| 其他摘要 | Due to the peculiar morphological structure and biological characteristics, mosses have been widely employed in indicating and monitoring environmental change and atmospheric deposition. The stable isotopes (e.g. 13C, 15N, 34S) are reliable tools in identifying the source and fate of elements in varying environments and understanding the relationships between plants and environment. By combining moss bioindicating method with isotopic technique, this thesis was mainly around a topic on using 13C and 15N in mosses for indicating environmental conditions and atmospheric N deposition. By investigating factors affecting moss isotopic signatures, the mechanisms of moss isotopic responses to environmental factors and N deposition were revealed. By using elemental contents and isotopes in epilithic mosses, and combining with photosynthetic pigments, N deposition at Guiyang area was systematically studied. This thesis has deepened the application of moss isotopes for monitoring environment and N deposition, and has provided geochemical references for the prevention and control of N pollutants at city area. Some significant innovations have been obtained.
1. Variations and implications of moss elements and isotopes under different habitats
Differences of water availability and light intensity will cause significant 13C variation of mosses even collected at the same site, and long-term retention of canopies on atmospheric deposition will lead to lower elemental contents and more negative isotopes (15N and 34S) in mosses, depending on canopy types and canopy thickness. Therefore, influences of habitats differences must be stressed when using moss isotopes for indicating regional environment and atmospheric deposition, elemental contents and isotopes of mosses distorted by growing conditions would possibly lead to wrong interpretation of environmental variation and atmospheric deposition, epilithic mosses at open sites were more suitable and reliable tools for bioindicating study. It is one of the innovations in this thesis to investigate the differences of bioindicating information in mosses by using isotopic methods, which has not really documented before. This work not only deepened the understanding of moss bioindication of environmental conditions and atmospheric inputs, but also contributed to sample selection in moss bioindication of atmospheric deposition.
2. Carbon and N contents and isotopes in new and old tissues of mosses in different areas
Mosses at Guiyang urban have higher C contents and more negative 13C than those at Mt. Gongga, showing that city mosses were more influenced by anthropogenic CO2 emission and mosses at Mt. Gongga might be related to its higher altitude and lower annual temperature. Higher moss N content and more negative 15N at Guiyang urban indicated higher N deposition, which was mainly derived from sewage NH3 release. New tissues have generally higher C and N than old tissues, but no significant isotopic difference was found, suggesting that no isotopic effect occurred during the physiological malfunction and elemental translocation in old moss tissues, but N in new tissues could quantify the level of N deposition more reliably, showing a level of 30.18 kg/ha/yr at Guiyang and 8.46 kg/ha/yr at Mt. Gongga. As a result, the responses of moss isotopes to environmental factors and N deposition were broaden in different areas. For the first time, this work discussed the influences of senescence on moss elements and isotopes, which has laid a foundation for further understanding of the isotopic fractionation during the senescence of nonvascular plants.
3. The spatial variation of atmospheric N deposition at Guiyang area
Through the relation between N deposition and moss N (y=0.052x+0.7325), the level of N deposition at Guiyang area could be quantified by the N contents (0.85% ~ 2.97%) of epilithic mosses. The general level of N deposition at Guiyang area (0.91 ~ 44.69kg/ha/yr) has exceeded the critical load for the vulnerable terrestrial ecosystems (5 ~ 10kg/ha/yr), the mean N deposition at urban area (29.21±6.17 kg/ha/yr) was consistent with the former value (30.18 kg/ha/yr) obtained in a urban site, which was higher than the critical load (15 ~ 20 kg/ha/yr) for acidic coniferous forest and deciduous forest protection. Besides, the level of N deposition (14.31±5.11 kg/ha/yr) in the rural area was consistent with the mean regional NHx deposition (14.3 kg/ha/yr). For the first time, the level and spatial variation of N deposition were quantified by N in epilithic mosses, which is important for further studying the ecological effects of atmospheric N deposition and helpful for the protection of the fragile ecosystems in Karst region.
