中国北方草本植物及表土有机质碳同位素组成

Recently, more and more attention has been paid to stable isotope ratios in terrestrial depositional systems. Among them, δ~(13)C value is mainly determined by the surface vegetation, while vegetation is directly related to climate, therefore, carbon isotope ratio in soil organic matter and pedogenic carbonate has been employed as an important paleoecological indicator. In order to test the paleoecological information extracted from stable isotope ratios in terrestrial depositional systems, it is necessary to study the relationships between δ~(13)C value in standing terrestrial plants and today climate, as well as between δ~(13)C value in modern surface soil organic matter and standing vegetation. Thus, these relationships were studied in this paper by means of analysing δ~(13)C in standing plants and modem surface soil organic matter in North China. The main results and conclusions are presented as following: 1. According to their δ~(13)C values, 40 C-4 species represent about 16% of the 257 plant species sarnpled from the North China. C-4 photosynthesis mainly occurs in Poaceae, Cyperaceae and Chenopidaceae families, and percentage representation of C-4 photosynthesis is up to 56% in Poaceae family. 2. The δ~(13)C values of C-3 plant species in North China vary from -21.7‰ to -32.0‰ with an average of -27.1‰, and 93% focus on the range of -24.0‰ ～ -30.0‰; δ~(13)C values of C-4 plant species in North China are between -10.0‰ ～ -15.5‰ with an average of -12.9‰, and 90% concentrate on the range of -11.0‰ ～ -15.0‰. 3. The δ~(13)C composition of C-3 plant species collected from Beijing, a semi-moist district, mainly vary between -27.0‰ ～ -30.0‰, and the average is -28.7‰; the δ ~(13)C values of plants in the semi-arid district, east and west to the Liu Pan Moutain, focus on the range of-26.0‰ ～ -29.0‰ and -25.0‰ ～ -28.0‰, respectively, with the mean value of -27.6‰ and -26.6‰, respectively; the δ~(13)C composition in the arid district dominantly vary from -24.0‰ to -29.0‰, with the average of -26.2‰, and among them, the δ~(13)C values of C-3 plant species in deserts are often between - 22‰ ～ -24‰; the δ~(13)C values in the cold mountain district concentrate on the range of -24.0‰ to -29.0‰, with the average of -26.3‰. 4. The main range of δ~(13)C composition of C-4 plant species, derived from Beijing, a semi-moist district, are -13.0‰ ～ -15.5‰; the semi-arid district, -11.0‰ ～ -14.0‰; the arid district, -11.0‰ ～ -14.0‰. The mean values of them are -14.0‰, -12.4‰,-12.7‰, respectively. 5. From east to west in North China, δ~(13)C values of C-3 plant species increase with longitude. The correlation between δ~(13)C ratios of C-3 plant species and longitude is linear. Changing temperate and precipitation and changing atmosphere pressure are spossible explanations. 6. Almost all C-3 plant species have the trends that their δ~(13)C values gradually increase with decreasing precipitation, decreasing temperature and increasing altitude. Our results show the increases of the δ~(13)C value by 0.30 ～ 0.45‰, 0.19 ～ 0.27‰ and 1.1 ～ 1.2‰ per 100 mm, I℃ and 1000 m, respectively, for all C-3 plant species together. 7. The δ~(13)C values of all C-3 plant species together and a part of C-3 species show highly significant linear correlation with the mean annual temperature, the mean annual precipitation and the altitude, and the results suggest that they can be used as proxies of these environmental variables, while, those without highly significant correlation, may be not suitable as the proxies. 8. The extent, which of responses of δ~(13)C composition to environmental variables, is different for each C-3 plant specie. 9. The δ~(13)C variations along altitude and longitude may be non-linear for C-4 p1ant species in North China. The mean annual temperature may be not important influential factor, thus, it suggests that the δ~(13)C composition of C-4 plant species may be not suitable as the proxy of the mean annual temperature. The influences of summer temperature on δ~(13)C values are much bigger than that of annual temperature, among them, the influence of September temperature is biggest. The mean annual precipitation may be one of the dominant influential factors, and it shows a highly significant non-linear relationship with δ~(13)C values, and the result indicates that δ~(13) C composition of C-4 plant species can be employed as the proxy of the mean annual precipitation. 10. The variations of δ~(13)C ratios do not show systematic trends along longitude, latitude and altitude for modern surface soil organic in Northwest China. ll. The δ~(13)C ratios of modern surface soil organic do not exhibit systematic patterns with temperature and precipitation in Northwest China, it suggests that, unless soil organic is transferred from pure C-3 or C-4 vegetation, the δ~(13)C composition of soil organic may be not used as proxies of climatic variables. 12. The δ~(13)C values of modem surface soil organic are heavier than that of standing vegetation, and the difference ofrnean δ~(13)C between them is -2.18‰. 13. Without considering the δ~(13)C difference between vegetation and soil organic, as well as the δ~(13)C drift in various enviromnent, we may not obtain the valuable information of C-3, C-4 relative biomass in vegetation. 14. The C-4 biomass contribution in vegetation increase with decreasing latitude, increasing longitude and decreasing altitude in Northwest China. The C-4 biomass almost are zero in those regions north to 38 ° N, or west to 100°E, or above 2400 m. 15. The C-4 relative biomass in vegetation increase with growing temperature and precipitation. and, C-4 plants are rare at those regions where the mean annual temperature is less 4 ℃, or the mean annual precipitation is less 200 mm, and their biomass contribution in vegetation are almost zero. Both the mean annual temperature and the mean annual average precipitation may be the important influential factors of C-4 distribution, but the dominant factors.

