内蒙古典型草原的群落物种组成对割草、施肥、増雨以及加热的响应

随着温室气体的大量排放、土地利用方式的改变以及气温的逐步升高，全球变化对自然生态系统的影响也日益加强，全球变化背景下植物群落物种组成的改变能够引起整个生态系统功能的变化。本论文的研究以内蒙古多伦县十三里滩全球变化多因子实验基地为依托，研究割草、施肥、増雨以及红外加热对内蒙古温带典型草原群落物种组成的影响。本研究采用传统的植物调查方法，比较样方中所有物种和功能型的变化，最终揭示植物群落结构对上述四个处理的响应状况。 研究结果表明：氮素作为中国北方典型草原生产力的重要限制因子，增加了禾草优势物种克氏针茅和冰草的盖度，提高了禾草植物盖度和群落总盖度。施磷提高了群落中豆科植物的盖度。 种群水平上，割草提高了乳白花黄芪、苔草和冷蒿等矮小植物盖度；群落水平上，提高了群落总盖度，增加了群落的物种数。但是仅仅在施氮处理下，割草显著地影响了种群和群落水平的指标。 除了氮以外，土壤水分含量也是草原生态系统的限制因素之一。虽然不同增雨方式之间对群落的影响无显著差异，但是增雨都显著地增加了群落中禾草植物和群落的总盖度。在氮肥处理样地，增雨对禾草植物的正效应更加显著。增雨单独对群落的物种丰富度没有显著的影响。 加热减少了土壤水分含量，降低了群落物种数和群落的总盖度。加热提高了群落中非禾草杂类草的比例，降低了群落中禾草植物的比例。 总之，土壤水分和活性氮含量左右了草原植物群落的物种组成。全球变化的四个主要驱动因子正是通过影响生态系统中水分和氮的有效含量来影响群落结构。施氮和增雨降低了土壤水分和氮含量的限制，割草、加热增强了土壤水分和氮的限制。基于四个驱动因子对草原生态系统相反的作用和影响，有必要进行长期的控制实验，充分了解全球变化下，内蒙古典型草原群落物种组成的变化规律。

气候变化对高纬度流域中汞和甲基汞迁移转化的影响——以降水量增加为例

全球变化的研究发现：19世纪以来全球平均气温大约上升了0.6℃，并预计在21世纪会升高更多。温室效应的直接结果是导致全球地面增温，温度的升高会增加下垫面的蒸发量。根据全球水平衡原理，蒸发量应该与降水量相等，因而降水量也会相应增加。气候变化的情景模式研究表明，世界各地雨量变异大，在高纬度地区，包括北欧地区，降水会增加。 流域是水生系统的一个重要的汞和甲基汞的输入源。欧洲淡水系统中，鱼体汞含量过高的湖泊主要局限于北方的高纬度森林地区。在斯堪的纳维亚半岛，就有上万湖泊中的鱼体汞含量超过了健康食用标准0.5 mg/kg。水生系统甲基汞输入源包括：水体或沉积物通过甲基化形成甲基汞，通过大气干、湿沉降输入的甲基汞和陆地生态系统的地表径流输入的甲基汞。 本研究的主要目标是搞清楚未来气候变化引起的大气降水量的增加对北方高纬度森林流域土壤中汞的迁移和形态转化有何影响。未来大气降水量的增加可能会增强森林地区流域土壤中汞的迁移，进而对水生系统的汞循环产生影响。一方面，如果地下水水位上涨，当水流过富含有机质结合含汞土壤层时，将会增加土壤中汞和甲基汞的迁移性，由此而导致流域向河流、湖泊等水生系统中汞输出通量的增加。另一方面，氧化还原电位改变、DOC和营养物质的释放会加剧汞在土壤中的释放过程，并且可能会促进汞的形态转化，可能增加毒性较强的甲基汞的产量，最终导致甲基汞从土壤向水生系统的输出通量的增加。 本研究选择瑞典西海岸花园湖（Gårdsjön）的G1流域作为研究对象，在流域内进行人工模拟增加降雨试验，模拟未来气候变化降水量增加的情景，观测流域地表径流中汞和甲基汞的变化。并在流域内选择四个点（1×1m²），进行汞同位素加入示踪试验。测定土壤剖面甲基汞和总汞及其同位素组成，来分析汞的迁移和形态转化。主要研究结果如下： 1. 通过对G1流域土壤中甲基汞分布的调查，发现甲基汞含量范围为0.020-3.1ng/g，甲基汞含量与TOC含量具有正相关的关系，表明土壤甲基汞的迁移受TOC迁移的影响。估算G1流域的甲基汞储库通量为123g/km2，G1流域甲基汞储库容为0.65g。 2. 通过甲基汞含量和甲基汞储库在土壤剖面的分布，可以看出表层腐殖质层甲基汞浓度最高，甲基汞最富集，而在较深的土壤层位甲基汞储库容量最大，表明甲基汞具有向下迁移的趋势。 3. 根据总汞同位素组成结果，汞同位素（198Hg(NO3)2）加入土壤中后，202Hg/198Hg比值迅速下降，然后随时间推移而上升，表明加入土壤中的汞与土壤中原始汞形态不同，具有不同的迁移方式，且加入土壤中二价汞的迁移性比土壤中的原始汞更强。计算表明，外界加入的汞，将有50-60%长期滞留在土壤中。 4. 根据甲基汞的同位素组成结果，Wet点的新加入的198Hg甲基汞分数要大于其它三点，表明湿润土壤中汞甲基化速率要快于较干燥的土壤中。另外，Wet点的甲基汞分数比以前无人工模拟增加降雨的同位素示踪结果高很多，表明降水量增加会导致汞的甲基化作用在土壤中的形态转化过程中占主导地位，最终导致甲基汞产率的提高。 5. 人工模拟增加降雨试验进行后，地表径流中总汞的浓度没有发生变化，而甲基汞的浓度增大了很多，从0.03ng/L增大到了1ng/L以上，表明降水的增加可以促进土壤中汞的甲基化作用。 6. 地表径流向流域外总汞和甲基汞的输出量的计算显示，受人工模拟增加降雨的影响，流域地表径流量也会增加，最终年地表径流中总汞和甲基汞的输出通量都变大。

