Heterogeneity of neutral phosphate buffer extractable soil organic matter     

Kae MIYAZAWA  Tohru MURAYAMA 《Soil Science and Plant Nutrition》2007,53(1):1-6

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一种新的竞争指标——树冠影响度     

王鑫  葛宏立  辛营营 《浙江林业科技》2011,(2)

可变竞争区域类竞争指标根据对象木某个描述型指标函数确定竞争区域;而树冠在树木的生长过程中具有重要的作用,并且反映了树木的长期竞争水平,根据这两种思想提出一种新的竞争可变区域类竞争指标——树冠影响度。采用典型选样法设置研究样地,并对样地内树种的各个测树因子进行调查统计,应用树冠影响度对数据进行分析讨论,结果显示:样地树种平均树冠影响度为0.359 4,样地中群落竞争相对较弱,群落结构相对稳定,其中马尾松(Pinus massoniana)的树冠影响度最小,为0.261 3;枫香(Liquidambar formosana)的树冠影响度最大,为0.510 1。优势种群树种平均树冠影响度小于非优势种群树种,优势种群受到的竞争较小,竞争能力较强,针叶树种平均树冠影响度小于阔叶树种树种,针叶树在群落中处于优势地位。  相似文献   

Drought and fire suppression lead to rapid forest composition change in a forest-prairie ecotone   总被引：1，自引：0，他引：1  

Ryan D. DeSantis  Stephen W. Hallgren 《Forest Ecology and Management》2011,261(11):1833-1840

Altered fire regimes and increased drought can lead to major vegetation changes, especially in ecotones. A decrease in fire can lead to woody species encroachment in prairies and increasing forest stand density. The threat of global climate change raises questions about potential increases in the length, severity, and incidence of droughts substantially altering species composition. Re-measured upland forests in south-central North America's midcontinent forest-prairie ecotone exhibited major changes in woody species composition and structure over fifty years and successional trajectories appeared to favor invasive Juniperus virginiana L. over the previous dominant Quercus species. The objective of this study was to determine whether climate and fire exclusion affected the recruitment history of dominant woody species in these upland forests located near the xeric western edge of the eastern deciduous forest biome of North America. We removed cores and cross-sections from 992 J. virginiana, Quercus marilandica Münchh. and Q. stellata Wangenh. trees from eleven forest stands located across central and northwest Oklahoma, and determined their ages using standard dendrochronological methods. Recruitment of all species increased following a severe mid-20th century drought, but a rapid increase in J. virginiana recruitment and decrease in Quercus recruitment appeared to be linked to a decrease in fire. Future fire regime changes and increased drought due to global climate change could lead to widespread shifts from Quercus- to Juniperus- dominated forests and cause substantial changes to ecosystem services.  相似文献   

Quantification of the biocontrol agent Pseudomonas fluorescens Pf153 in soil using a quantitative competitive PCR assay unaffected by variability in cell lysis- and DNA-extraction efficiency     

Davide Gobbin  Fabio Rezzonico  Cesare Gessler 《Soil biology & biochemistry》2007,39(7):1609-1619

Although often neglected, variability in cell lysis efficiency and DNA extraction yield represents the major hurdles of any polymerase chain reaction (PCR)-based quantification protocol in soil and other natural environments. In this study we developed a technique that minimizes the effects of these constraints, providing at the same time a reliable internal control to distinguish between PCR-inhibition and negative results. We used Pseudomonas fluorescens Pf153, a root-colonizing bacterium that shows biocontrol activity against tobacco and cucumber black root rot, as the target organism for PCR quantification. Prior to DNA extraction, the genetically engineered, cognate reference strain P. fluorescens CHA0/c2 was inoculated in a reference soil. CHA0/c2 in the reference soil and Pf153 in the soil sample were lysed in parallel and afterward the lysates were mixed in known proportions. CHA0/c2 carries the plasmid pME6031-cmp2 that contains an allelic variant (competitor) of the Pf153 specific sequence Pf153_2. In a quantitative competitive PCR (QC-PCR) assay the competitor allows the quantification of the target strain down to 0.66 Pf153 CFU/mg soil. Processing the reference strain in the same way as Pf153 enables the exact quantification of the target strain in biocontrol assays performed in natural soil, overcoming differences in DNA extraction efficiency and PCR amplification from different soil environments. This technique is easily adaptable to other Pseudomonas strains simply by replacing the competitor used here with one derived from a SCAR-marker which is specific for the strain of choice.  相似文献   

