Restoration and Rehabilitation of Arid and Semiarid Mediterranean Ecosystems in North Africa and West Asia: A Review

Mediterranean-type vegetation and ecosystems have undergone intense processes of degradation for decades, centuries, or millennia under heavy and prolonged pressure from human and livestock populations. An extensive literature on exclosures, afforestation, reafforestation, rehabilitation, and other regeneration operations over several million hectares in Mediterranean bioclimatic areas from the Atlantic Ocean to the Aral Sea, combined with 50 years of personal field experience, allowed us to draw a number of conclusions on the consequences of these efforts, constraints, and limitations: (1) Exclosure usually permits the restoration and biological recovery of vegetation structure, composition, biomass, and productivity in a time span of 3-5 years in steppic ecosystems and 25-30 years in coniferous or sclerophyllic vegeta tion areas. There are exceptions, however, when vegetation is so degraded to the level that it has reached a new metastable equilibrium, characterized, e.g., unpalatable range weeds, perennial dwarf ephemeroids, cryptogams, or soil surface sealing by raindrop splash or other physical or biological factors. (2) Controlled access and rationally managed utilization of the land may achieve similar and sometimes better results than full exclosure. (3) Afforestation and reafforestation are usually successful above the isohyet of 200 mm a-1 precipitation and, occasionally, at a lower annual precipitation, where the species introduced or reintroduced is appropriate and the causes for degradation have been discontinued or seriously mitigated. (4) Rehabilitation operations, including water and/or soil conservation, may quickly achieve spectacular results but at a higher cost and subjected to a number of constraints pertaining to the techniques utilized and adequate subsequent management. (5) The main constraint for success is the discontinuation of situations that have caused degradation. The most difficult to overcome usually are of a socioeconomic and/or sociocultural nature. The speed of biological recovery is commensurate (inter alia) with the rate to which this constraint is overcome. (6) The present land surface concerned with regeneration (restoration + rehabilitation) in this part of the world represents ca. 4 X 10 6 ha, representing some 6% of the actual ''forest and woodland'' in these areas. The National Regeneration Effort (NRE), a novel concept, is evaluated by using the ratio between the Annual Regeneration Expenditure (ARE) and both the Annual National Budget Expenditure (ANBE) and Gross National Product (GNP).     

某柴油机配气机构数值模拟

利用GT—VTrain对某发动机配气系统进行了计算,对比了采用两种凸轮轴下各零件的动力学结果,分析了配气机构中的气门运动情况、凸轮挺柱接触应力、活塞气门最小间隙、弹簧受力等结果,找出了其中更合适的凸轮轴．为发动机的设计提供了理论依据。     

Molecular dynamics of shoot vs. root biomarkers in an agricultural soil estimated by natural abundance C labelling

The aliphatic biopolyesters cutins and suberins have been suggested to significantly contribute to the stable pool of soil organic matter (SOM), and to be tracers for the above- or belowground origin of plant material. Contrary to other plant-derived aliphatic molecules found in the lipid fraction of soils, the stable isotope derived estimates of turnover of cutins and suberins have never been studied in soils. The aim of this study was to analyse the dynamics of shoot- and root-derived biomarkers in soils using a wheat and maize (C₃/C₄) chronosequence, where changes in the natural ¹³C abundance can be used to evaluate the incorporation of new carbon into SOM at the molecular level. The relative distribution of aliphatic monomers in wheat and maize roots and shoots suggested that α,ω-alkanedioic acids can be considered as root-specific markers and mid-chain hydroxy acids as shoot-specific markers.The contrasting distribution of the plant-specific monomers in plants and soils might be explained by different chemical mechanisms leading to selective degradation or stabilization of some biomarkers. The changes of the ¹³C isotopic signatures of these markers with years of maize cropping after wheat evidenced their contrasted behaviour in soil. After 12 years of maize cropping, shoot markers present in soil samples probably originated from old C₃ vegetation suggesting that new maize cutin added to soils was mostly degraded within a year. The reasons for long-term stabilization of shoot biomarkers remain unclear. By contrast, maize root markers were highly incorporated into SOM during the first six years of maize crop, which suggested a selective preservation of root biomass when compared to shoots, possibly due to physical protection.     

