| The hillslope region of Loess Plateau is one of the serious soil erosion regions in our country. soil erosion have been a certain controlling soil loss after Grain to Green Project in these region, resulting that soil carbon sequestration effect has been increased. Vegetation will also absorb in CO2 in the atmospheric through photosynthesis to decreasing CO2. However, vegetation also breathe, leading to elevated atmospheric CO2, increasing soil CO2 emission finally. A little Changes will significantly increase soil organic carbon storage in slope fields’ landscape covered by forest vegetation, and converting cultivated land into forests induced changes of vegetation cover will significantly increase soil organic carbon storage in the sloping land landscape. However, the spatial change of soil CO2 emission and its control mechanism are poorly understood and can thus lead to further uncertainties in the quantitative estimations of soil carbon sequestration in these reforested areas. A typical planted forest hillslope(250m) was selected to study the temporal and spatial variation of soil CO2 emissions to atmosphere and its controlling mechanism in ecological forest full-hillslope in the Loess Plateau. We measured soil CO2 emission rate using LI-8100 soil automated Soil CO2 flux system during 2012 to 2013 and analyzed the relationship of soil CO2 emission rate with soil temperature(2cm、5cm), soil moisture content, slope gradient, SOC stock, 137 Cs inventory and root density. To provide the scientific basis for further understanding of the Loess Plateau organic carbon turnover and estimation of terrestrial ecosystem carbon balance. And the research contents and results are as follows:(1) Slope gradients between 4 slope positions and the hilltop(reference)reached a significant level(p<0.05), the changes are: upper slope(31.14±1.30°) >shoulder slope(22.77±0.41°) > middle slope(22.19±2.20°) > lower slope(15.14±0.07°) > hilltop(reference)(1.75±0.36°), upper slope is largest; Which in addition to shoulder slope and middle slope had no significant difference, the rest of each two slope position had significant difference(p<0.05). The slope vegetation cover percentage was tree(40.64±3.26) >shrub(29.72±3.18)> grass(25.88 ±2.10) > bare land(3.76±0.52). Spatial variation analysis of re-forests in different diameters of root density is only <1mm along the direction of the slope was significantly decreased(p<0.05). and root density of <1mm respectively of all root density 59%, 53%, 22%, 32%and 33%in hilltop(reference), shoulder, upper, middle and lower. The remaining of four diameter root density had no significant difference in the different slope positions(p>0.05).(2) The 137 Cs inventory in different slope positions in the hilltop(reference) 、lower slope and shoulder slope 、 upper slope and middle slope showed significant difference(p<0.05), compared with hilltop(reference), 137 Cs inventory in upper slope and middle slope were decreased by 46% and 29%; 137 Cs inventory of shoulder slope and lower slope area were increased by 88%and 52%. It showed that upper of the hillslope is serious soil erosion, soil erosion also occurred middle slope, however shoulder slope and lower slope were soil accumulation, especially shoulder slope was the most. The spatial changes of SOC stock was not similar with137 Cs inventory, it was no significant difference between other slopes except upper slope(p>0.05). The change regularity of SOC stock was: upper slope(68.08±3.01 t/ha) > shoulder slope(59.29±1.70 t/ha) > middle slope(55.35±2.96 t/ha) > lower slope(52.21±2.96 t/ha) > hilltop(reference)(49.03±0.97 t/ha). The most was upper slope, and the lowest was lower slope.(3) Seasonal variation regularity of 4 different slope positions and hilltop of soil CO2 emission rate were basically the same in different months between 2012 and 2013, and it showed that a “single peak” trend, reached the highest values in June – August of each year. And soil CO2 emission rate of the same slope in different months reached significant level(p<0.05). The slope of the average daily change was 1.83 μmol/(m2·s). Overall of hillslope, 4 different slope positions and hilltop slope of soil CO2 emission rates were similar in the years 2012- 2013 annual seasonal variation, the rules was: summer(6-8 months) > autumn(9-11 months).> spring(3-5months). The seasonal variation of soil CO2 emission and soil temperature(2 cm、5 cm) showed a significant exponential relationship, and with soil moisture content showed a linear positive correlation in this study. it also showed that soil temperature in 2 cm soil temperature and 5cm respectively can to explain soil CO2 emission rate of 16%~ 61% and 21%~ 64%. This study showed that soil temperature sensitivity of Q10 in between 1.58~2.18 in yangjuangou small watershed of Loess Hilly Gully, and Q10( 5 cm) >Q10( 2 cm), it would gradually increase with soil depth. Soil moisture can explain temporal changes in soil CO2 emission rate of the area of the 3%~ 17%, And through the soil temperature and soil moisture content of the two factor model Rs = a+b × T+c × W and Rs = a + b ×(T × W), Rs = a+b× T+c× W could better explain the two factor influence on the changes of soil CO2 season, and it could explain the seasonal variation 28%~58%, this study showed that the soil moisture limits the seasonal variation of soil CO2 emission and soil temperature. and soil temperature was more influence changes of soil CO2 emission than soil moisture content.(4) The result showed that the spatial patterns of soil CO2 emission rate were similar among Spring, Summer and Autumn, the average soil CO2 emission rates of the three seasons decreased as following order: hilltop(reference)[2.51±0.07 μmol/(m2·s)] > shoulder(2.19±0.17 μmol/(m2·s)]>lower [1.88±0.12 μmol/(m2·s)]> middle [1.71±0.09 μmol/(m2·s)]>upper [1.62±0.12 μmol/(m2·s)]. We found that soil CO2 emission rate were significantly correlated with slope gradient(p<0.01) and 137 Cs inventory(p<0.01), while, were significantly correlated with soil moisture, soil temperature(5 cm) and SOC stock only in summer(p<0.01). No significant regression relationship was found between soil CO2 emission and root density. These results suggested that soil erosion and deposition processes induced by the change of topographic slope are the main factors controlling the spatial variation of soil CO2 emission rate on the loess plateau ecological forest slope, and it should be considered in the quantitative evaluation of effectiveness of soil carbon sequestration by Grain to Green Project. |
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