Effect of PbO on precipitation of laser-induced gold nanoparticles inside silicate glasses

We report on three-dimensional precipitation of Au nanoparticles in gold ions-doped silicate glasses by a femtosecond laser irradiation and further annealing. Experimental results show that PbO addition plays the double roles of inhibiting hole-trapped centers generation and promoting formation and growth of gold nanoparticles. Additionally, glass containing PbO shows an increased non-linear absorption after femtosecond laser irradiation and annealing. The observed phenomena are significant for applications such as fabrications of three-dimensional multi-colored images inside transparent materials and three-dimensional optical memory, and integrated micro-optical switches. (c) 2007 Elsevier B.V. All rights reserved.

Study on the hydrolysis/precipitation behavior of Keggin Al-13 and Al-30 polymers in polyaluminum solutions

The hydrolysis/precipitation behaviors of Al3+, Al-13 and Al-30 under conditions typical for flocculation in water treatment were investigated by studying the particulates' size development, charge characteristics, chemical species and speciation transformation of coagulant hydrolysis precipitates. The optimal pH conditions for hydrolysis precipitates formation for AlCl3, PAC(A113) and PAC(A130) were 6.5-7.5, 8.5-9.5, and 7.5-9.5, respectively. The precipitates' formation rate increased with the increase in dosage, and the relative rates were AlCl3 >> PAC(A130) > PACA113. The precipitates' size increased when the dosage increased from 50 mu M to 200 mu M, but it decreased when the dosage increased to 800 AM. The Zeta potential of coagulant hydrolysis precipitates decreased with the increase in pH for the three coagulants. The isoelectric points of the freshly formed precipitates for AlCl3, PAC(A113) and PAC(A130) were 7.3, 9.6 and 9.2, respectively. The Zeta potentials of AlCl3 hydrolysis precipitates were lower than those of PAC(A113) and PAC(A130) when pH > 5.0. The Zeta potential of PAC(A130) hydrolysis precipitates was higher than that of PACA113 at the acidic side, but lower at the alkaline side. The dosage had no obvious effect on the Zeta potential of hydrolysis precipitates under fixed pH conditions. The increase in Zeta potential with the increase in dosage under uncontrolled pH conditions was due to the pH depression caused by coagulant addition. Al-Ferron research indicated that the hydrolysis precipitates of AlCl3 were composed of amorphous AI(OH)3 precipitates, but those of PACA113 and PACA130 were composed of aggregates of Al-13 and Al-30, respectively. Al3+ was the most un-stable species in coagulants, and its hydrolysis was remarkably influenced by solution pH. Al-13 and Al-30 species were very stable, and solution pH and aging had little effect on the chemical species of their hydrolysis products. The research method involving coagulant hydrolysis precipitates based on Al-Ferron reaction kinetics was studied in detail. The Al species classification based on complex reaction kinetic of hydrolysis precipitates and Ferron reagent was different from that measured in a conventional coagulant assay using the Al--Ferron method. The chemical composition of Al-a, Al-b and Al-c depended on coagulant and solution pH. The Al-b measured in the current case was different from Keggin Al-13, and the high Alb content in the AlCl3 hydrolysis precipitates could not used as testimony that most of the Al3+ Was converted to highly charged Al-13 species during AlCl3 coagulation.

Synthesis and magnetic characterization of nickel ferrite nanoparticles prepared by co-precipitation route

Magnetic nanoparticles of nickel ferrite (NiFe2O4) have been synthesized by co-precipitation route using stable ferric and nickel salts with sodium hydroxide as the precipitating agent and oleic acid as the surfactant. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses confirmed the formation of single-phase nickel ferrite nanoparticles in the range 8-28 nm depending upon the annealing temperature of the samples during the synthesis. The size of the particles (d) was observed to be increasing linearly with annealing temperature of the sample while the coercivity with particle size goes through a maximum, peaking at similar to 11 nm and then decreases for larger particles. Typical blocking effects were observed below similar to 225 K for all the prepared samples. The superparamagnetic blocking temperature (T-B) was found to be increasing with increasing particle size that has been attributed to the increased effective anisotropy energy of the nanoparticles. The saturation moment of all the samples was found much below the bulk value of nickel ferrite that has been attributed to the disordered surface spins or dead/inert layer in these nanoparticles. (c) 2008 Elsevier B. V. All rights reserved.

