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DOI: 10.12357/cjea.20220752董心亮, 王金涛, 田柳, 娄泊远, 张雪佳, 刘彤, 刘小京, 孙宏勇. 盐渍化土壤团聚体和微生物与有机质关系研究进展[J]. 中国生态农业学报 (中英文), 2023, 31(3): 364−372DONG X L, WANG J T, TIAN L, LOU B Y, ZHANG X J, LIU T, LIU X J, SUN H Y. Review of relationships between soil aggreg-ates, microorganisms and soil organic matter in salt-affected soil[J]. Chinese Journal of Eco-Agriculture, 2023, 31(3): 364−372盐渍化土壤团聚体和微生物与有机质关系研究进展*董心亮, 王金涛, 田 柳, 娄泊远, 张雪佳, 刘 彤, 刘小京, 孙宏勇**(中国科学院遗传与发育生物学研究所农业资源研究中心/中国科学院农业水资源重点实验室/河北省节水农业重点实验室 石家庄 050022)摘 要: 土壤有机质是耕地质量的核心, 不仅促进土壤团聚体形成, 也为植物和微生物提供养分。
土壤有机质的形成和分解过程都离不开微生物的参与, 而土壤团聚体不仅为微生物提供了栖息环境, 也对有机质进行物理保护。
在高盐分含量的土壤中, 有机质的积累和分解过程变得更加复杂, 因此本文总结了土壤盐渍化及其危害, 分析了土壤盐分对土壤团聚结构和微生物特征的影响、盐碱土壤有机质特征及积累规律, 进而综述了土壤盐分对土壤有机质影响规律的研究进展, 旨在揭示盐碱土壤碳封存的潜在机理。
以往研究表明盐渍化土壤有机质含量低、团聚结构差、微生物活性低, 这些都与土壤盐分含量高和外源有机物质输入量低有关。
恶劣的土壤结构导致盐渍化土壤有机质暴露而较易分解, 低量的外源有机物质输入导致盐渍化土壤有机质较难积累。
第一章1关键词：Nano materialsScanning Tunn eli ng Microscope (STM)Zero (one, two, three)-dime nsionSize Effect - Kubo theory(1)Quan tum size effect⑵ Small size effectSurface effectCoulomb blockade and Quan tum tunn eli ng effectDielectric confin eme nt effect纳米材料扫描隧道显微镜(STM零(一，二，三)维尺寸效应-久保理论(1)量子尺寸效应(2)小尺寸效应表面效应库仑阻塞和量子隧道效应介电限域效应2名词解释：(1) Nan6' —From the Greek word for“dwarf ” and means for 10 -9or one billio nth. In this case it refers to 10 -9 meters, or 1 nano meter (nm). 1 nm is about 3 atoms long.(2) Nano materials - Materials hav ing a characteristic len gth scale less tha n about a hun dred nano meters.(1)纳米“-从希腊字为”矮“和10-9，或十亿分之一的手段。
在这种情况下，它指的是10-9米，或1纳米(nm)。
1纳米是关于三原子长。
(2 )纳米材料-材料特征长度有规模比大约一百纳米以下。
这可能是一个尺度粒径，粒度，层厚，或在电子芯片导体线宽。
1. Size Effect - Kubo theory (课堂笔记)：尺寸效应：当纳米材料组成的尺寸如晶粒的尺寸，第二相离子的尺寸减小时，纳米材料的性能会发生变化，当组成相的尺寸小到于某一临界尺寸相当时，材料的性能将发生明显的变化或突变2. snall size effect 小尺寸效应：随着颗粒尺寸的量变，在一定条件会引起颗粒性质的质变，由于颗粒尺寸变小所引起的宏观物理性质的变化就成为小尺寸效应3. Coulomb blockade and Qua ntum tunn eli ng 库伦阻塞量子隧穿效应：体系进入纳米体系是电荷量子化的，即充电和放电过程是不连续的，充入一个电子所需的能量Ec=e2/kc这个能量就成为库伦阻赛能，这就导致了一个销体系的充放电过程，电子不能集体传输，惹事一个个单电子的传输。
小学上册英语第一单元测验试卷英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.I like to ride my ______ (scooter) around the block.2.What is the name of the famous novel written by J.R.R. Tolkien?A. The HobbitB. The Lion, the Witch and the WardrobeC. Harry PotterD. The Chronicles of Narnia3.Flowers are often given as _____ (礼物) on special occasions.4.The newt can regenerate its ______ (尾巴) if lost.5.Which fruit is red and often mistaken for a vegetable?A. OrangeB. AppleC. TomatoD. BananaC6.The gerbil is a popular ______ (宠物) in many homes.7.What is the capital city of Brazil?A. Rio de JaneiroB. BrasiliaC. Sao PauloD. Salvador8. A simple sugar is known as a _____ (monosaccharide).9.The chemical formula for -heptanoic acid is ______.10.What is the opposite of hot?A. WarmB. ColdC. CoolD. FreezingB11. A parrot can live for many _______ (年).12.小鹿) leaps gracefully over the stream. The ___13.How many legs does a spider have?A. SixB. EightC. TenD. Twelve14.What do we call the time it takes for the Earth to complete one rotation?A. YearB. DayC. MonthD. HourB15.The ______ teaches us about different countries.16.The __________ was a time when the world faced a global pandemic. (流感大流行)17.What is the main ingredient in ice cream?A. SugarB. CreamC. MilkD. All of the aboveD18.My teacher gives us ______ (奖励) when we do well on our tests. It makes me______ (开心).19.What is the main language spoken in the United States?A. FrenchB. SpanishC. EnglishD. Chinese20.ts are ______ (有毒) and should not be touched. Some pla21.I like to watch ______ (体育) events on TV.22.What do you call the time when the sun sets?A. DawnB. NoonC. DuskD. MidnightC23.What is the name of the largest type of galaxy?A. Irregular GalaxyB. Elliptical GalaxyC. Spiral GalaxyD. Lenticular Galaxy24.Chemical changes often involve the formation of ________.25. A ______ (海狸) builds dams in rivers.26.The __________ (花香) is a delight in the garden.27.In 1492, Christopher Columbus sailed the ocean _______. (蓝)28.What is a group of stars called?A. GalaxyB. UniverseC. Solar SystemD. ConstellationD29.The _____ (果实) of a pumpkin is very large.30.Hydrochloric acid is found in the __________.31.What do you call a person who draws pictures?A. ArtistB. PainterC. IllustratorD. SculptorA32.What is the capital of Spain?A. MadridB. BarcelonaC. ValenciaD. Sevilla33.What do you call a place where animals are kept for public display?A. ZooB. AquariumC. FarmD. SanctuaryA34.My _____ (妈妈) is a great cook.35.I like to _______ (参加) music festivals in summer.36.How many zeros are in one thousand?A. OneB. TwoC. ThreeD. Four37._____ (evergreen) trees provide shelter year-round.38.The chemical symbol for sodium is ____.39.What do we call a large body of freshwater?A. OceanB. RiverC. LakeD. Sea40.What do you call the main character in a movie?A. ActorB. ProtagonistC. DirectorD. ExtraB41.ts have developed traits to survive in nutrient-poor ______. (某些植物已发展出特征，以在养分贫乏的土壤中生存。