Relationship between molecular characteristics of soil humic fractions and glycolytic pathway and krebs cycle in maize seedlings     

Serenella Nardi  Adele Muscolo  Salvatore Baiano  Alessandro Piccolo 《Soil biology & biochemistry》2007,39(12):3138-3146

A humic acid (HA) isolated from a volcanic soil was separated in three fractions of decreasing molecular size (I, II and III) by preparative high performance size exclusion chromatography (HPSEC). The molecular content of the bulk soil HA and its size fractions was characterized by pyrolysis-GC-MS (thermochemolysis with tetramethylammonium hydroxide) and NMR spectroscopy. All soil humic materials were used to evaluate their effects on the enzymatic activities involved in glycolytic and respiratory processes of Zea mays (L.) seedlings. The elementary analyses and NMR spectra of the humic fractions indicated that the content of polar carbons (mainly carbohydrates) increased with decreasing molecular size of separated fractions. The products evolved by on-line thermochemolysis showed that the smallest size fraction (Fraction III) with the least rigid molecular conformation among the humic samples had the lowest content of lignin moieties and the largest amount of other non-lignin aromatic compounds. The bulk HA and the three humic fractions affected the enzyme activities related to glycolysis and tricarboxylic acid cycle (TCA) in different ways depending on molecular size, molecular characteristics and concentrations. The overall effectiveness of the four fractions in promoting the metabolic pathways was in the order: III>HA>II>I. The largest effect of Fraction III, either alone or incorporated into the bulk HA, was attributed to a flexible conformational structure that promoted a more efficient diffusion of bioactive humic components to maize cells. A better knowledge of the relationship between molecular structure of soil humic matter and plant activity may be of practical interest in increasing carbon fixation in plants and redirect atmospheric CO₂ into bio-fuel resources.  相似文献   

应用竞争排斥法防制禽沙门氏菌感染     

韦平  李康然 《广西农业生物科学》1993,(2)

本文介绍了竞争排斥的概念,并概述了其在控制禽沙门氏菌感染中的效果、作用机理和实际应用等方面的研究进展,并讨论了这一方法的主要优点及其在家禽生产及人医上的潜在应用前景。  相似文献   

配体竞争法配合物稳定常数的测定     

曲祥金  张昌军 《山东农业大学学报(自然科学版)》1997,28(4):421-426

本文提出了测定配合物稳定常数的新方法，此适用于两配体竞争同一金属离子反应。通过测定以下两体系及其它体系的稳定常数进行验证，（１）偶氮胂Ｉ与ＥＧＴＡ竞争Ｍｇ（Ⅱ）体系，（２）４－（２－吡啶偶氮）－间苯二酚与ＥＤＴＡ竞争Ｃｏ（Ⅱ）体系等，结果令人满意。  相似文献   

中国大米国际竞争力研究 ——基于同印度、巴基斯坦、泰国和越南的比较分析     

胡中应 《勤云标准版测试》2006,(8)

稻谷是中国主要的粮食生产作物,具有较强的创造经济效益的能力。从联合国粮农组织和世界贸易组织的相关数据上来看,中国大米产量与中国大米的出口量存在着明显的不对称。笔者从研究国际竞争力的显示性指标出发,选择了亚洲最大的5个大米出口国作为研究对象,运用了国际市场占有率、贸易竞争指数、显示性比较优势指数、进出口价格比4个指标对中国大米的国际竞争力作了实证分析。结果表明,中国大米与印度、巴基斯坦、泰国和越南4国相比,不具有国际竞争力优势。  相似文献   

欠发达地区发展区域特色经济的思考     

洪燕云 《安徽农业大学学报》2003,(5):13-16

欠发达地区面临宏观环境的变化，使得地区在竞争中急需寻求一条把比较优势转化为竞争优势的有效途径。本文分析了发展特色经济的战略意义，提出了欠发达地区发展区域特色经济的基本思路和对策。  相似文献   

一类合作和竞争系统的全局渐近稳定性     

周兰  李小平  梁经珑 《湖南农业大学学报(自然科学版)》2006,32(5):557-559

利用负散度和不可约合作（竞争）系统的单调性质，分别研究二维和三维合作（竞争）系统的全局渐近稳定性．所得结果推广了已有文献的相关结论．  相似文献