Mechanism of nucleation and precipitation in Li containing Al-Zn-Mg-Cu alloys

Investigations on the aging hardening behavior of four Al-Li-Zn-Mg-Cu alloys were carried out using differential scanning calorimetry, transmission electron microscopy and hardness measurement. It is shown that the addition of Li inhibits the formation of Zn-rich G.P. zones in Al-Zn-Mg-Cu alloys. The dominant aging hardening precipitates is delta'(Al3Li) phase. Coarse T ((AlZn)(49)Mg-32) phase, instead of MgZn2, precipitates primarily on grain boundaries, and provides little strengthening. The multi-stop aging involving plastic deformation introduces in the matrix a high concentration of structural defects. These defects play different role on the nucleation of Zn-rich G.P. zones in different alloys. For the Li free alloy, structural defects act as vacancy sinks and tend to suppress the homogeneous precipitation of G.P. zones, while for the Li containing alloys, these defects promote the heterogeneous nucleation of G.P. zones and metastable MgZn2. A significant aging hardening effect is attained in deformed Li containing alloys due to the extra precipitation of fine MgZn2 in the matrix combined with deformation hardening.

Effect of cerium oxide on the precipitation of silver nanoparticles in femtosecond laser irradiated silicate glass

We investigated the effect of cerium oxide on the precipitation of Ag nanoparticles in silicate glass via a femtosecond laser irradiation and successive annealing. Absorption spectra show that Ce3+ ions may absorb part of the laser energy via multiphoton absorption and release free electrons, resulting in an increase of the concentration of Ag atoms and a decrease of the concentration of hole-trapped color centers, which influence precipitation of the Ag nanoparticles. In addition, we found that the formed Ag-0 may reduce Ce4+ ions to Ce3+ ions during the annealing process, which inhibits the growth of the Ag nanoparticles.

Precipitation of nanocrystals in glasses by electron irradiation: An alternative path to form glass ceramics?

This letter demonstrates an alternative method to form gallium silicate glass ceramics using high-energy electron irradiation. Compared with glass ceramics obtained from the conventional thermal treatment method, the distribution and crystal sizes of the precipitated Ga2O3 nanoparticles are the same. An advantage of this method is that the spatial distribution of the precipitated nanoparticles can be easily controlled. However, optically active dopants Ni2+ ions do not participate in the precipitation during electron irradiation. (c) 2007 American Institute of Physics.

Space-selective co-precipitation of silver and gold nanoparticles in femtosecond laser pulses irradiated Ag+, Au3+ co-doped silicate glass

We report on space-selective co-precipitation of silver and gold nanoparticles in Ag+, Au3+ co-doped silicate glasses by irradiation of femtosecond laser pulses and subsequent annealing at high temperatures. The color of the irradiated area in the glass sample changed from yellow to red with the increase of the annealing temperature. The effects of average laser power and annealing temperature on precipitation of the nanoparticles were investigated. A reasonable mechanism was proposed to explain the observed phenomena. (c) 2006 Elsevier Ltd. All rights reserved.

Enhanced luminescence from transparent Ni2+-doped MgO-Al2O3-SiO2 glass ceramics by Ga2O3 addition

Transparent Ni2+-doped MgO-Al2O3-SiO2 glass ceramics without and with Ga2O3 were synthetized. The precipitation of spinel nanocrystals, which was identified as solid solutions in the glass ceramics, could be favored by Ga2O3 addition and their sizes were about 7.6 nm in diameter. The luminescent intensity of the Ni2+-doped glass ceramics was largely enhanced by Ga2O3 addition which could mainly be caused by increasing of Ni2+ in the octahedral sites and the reduction of the mean frequency of phonon density of states in the spinel nanocrystals of solid solutions. The full width at half maximum (FWHM) of emissions for the glass ceramics with different Ga2O3 content was all more than 200 nm. The emission lifetime increased with the Ga2O3 content and the longest lifetime is about 250 mu s. The Ni2+-doped transparent glass ceramics with Ga2O3 addition have potential application as broadband optical amplifier and laser materials. (C) 2007 Elsevier Ltd. All rights reserved.

Space-selective precipitation of Au nanoparticles inside silica gel

Au colloids were prepared by irradiation with a Nd:YAG laser. Au nanoparticles were characterized by absorption spectra, transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. It is found that the wavelength of the laser has no effect on the size but the number of the Au nanoparticles. By irradiating a transparent silica gel doped with gold ions with the focused laser in the gel and subsequent exposing in air, a space-selective pattern of letter "P" consisting of Au nanoparticles was observed inside the silica gel.