全卷满分：150分考试时间：120分钟2024.08第一部分：听力（共两节，满分30分）（略）（答案在最后）第二部分：阅读（共两节，满分50分）第一节（共15小题；每小题2.5分，满分37.5分）阅读下列短文，从每题所给的A、B、C、D四个选项选出最佳选项。
ABrecon Beacons YAC has an amazing opportunity for budding(崭露头角的) archaeologists(考古学家)of all ages,in Sunderland in Tyne and Wear.Thanks to funding from the Heritage Lottery Fund,professional archaeologists from Wardell Armstrong will be investigating an archaeological mystery and they need your help.Romans?“The site is a bit of a mystery”,Frank Giecco from Wardell Armstrong said.“There is a very nice cropmark recorded on the site that has got lots of people very excited.Geophysicists (地质物理学家)failed to find anything related to the cropmark.There is anecdotal(趣闻的) evidence of Roman material being found,but nothing is officially recorded.We hope to finally answer the question of what is in this field,during the two weeks on site.Is there evidence of any Roman occupation on the site?Can you help us find out?Get involvedThis is an opportunity for both adults and children to take part in field walking and trial trenching(small hand-dug test pits for children).No experience is necessary,training will be provided by the professional archaeologists from Wardell Armstrong,and all equipment will be provided.Volunteers can attend for as few or as many days as they wish,but they do need to book a place.For more information,and to book your place,please contact Norman Kirtlan at***********************************.21.What’s the main purpose of the activity?A.To seek funding for archaeological research.B.To find volunteers to help solve an archaeological mystery.C.To appeal for help in proving findings based on anecdotal evidence.D.To organize volunteers to help sort out data on the Roman occupation.22.Which of the following statement might be true if you take part in the activity?A.You will be coached by experts in the field.B.You should have worked with archaeologists before.C.You should spare two weeks to stay at the site.D.You will be charged for using equipment.23.Who is the activity probably intended for?A.Romans.B.Experts.C.Officials.D.FamiliesBKiska,the last captive(圈养的)killer whale in Canada---also known as“the loneliest whale in the world”----has died,according to local media.The whale,believed to be47years old,died on March9,2023.Ontario’s Solicitor General Ministry spokesperson Brent Ross confirmed to CBC.Kiska,who spent overfour decades in captivity,lived at Marineland,a zoo and amusement park in Niagara Falls, after being captured in Icelandic waters in1979alongside Keiko,the star of the movie Free Willy.Marineland said that Kiska’s health had declined in recent weeks.The news comes four years after the country officially banned whales and dolphins from being bred or held in captivity.However,marine mammals like Kiska that were already held in captivity would remain so,according to the new law.Kiska’s death marks the end of captive whales in Canada.People for the Ethical Treatment of Animals(PETA)said Kiska“spent her final years isolated in a tank,swimming in endless circles or sometimes floating listlessly.”Kiska’s life of continued isolation led the Whale Sanctuary Project to describe her as “the loneliest whale in the world.”Both animal rights groups reported that Kiska died of a bacterial infection.Ross told CBC that animal welfare officials had conducted a necropsy(尸检)of the mammal.Kiska underwent several hardships while living at the amusement park,according to PETA.These included the deaths of all five of her children and the departure of her sole tankmate Ikaika in2011.Videos of Kiska slamming her head against her tank wall multiple times began circulating on social media in2021.Kiska was one of the last survivors of the Icelandic captures back in the1970s.She lived the last12years of her life completely along,no other whales or in fact any other species were there to keep her company.