[Carbon Sequestration Characteristics Under Natural Vegetation Restoration in Ziwuling Area of the Loess Plateau].

The aim of this study was to analyze the carbon sink effect under natural vegetation restoration and the influence of changes in vegetation community characteristics on ecosystem carbon density in ecologically fragile areas of the Loess Plateau. In this study, the changes in carbon sequestration of a vegetation-soil system under eight successional stages[slope cropland, abandoned cropland for 10 years, abandoned cropland for 20 years, Sophora davidii (Franch.) Skeels., Betula platyphylla Suk., Pinus tabulaeformis Carr., Quercus wutaishanic Mary+P. tabulaeformis Carr mixed forests, and Q. wutaishanic Mary] in Ziwuling area over 150 restoration periods were investigated using the common method of spatial and temporal substitution. This study also discussed the relationship between changes in vegetation community characteristics and vegetation-soil system carbon density. The results showed that the community coverage of the investigated vegetation fluctuated from 85% in the slope cropland stage to 100% in the arbor stage. The number of species, Margalef index, Shannon-Wiener index, Pielou index, and Simpson index initially increased rapidly, then declined slowly until becoming stable, and reached a peak in the middle of the succession (B. platyphylla Suk.). The biomass and carbon density of vegetation components (above-ground biomass, below-ground roots, and litter) increased exponentially during the succession, i.e., increased slowly before B. platyphylla Suk. but increased significantly in B. platyphylla Suk. and P. tabulaeformis Carr.(P<0.05). The biomass and carbon density reached the maximum values of 27858.08 g·m-2 and 13232.51 g·m-2, respectively, in Q. wutaishanic Mary+P. tabulaeformis Carr mixed forests and tended to be stable in the late succession stage. Soil organic carbon density showed a power function relationship with vegetation restoration, with the greatest increase in the stages of abandoned cropland for 10 years and B. platyphylla, but no significant changes in the subsequent stages (P>0.05). In the early succession stage, the carbon density of the farmland ecosystem was the lowest (4395.70 g·m-2), whereas the other seven stages increased by 55.54%, 40.37%, 69.96%, 202.48%, 326.35%, 357.43%, and 351.07%, respectively, compared with the farmland ecosystem. Community coverage, Margalef index, Shannon-Wiener index, above-ground biomass, root biomass, and litter biomass were significantly positively correlated with vegetation-soil system carbon density (P<0.05). The carbon sink effect of long-term natural restoration in Ziwuling Region was significant, and the carbon density of the vegetation-soil system under interspecific competition tended to be stable in the late succession stage. Dynamic changes in the vegetation community structure and plant diversity during the succession process increased vegetation carbon density and soil carbon density. This study helps to clarify the carbon sink effect of natural vegetation restoration in ecologically fragile areas of the Loess Plateau and provides a theoretical basis for promoting natural forest conservation and achieving carbon neutrality.

[Carbon Sequestration Characteristics of Different Restored Vegetation Types in Loess Hilly Region].

Since 1999, the "Grain for Green" Program has been extensively implemented in the Loess Plateau region. This measure has largely been of concern not only for its contribution to soil erosion reduction but also for its effects on carbon sequestration. The aim of this study was to assess the carbon sequestration characteristics of different restored vegetation types in areas with severe soil erosion on the Loess Plateau and to compare the effects of restoration age and slope direction on the vegetation carbon sequestration. To evaluate the carbon density and composition characteristics of different ecosystem types, six typical vegetation types (including farmland, grassland, Hippophae rhamnoides Linn., Caragana korshinskii Kom., Robinia pseudoacacia L., and Populus davidiana Dode.) were selected in the Loess Hilly Region, i.e., Wuqi County and Zhidan County in Northern Shaanxi province, which is a typical area for the implementation of artificial vegetation restoration. The results showed that:① vegetation restoration in the semi-arid loess region had a profound impact on carbon sequestration. The carbon density of different vegetations, as well as different vegetation components including above-ground vegetation, below-ground roots, and litter, shared the same pattern as tree>scrub>grassland>farmland. The 0-40 cm soil layer of the farmland showed the lowest soil organic carbon density (1355.5 g·m-2), compared to which those of grassland, H. rhamnoides Linn., C. korshinskii Kom., R. pseudoacacia L., and P. davidiana Dode. were higher by 91.4%, 125.2%, 144.0%, 124.5%, and 232.6%, respectively. ② It was common in grassland, H. rhamnoides Linn., C. korshinskii Kom., and P. davidiana Dode. for the carbon density of different vegetation components as well as soil organic carbon density of different soil layers (0-5, 5-20, and 20-40 cm) to generally show an increasing trend with increased restoration age. ③ Slope direction had a significant impact on the vegetation carbon density only for H. rhamnoides Linn., C. korshinskii Kom., R. pseudoacacia L., and P. davidiana Dode., while showing the contrary for farmland and grassland. Soil organic carbon densities for sunny slopes were significantly lower than those for shaded slopes by 22.9%, 34.3%, 75.8%, 49.1%, 22.4%, and 69.4%, respectively, for farmland, grassland, H. rhamnoides Linn., C. korshinskii Kom., R. pseudoacacia L., and P. davidiana Dode. ④ Ecosystem carbon density varied significantly for different ecosystem types, among which farmland showed the lowest (2022.1 g·m-2), and grassland, H. rhamnoides Linn., C. korshinskii Kom., R. pseudoacacia L., and P. davidiana Dode. showed values higher by 48.7%, 152.8%, 125.1%, 166.3%, and 530.7%, respectively. The carbon density of each ecosystem component showed a pattern as follows:soil layer>above-ground vegetation layer>root layer>litter layer. Soil organic carbon constituted the main part of the ecosystem carbon density and accounted for 67.0%, 86.3%, 59.7%, 72.7%, and 56.5%, respectively, for farmland, grassland, H. rhamnoides Linn., C. korshinskii Kom., and R. pseudoacacia L. These results can provide an essential basis for scientific management of ecosystem carbon pools and promote ecological environment management on the Loess Plateau.