| 其他摘要 | Calcium is the most abundant alkaline earth element in Earth’s crust and is an essential macronutrient to all plants. There is growing support for the potential of Ca supply to significantly influence the structure and function of forested ecosystems. Acid rain still remains an important thread to many ecosystems in Southwest China. Guizhou province stands in the typical karst region and also is one of the most seriously acid rain areas in China. It has been suggested that in acidified soils, depletion of Ca may directly or indirectly impact forest productivity. Bedrock weathering and atmospheric deposition are the two primary sources of base cations (K+, Ca2+ and Mg2+) to forest ecosystems. Therefore, the key problem is to understand the relative inputs from these two sources and the cycling in the ecosystem. In order to understand forest health and to evaluate the effects of acid rain on forest ecosystems, it is important to identify and differentiate between various plant nutrient sources.
Forest ecosystem of karst areas is an open system, which exchange matter and energy with the environment rapidly and intricately. This study focuses on the effects of acid deposition on cation cycling in a typical small forested Karstic catchment in Guizhou Province. Sr isotope ratios were used as a tracer for understanding the transport process between the different cation pools: rock, soil, surface water, atmospheric deposition and plant. The samples of wet deposition, throughfall, surface water, vegetation, and soil were monthly collected. By making use of chemical mass equilibriums and strontium isotope geochemical approaches, the temporal and spatial variations of nutrient element and its strontium isotopic composition in the water cycling, the effect of acid deposition on base cation cycling and its strontium isotopic composition in the karstic forested catchment are discussed / investigated. The contribution of the two nutrition element’ sources (bedrock weathering and atmospheric deposition) to plants have been calculated on quantitative basis. Meanwhile, the study which use strontium isotope as a tracer of differential weathering of minerals by microorganism has been explored. The source, distribution and transferring laws of nutrient element in karstic forested ecosystem have been revealed, which could be used as an important scientific theoretical base for understanding the forest ecosystem environment quality appraising and environment protecting in karstic terrain. The main conclusions have been summarized as follow:
1. The pH value of rain water in Longli small forested catchment showed that the areas have been suffered by acid deposition. During the process from atmospheric deposition to surface water, the main ionic concentration of water solution has changed greatly. Acid rain that fall on the leaves leaches the nutrients from them. The effect of acid deposition on forest canopy mainly showed that the rainwater leach the base cations from leaf. Anions also have been leached from leaf other than NO3-. The strontium isotope composition of water solution in the small catchment is mainly contralled by carbonate weathering. The 87Sr/86Sr ratios of water solution range from 0.70716 to 0.71051. The Sr isotope composition of wet deposition, throughfall, and surface water vary each other showed that water solution change ions with environment in the water cycling.
2. The strontium isotope composition have changed among the soil exchangeable cations, soil carbonate associating cations, and whole soils in the same soil section, and that also have varied with soil depth. Vertical patterns of strontium isotope composition of different existing form cations in yellow soil have changed evidently than that in limestone soils. The variational characteristic of 87Sr/86Sr ratios of exchangeable cations in soil samples could be used to understand how the atmospheric deposition effected on the soil. The vertical distribution of 87Sr/86Sr ratios of exchangeable cations in yellow soil section show that the impact on soil of rainwater has reached up to 130 cm depth soil layer in this region.
3. The plants growing in yellow soils of karstic area have the characteristic of high concentration of Ca. The main character of nutrition element is Ca > K > Mg type. The mean concentrations of Ca, K and Sr present in plants in this region is higher than that of other terraneous plants in the world, but lower than that of the plants which grow in limestone soils weathering from limestone of other Karstic area of Guizhou. The concentration of Ca of plants ( contain bast, leaf, fine root, and stem ) growing in limestone soils is higher than that of the plants which grow in yellow soils, but the concentrations of Sr, Ba, and Mn behave in the opposite way. The nutrition element present in broadleaf is higher than that of conifer generally.
4. The sources of nutrient uptake by different plant species are different. The results based on mixing model of end-member 87Sr/86Sr ratios of rain water and exchangeable cations in soil samples show that the nutrition element which absorded by Lycopodium and Moss mainly derived from atmospheric deposition. The contribution of nutrition element coming from atmospheric deposition to Moss may come up to 88.66%, and that of Lycopodium may be 77.28%. Except for Lycopodium and Moss, about the nutrition element present in plants growing in yellow soils of karstic area were mainly (about 58.88%～85.64%) from soil weathering, and the atmospheric deposition sources only contribute relatively minor.
5. The strontium isotope composition of soil-exchangeable (plant-available) cations is the main factor that influences the isotope composition of plant. There have great difference among the 87Sr/86Sr ratios of bast, leaf, and fine root, and the 87Sr/86Sr ratios of fine root also vary with soil depth, which showed that there was difference in various sources’ nutrition element utilization efficiency between various plant organs. There are difference between the strontium isotope composition of fine root of Pinus massoniana and Betula luminifera which grow in limestone soils with soil depth, which maybe connected with their eco-physiological characteristics. While no evident changes of the 87Sr/86Sr ratios between old leaf and young leaf were observed.
6. Strontium isotope was used as a tracer to differentiate the sources of various base cations which were released by mineral weathering, and then quantify differential weathering of minerals by microorganism. The 87Sr/86Sr ratios of supernatants changed with the prolonging of culture time in the system mixing of apatite and calcite, which showed that the contribution of calcite and apatite weathering to the base cations’ sources has changed. The results based on mixing model of end-member 87Sr/86Sr ratios of supernatants suggested that, at the previous stage of weathering, the Ca in supernatants of Aspergillus niger groups comes from calcite weathering. The Ca in supernatants mainly sources from apatite weathering with prolonging the culture time as the weathering intensity of calcite by A. niger. has became weakly. On the other hand, the Ca in supernatants of Penicillium sp. groups mainly comes from apatite weathering at the previous weathering stage, and then the contribution of calcite weathering has been enhanced. |
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