“It is heartbreaking to know that Kiska will never have the chance to be relocated to a whale reserve and experience the freedom that she so deeply deserved.”Camille Labchuk, executive director of Animal justice,told CBC.There are at least55killer whales being held in captivity in marine parks around the world,according to the Whale and Dolphin Conservation.Killer whales are known to be very social mammals and are not suitable for captivity.Chronic stress can weaken the immune system and physiology of captive killer whales,leading to illness and even death.24.Which of the following statement is true about Kiska?A.She died as the last captive killer whale in the world.B.She was captured in the ocean more than40years ago.C.She was a star in a film called Free Willy.D.She should have been set free according to a new law passed in2019.25.Why Kiska was referred to as“the loneliest whale in the world”?A.Because Kiska’s death marks the end of captive whales in Canada.B.Because Kiska was kept alone after a bacterial infection.C.Because Kiska lived the last12years of her life without company.D.Because all five of Kiska’s children died ahead of her.26.What does the underlined word listlessly probably means in the text?A.Without enthusiasm.B.Without destination.C.Without breath.D.Without effort.27.What is the best title of the text?A.It’s time we stop catching killer whalesB.The death of a killer whale leads to a new law in CanadaC.Kiska,the last survivor of the Icelandic captures,has diedD.Kiska,“the loneliest whale in the world”,dies in CanadaCNow a company is launching a new robot on personal service machines.Rather than keeping you company at home,GITA(pronounced Jee-Tah)helps you go hands-free while you walk down the street,to the grocery store or to visit your neighbors.Like the name, which means“outing”in Italian,the robotic servant is for short trips out and about.Designed by the Piaggio Group,the smart device uses five cameras to see you,follow you around and carry up to40pounds of your belongings for four hours.It’s similar to those robotic suitcases that trail behind you at the airport,but with an entirely different outside. Picture a large container but on wheels.That’s what GITA looks like in the flagship orange color.Imagine having a hi-tech that can follow you around with your shopping inside.It’s round and strong yet relatively smart and convenient.The robotic helper is a joy to engage with,sort of like a dog.Operating GITA is simple.You just press one button to turn it on,one button for the cameras to scan your legs,and you’re off.It actually does pretty good job of following you and recognizing when you change directions.Its movement is oddly natural.When you speed up it falls behind a bit before trying to catch up,and when you stop.it stops-usually.GITA operates best on hard surfaces.However,it can’t go upstairs.It can roll on slight inclines(斜坡)and keep up with a walking person at speeds up to6miles per hour.A system of sounds and lights will let you know whether it is unpaired or needs a charge. And an app lets you share your robot with your“crew”,so it can follow them,too.With a speaker,your GITA can carry candy and play theme music as you take the kids trick-or-treating.A USA TODAY tech reporter gave GITA a test run in2019.He said,“GITA certainly made quite an impression on passers-by as it rolled down New York’s busy Madison Avenue.”Yet having tech trail behind you takes some getting used to.You can’t help but look behind you to make sure it’s keeping up,somewhat like monitoring a pet.28.For what purpose is the new robot designed?A.To instruct you where to go.B.To serve you as you walk around.C.To accompany you at home.D.To protect you when you’re traveling.29.What does the new robot probably look like?A.It looks like a royal dog.B.It has flags and smart panels.C.It is round,orange and can roll of itself.D.It resembles cameras in appearance.30.Which of the following is true about GITA?A.It can roll up a sharp slope at a high speed.B.It can only recognize and follow its owner.C.It can adjust its pace automatically.D.It can play tricks for children.31.What can we learn about GITA from the last paragraph?A.People are heavily dependent on it.B.It is far from being put into use.C.It is easy to get lost and needs to be improved.ers have difficulty accustoming themselves to it at first.DSalad plants have already been grown in old shelters and tunnels.Urban farming is a regular topic of interest at places like the World Economic Forum(WEF)in Davos,where leaders consider whether the world’s food system,blamed for causing both obesity and malnutrition,can be fixed.There are already plenty of urban farming projects around the world,particularly in the US,Japan and the Netherlands,from urban fish and plant farms to vertical farming.