4. The main sources and N form of atmospheric N deposition at Guiyang area
15N of epilithic mosses at Guiyang area (-12.50‰ ~ -1.39‰) exhibited a significant bi-modal distribution (-12‰ ~ -6‰ and -5‰ ~ -2‰), indicating there are two main sources for N deposition in the study area. More negative 15N for urban mosses (-8.87±1.65‰ within 5km and -8.59±2.28‰ at 5 km ~ 10km) mainly indicated NH3 released from city excretory wastes and sewage, while less negative 15N away from urban to rural area (-3.83±0.82‰ ~ -2.48±0.95‰, >15km) was mainly influenced by agricultural NH3. According to the 15N of rainfall and urban mosses, NHx-N apportioned about 76% of N in urban mosses while the contribution of NOx-N was about 23%, thus atmospheric N deposition at Guiyang area was dominated by NHx deposition. This study firstly found the regulation of moss 15N variation (more negative at urban than rural) around a city dominated by NHx deposition, which was opposite to the pattern (more positive at urban than rural) in cities dominated by NOx-N deposition. Besides, according to the mechanism of moss 15N variation with NHx-N/NOx-N ratio in atmospheric deposition, we recognized that variations of moss N and 15N at Guiyang area were actually controlled by urban-derived NHx and agriculture-derived NHx, thus further evidences for NHx deposition at Guiyang area could be obtained from moss N and 15N variations.
5. The deposition mechanisms and patterns of the main atmospheric N sources (NHx) at Guiyang area
Tissue N of epilithic mosses at Guiyang area decreased exponentially with distance from the urban center (y=1.5e-0.13x+1.26), while 15N increased logarithmically [y=2.54Ln(x)-12.227] with distance, revealing that the deposition of urban-derived NHx followed the pattern of point NH3 source characterized by an exponential decline with distance from the urban center, and Guiyang urban could be seen as a point NH3 source in the whole study area. However, in the rural area, moss N did not vary with distance and there was no significant difference between mean 15N values, reflecting the non-point source pattern of agriculture-derived NHx. By limit calculation method, it could be identified that N deposition was dominated by urban-derived NHx at sites within 17.2km from the urban center and by agriculture-derived NHx at sites beyond 17.2km. It could be indicated that the maximum distance of urban-derived NHx deposition averaged 41km from the urban center, the variation of urban-derived NHx deposition with distance from the urban center could be modeled as y=56.272e-0.116x-0.481 at Guiyang area. Besides, the varying gradients of moss N and 15N with distance were different along different directions, implying that the atmospheric transport of urban-derived NHx was not equipotent around the city, which was mainly related to the differences of geographic condition, vegetation coverage, wind direction and urbanization in different directions. For the first time, the deposition of urban-derived NHx was identified as a point-source model through moss bio-monitoring in a city where N deposition was dominated by NHx, it was one of innovations in the thesis to reflect the distribution and mechanisms of NHx deposition from evidences of moss N and 15N.
6. Implications of tissue C and 13C variations of epilithic mosses at Guiyang area
Tissue C of epilithic mosses (34.47% ~ 52.76%) at Guiyang area decreased exponentially with distance from the urban center (y=9.206e-0.042x+38.024) and strongly correlated with tissue N, showing the enhancing effect of atmospheric N inputs on tissue C. Moss 13C (-30.69‰ ~ -26.96‰) increased significantly with distance [y=0.674Ln(x)-30.03], indicating that the influence of anthropogenic CO2 emission decreased from the urban to rural area. Moreover, moss 13C became more negative with increasing C and N content, good correlation between moss 13C and 15N was observed, showing the photosynthesis and N assimilation were correlated in epilithic mosses, higher N deposition at Guiyang urban had caused greater 13C discrimination by enhancing the photosynthesis and C fixation of mosses. This thesis firstly found that higher N deposition in city area had enhanced the CO2 fixation ability of epilithic mosses, moss 13C could be an indicator of city anthropogenic CO2 and the biological effect of N deposition.
7. Other innovations
For deepening the understanding of biological responses to N deposition variation at Guiyang area, the concentrations of photosynthetic pigments in epilithic mosses were investigated and the correlations between photosynthetic parameter and moss C and N were discussed. Urban mosses had higher photosynthetic pigments than rural mosses and there were positive correlations between chlorophyll and C and N contents, showing photosynthetic pigments in epilithic mosses could respond to the environmental stress, C fixation and N supply. The negative correlation between chlorophyll and 13C values indicated the enhancing mechanism of N deposition effect on photosynthesis of epilithic mosses. |
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