喀斯特丘原区石漠化过程中土壤退化的表征与分析——以清镇王家寨小流域为例

喀斯特石漠化问题是贵州省乃至整个西南地区最大的生态环境问题，对当地的经济发展和人类生存造成极大障碍，虽然石漠化问题日益受到重视，但生态建设超前、基础研究落后却是不争的事实。石漠化过程中土壤性质变化、人为干扰方式对土壤质量的影响、土壤——植被系统的关联退化是石漠化研究中非常重要的基础理论，可作为石漠化等级划分的参考，也是石漠化治理及生态恢复的重要依据。本文以王家寨小流域为研究区域，对喀斯特石漠化过程中土壤、植被的退化进行定量分析研究，以找出石漠化过程中土壤质量演变的规律以及人为干扰对其产生的影响，初步探讨了土壤——植被系统的退化特征及其协变关系，分析了坡位等流域内部影响因素对土壤质量退化的影响，并采用土壤退化指数对其进行评价，取得了以下几点认识： （1）研究区内土壤基本特征为：土层浅薄，剖面层次分化不明显，土壤呈中性至微碱性反应，CEC及盐基饱和度较高，有机质、氮素含量丰富，C/N较低，供氮潜力高，土壤营养元素较为丰富，但P、K、Na相对不足。 （2）石漠化过程中，土壤容重变大，孔隙度降低，土壤质地粘重，水稳性大团聚体减少，小团聚体相对增加；土壤有机质流失，全N含量降低，土壤中易于移动的营养元素，如P、K、Na、Ca、Mg等含量随石漠化程度加深而降低，难于移动的元素，如Fe、Mn、Cu等则表现出增加的趋势，土壤pH及CEC也基本呈降低趋势，土壤养分有效性降低，各元素有效态含量均下降，仅有效Mn例外。 （3）土壤剖面构型多为A—D，A—C型，也有少数已形成较为明显的心土层。土体A层营养状况相对较好。石漠化过程中，粘粒含量增加，尤其在土体中下部聚集更为明显，地球化学性质相对活跃的元素在土壤上、下层含量均降低，流失量上层大于下层，而相对较稳定的元素聚集量下层大于上层，土壤表层CEC降低，下层升高，有效Mn含量增加，有效铁含量仅在土壤下层增加，其他元素均降低。 （4）石漠化过程中土壤理化性质恶化与植被退化相互促进、互为因果，形成一种相互反馈的恶性循环。随石漠化程度加深，植被演替趋势为乔→草灌→灌草→稀疏灌草，其特征主要表现为：植被覆盖度较低，群落结构简单，富N、Ca、Mg，缺P、K、Na。通过对比及相关性分析，得出植物生长的主要限制因子为P、K、Na、Mn、Zn。 （5）石漠化经过一定时期发展，地势较平坦的区域易形成黄色石灰土，其理化性质介于石灰土与黄壤之间，随石漠化发展土壤性质的变化规律也更为明显；而地势陡峭，地形崎岖的区域则易在负地形中残留土壤形成年轻的黑色石灰土，其理化性质较好，剖面没有黄色石灰土分化明显，土壤性质也差异极大，因而在石漠化等级划分及综合治理上应予以区分，不应一概而论。 （6）筛选出土壤退让评价指标并赋予权重，主要包括平均土厚、有机质、全氮、pH、CEC、容重、密度、颗粒组成、水稳性团聚体、全磷、全钠、全钙、全镁、全铜、碱解氮、速效钾、有效铁、有效铜、有效锌等。评价结果显示土壤退化与石漠化发展具有同步性，其中化学性质的退化较物理性质明显，仅石漠化初期表现出相对较强的物理性质退化。轻度至中度是土壤退化最敏感的阶段，退化最为强烈，应予以重点防治。 （7）不同的人为干扰方式对土壤质量产生的影响不同，开垦和放牧作用下的土壤较为粘重，容重、密度都较樵采作用下的土壤大。通过退化评价指数可知，开垦和放牧影响下的土壤退化最为严重，主要表现为开垦、放牧后土壤养分的大量消耗和流失，土壤化学性质的变异引起土壤质量强烈恶化，而樵采和火烧则在一定程度上通过改善土壤的化学性质使退化延缓，因而土壤质量相对较好。 （8）小流域的研究方法虽然可以排除各种外在因素影响，但却无法排除流域内的影响因素，如坡位、坡度、坡向、坡形等。研究结果显示坡位对土壤密度、容积含水率、气相率、水稳性团聚体、C/N、元素全量及有效态含量都有一定影响。土壤退化指数显示土壤退化程度上坡>中坡>下坡。

纳秒激光诱导C60分子碎裂中轻碎片离子C+n(n＜30)的产生机理研究

The time of flight mass spectrometric technique was used to determine the initial mean kinetic energy of small fragment ions C-n(+) (n <= 11) produced from C-60 excited by 532 nm nanosecond laser pulses. The measured kinetic energy shows little variation with the fragment mass and the laser fluence in a broad range. Based on the assumption that C-30(+) is produced predominantly by a single electron emission followed by successive C-2 evaporation from hot C-60 in the nanosecond laser field, the formation of small fragments is interpreted as the complete breakup of the unstable C-30(+) cage structure. The interpretation is consistent with the previously observed results.