Dynamics of carbon and nitrogen in an extreme alkaline saline soil: A review

The soil of the former lake Texcoco is an ‘extreme’ alkaline saline soil with pH > 10 and electrolytic conductivity (EC) > 150 dS m⁻¹. These conditions have created a unique environment. Application of wastewater sludge to Texcoco soil showed that large amounts of NH₄⁺ were immobilized, NO₃⁻ was reduced aerobically, NO₂⁻ was formed and the mineralization of the organic material in the sludge was inhibited. A series of experiments were initiated to study the processes that inhibited the decomposition of organic material and affected the dynamics of mineral N. The large EC and pH inhibited the decomposition of easily decomposable organic material such as glucose and maize, although cellulolytic activity was observed in soil with pH 9.8 and EC 32.7 dS m⁻¹. The high soil pH favoured NH₃ volatilization of approximately 50 mg N kg⁻¹ soil within a day and a similar amount could be fixed on the soil matrix due to the dispersed minerals and their volcanic origin. Soil microorganisms immobilized large amounts of NH₄⁺ within a day when glucose was added to soil in excess of what was required for metabolic activity. Removal of NO₃⁻ from soil amended with glucose was not inhibited by 100% O₂ and NH₄⁺ indicating that the contribution of denitrification and assimilatory reduction to the reduction of NO₃⁻ was minimal while the formation of NO₂⁻ was not inhibited by 0.1% acetylene, known to inhibit nitrification. Additionally, the reduction of NO₃⁻ in the glucose-amended alkaline saline Texcoco soil was followed by an increase in the amount of NH₄⁺, which could not be due to denitrification. It was concluded that the reduction of NO₃⁻ and the formation of NO₂⁻ and NH₄⁺ in the glucose-amended soil was a result of aerobic NO₃⁻ reduction. A phylogenetic analysis of the archaeal community in the soil of the former lake Texcoco showed that some of the clones identified were capable of reducing NO₃⁻ aerobically to NO₂⁻ when glucose was added. A study of the diversity of the bacterial dissimilatory and respiratory nitrate-reducing communities indicated that bacteria could have contributed to the process.     

梁结构等效模型的研究与应用

基于梁的基本理论和结构灵敏度分析，讨论了材料不一致的比例模型的研制方法，并将这一方法应有于梁或由梁组成的框架结构中。有限元法的验证表明了该方法的正确性，这一方法将对农业机械动力学试验的简化具有一定的指导意义。     

诃子总多酚恒温超声辅助提取与过程动力学研究

采用三频恒温数控超声辅助提取技术优化了诃子总多酚提取工艺;通过比较试验和扫描电镜分析了该技术高效提取的原因;从Fick第一定律出发建立了诃子总多酚扩散动力学方程,并进行了验证。结果表明,恒温超声提取诃子总多酚最佳工艺条件为:乙醇体积分数60%、超声频率80 k Hz、超声功率180 W、提取温度70℃;该工艺提取效率显著高于水浴振荡提取工艺(P0.001);扫描电镜观察发现,恒温超声提取技术对原料粉末微观结构破坏更严重。结合Fick第一定律及超声提取动力学推导了恒温超声辅助提取诃子总多酚动力学方程;验证试验表明,在相应的范围内,提取时间对数和提取功率对数均与诃子总多酚浓度对数呈正比关系。结果表明:恒温超声辅助技术可用于诃子总多酚高效提取。     

永磁振动钻削排屑分析

通过普通钻削与振动钻削情况下切屑单元受力情况的分析，建立了在两种切削状况下单位时间内切屑的位移公式，解释了振动钻削情况下排屑顺利的原因。介绍了一种新型的振动钻削装置——永磁振动钻削装置。在此新型的振动钻削装置上进行试验，结果证明振动钻削的排屑效果优于普通钻削。     

黑龙江农村主要产业协调发展研究SD模型

应用系统动态学(SD)方法建立模型,研究了黑龙江省农村各主要产业之间和各产业同国民经济系统中其它产业之间存在的相互影响和相互作用关系及各产业内部的结构特性。此模型的研究结果为制定相应的中长期发展战略和政策提供了较为科学和可靠的依据,并在实际中得到了应用。     

红壤稻田土壤溶解有机碳含量动态及其生物降解特征

本通过田间采样分析和室内培育试验,研究了不同利用年限的3种红壤水田土壤中溶解有机C(DOC)的含量动态和生物降解特征。结果表明：土壤DOC的含量随土壤深度而降低,二具有显负相关性。0-30cm土壤DOC的含量及其占总C比例随土壤有机C含量的升高而增大。DOC的季节变化明显,主要与气候因素有关：降雨和灌水可显提高DOC的含量,是落干时含量的1．44-2．50倍。淋溶试验结果表明,从试验开始,淋滤液中DOC的浓度呈增加趋势,至2l天时达到最大,其后又趋下降。在49天的培养期内,DOC的分解速率为3l％-58％,其中低分子量组分能在数天内降解掉。不同土壤间DOC的分解速率有明显差异。     

离子交换树脂膜吸附土壤养分的动力学研究

采用从英国进口的离子交换树脂膜和室内埋置法研究了取自河北玉田县的 4个土壤吸附土壤磷、钾、钙、镁和锰的动力学。结果表明 ,在 0 .5～ 2 4h的吸附时间内 ,随着提取时间的延长 ,树脂膜吸附的土壤磷、钙量和锰量显著增加 ,而吸附的钾量减少。