| 其他摘要 | As one of the most severe eco-environmental problems in southwest China, the Karst rocky desertification has greatly influenced the regional sustainable development. Therefore, researches on the rocky desertification and associated impacts have attracted more and more attention from scientists of geosciences, ecology and environment, and it has currently become an important topic for environmental geochemical researchers to explore the mechanisms and processes of the Karst rocky desertification. As the rocky desertification is substantially a process of land degradation involved the decline in soil fertility and productivity, it is a major challenge to manage terrestrial ecosystems to keep the stability of original carbon pool and sequestrate more carbon in the Karst areas.
Peak-cluster depression is one of typical landscapes in the Karst region. The slopes are the most vulnerable components of this landscape in the Karst area, with a large number of exposed bedrock, the most serious land and vegetation degradation. This thesis selected the four typical slope lands with different vegetation (grass, brush – grass, brush and forest) in southwest Karst areas to focus on the mechanisms of carbon cycle in Karst areas. Surface soils and soils in profiles were systematically collected at the different topographic locations of four typical slope lands in Huan Jiang and Pu Ding. Soil dissolve organic carbon (DOC), soil organic carbon (SOC) in bulk soil and particle-size fractions, and stable carbon isotope (δ13C) were analyzed, fresh leaves of dominant plants and litters were also sampled for δ13C determination. The main results can be summed up as follows:
1. The SOC content and C/N ratio have significant differences between different topographic locations for grass slope and brush-grass slope; upslope soils have higher SOC and C/N ratios. But in the brush slope and forest slope, SOC content and C/N ratio showed little change trend with slope gradient.
2. DOC concentrations in soil profiles were mainly regulated by the inputs of organic matters, topographic locations and soil quality. In our study, DOC in upper soil layers (20cm) at different topographic locations of four slopes were surveyed, the DOC decreased as the order: forest slope>brush slope>brush-grass slope>grass slope. Meanwhile, comparisons were conducted between DOC and SOC in soil profiles of the grass slope, brush-grass slope and forest slope. The lower of DOC/SOC value was seen in profiles of upper slope area except that brush slope had lower slope locations (PG3), the lowest DOC/SOC value in the soil profiles (HC1, HG1, HG2), with all values lower than 0.2%, all these features showed obvious higher SOC but lower DOC concentrations. While the DOC/SOC value of soil profiles in the forest slope was higher, with all values higher than 0.3%, and change trend was similar.
3. Particle size distributions with depth are closely related to the evolvement of soil profiles. The soils in profiles(HC1、HG1 and HG2) of upper grass slope and brush-grass slope mainly derived from weathering materials of bedrocks and the accumulation of plants residues, thus with higher sand content. Its soil organic carbon was higher than other soil profiles down slope area, but organic carbon are mainly stored in the sandy soils with unstable carbon dynamic. While soil profiles down slope area were dominated by silt and clay fractions, although the soil organic carbon was relatively lower in soil profiles, organic carbon was mainly stored in the silt and clay fractions, the carbon pool was more stable. Because of the higher silt and clay content in soil profiles at the different topographic of the brush slope and forest slope, and change range in particle size of soil was smaller, organic carbon is mainly stored in the silt and clay fractions, which carbon pool was relatively stable.
4. δ13C of fresh leaves and litters of dominant plants were shown to be largely controlled by genetic factors (photosynthesis pathways). The δ13C of studied plants showed significant differences between each other; mean δ13C value of C3 plants was -28.2‰, and -12.7‰ for C4 plants. In addition, the leaves δ13C of the same plants from different slopes was less influenced by environmental factors.
5. According to the correlations between δ13C of litter and surface soil organic matter, it was significant for those of the brush-grass slope and forest slope (R2=0.93 and R2=0.77 respectively), but not significant in the grass slope and brush slope (R2=0.28 and R2=0.36 respectively). Because of different properties of soil profiles and vegetation, the δ13C of soil organic matter in grass slope and brush-grass slope got more positive than those of brush slope and forest slope, and δ13C variation in soil profiles at the different topographic locations was even more complicated. The δ13C value of soil organic matter in most profiles first increased with depth to a maximal value, then decreased gradually to keep stable at a certain depth. The variation of δ13C with soil depth reflected inputs and accumulation of plant residues, which could be a good indicator for SOC migration in soil profiles.
6. δ13CDOC and δ13CSOC of most soil profiles have similar variations. Moreover, the differences of △(δ13CDOC-δ13CSOC) indicated the correlation between DOC and SOC in soil profiles at different topographic locations, the δ13CDOC was higher than δ13CSOC in most soil profiles, and the lowest and highest △(δ13CDOC－δ13CSOC) were observed at the forest slope and the grass slope respectively.
7. Through analyzing the δ13C value of SOC in particle-size fractions of soil profiles at the different topographic locations of four typical slopes, decomposition degree and change of SOC were supposed to be main factors controlling the δ13Csoc variation in associated with all size fractions with depth in soil profiles, these results proved that the δ13C value in SOC associated with sand fraction was most sensitive to vegetation types, and could be a reliable indicator for new sources or sources due to slower decomposition of soil organic matter; and the δ13Csoc associated with silt and clay fraction indicated faster and higher decomposition rate of soil organic matter. Thus δ13Csoc in particle-size fractions of soil profiles was a useful parameter to evaluate the migration and decomposition degree of soil organic matter. |
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