the single-peak curve and the highest occurred at June. But for WUE, the curve
exhibited with a double-peak appearance, C. korshinskii reached its maximum in April and August and the WUE of A. ordosica reached its maximum in June and September. However, as shown in Additional file 1: Tables S1–S8, during the closed period of CACE system in 2010, the above measured
four physio-ecological parameters of C. korshinskii and A. ordosica at different pots have showed no significant difference (PÂ Â 0.05). However, when the relief valves turn on from January 2011, the elevated
root zone CO
2
affected the Pn, Tr, Gs, and WUE of C. korshinskii and A. ordosica greatly, and there have showed a significant difference between different soil CO
2
concentrations as seen in Additional file 1: Tables S1–S8 (P  0.05). In short time scales, such as in 2011, the Pn, Tr, Gs, and WUE of C. korshinskii and A. ordosica were all increased with evaluated root zone CO
2
concentrations under the threshold soil CO
2
concentration (0.554% for C. korshinskii and 0.317% for A. ordosica). But above the threshold, the values of each measured parameters were smaller than
those of the control groups which means that the high soil CO
2
concentrations would inhibited the plants growth and development. On the other hand,
in the 3Â year time scale, evaluated root zone CO
2
concentrations had a positive stimulation on plant growth, but after a period of adaptation,
they would return to the normal level although the parameters were still larger than
the control site, in addition, the difference between each CO
2
concentration gradients at different years was not significant (PÂ Â 0.05) But below the threshold, the growth of plants represented by the above four
measured parameters was all lower and they could not restored to normal levels. Specifically,
for C. korshinskii, before the threshold optimal soil CO
2
concentration (0.554%), each additional of 0.001% soil CO
2
concentration, the Pn, Tr, Gs, and WUE was increased by 2.1–31.3%, 0.04–0.5%, 0.8–6.2%,
0.9–3.8%, respectively, but after exceeding this threshold, the Pn, Tr, Gs, and WUE
was reduced by 5.4–15.4%, 0.07–0.2%, 0.8–6.2% and 0.2–0.7% for each additional of
0.001% soil CO
2
concentration. And for A. ordosica, before the threshold optimal soil CO
2
concentration (0.317%), each additional of 0.001% soil CO
2
concentration would lead to the Pn, Tr, Gs, and WUE increase by 3.2–41.5%, 0.03–0.2%,
2.1–23.7%, 0.02–0.8%, respectively, but after exceeding this threshold, the Pn, Tr,
Gs, and WUE was reduced by 0.2–1.2%, 0.006–0.07%, 0.09–0.5% and 0.01–0.06%.
Figure 1. The average Pn, Tr, Gs and WUE of C. korshinskii during the experimental period (2010–2013).
Figure 2. The average Pn, Tr, Gs and WUE of A. ordosica during the experimental period (2010–2013).
The phenophase of C. korshinskii and A. ordosica under different CO
2
concentrations
The elevated root zone CO
2
affected the phenophase of C. korshinskii and A. ordosica significantly as seen in Figures 3 and 4. When the CO
2
concentration in the soil was less than 0.554%, the phenological phase of C. korshinskii, including resurrection, leaf expansion, growing, leaf color change and leaf shedding
stages was advanced with increasing soil CO
2
concentrations, but when the CO
2
concentration in the soil was larger than 0.554%, the elevated root zone CO
2
would lead to a delayed phenophase comparing with the control groups. For A. ordosica as seen in Figure 4, this threshold of soil CO
2
concentration was 0.317%. Same conclusion was also obtained from the new twigs of
the both plants that the average annual length of the new branches of C. korshinskii and A. ordosica was 2.5 and 12.33Â cm, and then reached its maximum 3.55 and 18.16Â cm with increasing
CO
2
concentration until the threshold (0.554% for C. korshinskii and 0.317% for A. ordosica), but when exceeding the threshold value, they were lower than the normal plant growth
and maintained at a relatively low level 1.61–1.77 cm and 8.31–9.69 cm, respectively.
In addition, the phenophase for each treatment appeared almost simultaneously before
the leaf color change stage. The phenological phase of the both plants were advanced
or postponed for 1–4 days as the CO
2
concentrations increased. However, a significant difference was observed specifically
at the leaf shedding stages, and the phenological phase was advanced or postponed
for 1–6 days.
Figure 3. The difference of average phenodate every degree of soil CO
2
concentration in C. korshinskii from March 2011 to October 2013 (the vertical axis of the graph represents the advanced or postponed days of plant phenology, which
it was an integer value as 1, 2, 3 …, the dotted line was plant growing in the natural conditions, the upward means the plant phenology
was advanced, and conversely the downwards means plant phenology was postponed. The
horizontal axis represents the different CO
2
concentration gradient).
Figure 4. The difference of average phenodate every degree of soil CO
2
concentration in A. ordosica from March 2011 to October 2013 (Ditto).