“It’s becoming an expanding industry,”said Richard Ballard,one of the founders of the farm Growing Underground.“There are several other businesses starting up in London in containers,and there are other vertical farms around the country now.”Growing Underground is not a standard farm.The rows of crops could be in almost any tunnel,but these plants are100feet below Clapham High Street and show that urban agriculture is,in some cases at least,not a fad.The underground farm has occupied a part of the Second World War air-raid shelters for nearly five years,and Ballard is planning to expand into the rest of the space later this year.Growing Underground supplies herb and salad mixes to grocery shops,supermarkets and restaurants.Being in London creates an advantage,Ballard says,as they can harvest and deliver in an hour.He adds other advantages.Being underground means temperatures never go below15 ----surface greenhouses need to be heated.They can do more harvests:60crops a year, compared with about seven in a traditional farm.Electricity to power the lights is a major cost, but the company believes renewable energy will become cheaper.Similar British companies include the Jones Food Company in Lincolnshire,while in the US AeroFarms has several projects in New Jersey,and Edenworks in Brooklyn.32.What can we learn about urban farming?A.Different farming methods are used.B.Local governments pay efforts to develop it.C.It leads to a healthier lifestyle.D.It is rarely discussed at the WEF.33.Which of the following best explains“a fad”underlined in Paragraph3?A.A plan that has great potential.B.A tradition that appeals to a lot of people.C.An approach that can solve a serious problem.D.A fashion that’s popular for a short time.34.Which of the following is true about the underground farm?A.Its major products are herbs and salads.B.It produces more kinds of foods than a traditional farm.C.It is more productive than a traditional farm.D.It uses less energy than a greenhouse.35.What is the theme of the text?A.Traditional farming will be replaced soon.B.Growing Underground attracts more people.C.Current food system causes health problems.D.Urban farming is still thought costly and time-consuming.第二节（共5小题；每小题2.5分，满分12.5分）阅读下面短文，从短文后的选项中选出可以填入空白处的最佳选项。
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小学下册英语第6单元测验卷[有答案]英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.The first man-made satellite was launched by _______.2. A butterfly starts as a ________________ (幼虫).3.She is ______ her bike. (riding)4.What is the main ingredient in bread?A. FlourB. RiceC. SugarD. Salt5.The __________ (历史的情感共鸣) bridge divides.6.What do you call a group of fish?A. HerdB. SchoolC. PackD. Flock答案:B7.What is the name of the famous American singer known for "Thank U, Next"?A. Ariana GrandeB. Taylor SwiftC. Selena GomezD. Demi Lovato答案:A8.He is a pilot, ______ (他是一名飞行员), who flies airplanes.9.My favorite character in a movie is _______ (名字). 他/她是个 _______ (形容词)的人.10.What is the capital city of Norway?A. OsloB. BergenC. TrondheimD. Stavanger答案:A11.The __________ caused many trees to fall. (风)12.Which instrument has keys and is played by pressing them?A. GuitarB. DrumsC. PianoD. Violin13.Which of these is a dairy product?A. BreadB. YogurtC. RiceD. Pasta答案:B14.I want to grow a ________ for my mom's birthday.15.What is the term for a young ferret?A. KitB. PupC. CalfD. Chick答案:A16. A _______ is a chemical reaction that occurs in living organisms.17.The __________ is very cold today; make sure to wear a jacket. (天气)18.The capital of Peru is __________.19.The candy is ___ (colorful).20.The Battle of Hastings took place in the year ________.21.Chickens lay _______ (鸡蛋).22.Rush brought many people to ________ (加利福尼亚). The Grea23.The _____ (狐狸) has sharp senses and is very clever.24.What do you call a person who helps sick animals?A. VeterinarianB. DoctorC. NurseD. Zookeeper25.The ancient Greeks held the first Olympic Games in ________.26. A mineral’s ______ refers to its sh iny or dull appearance.27. A ________ is a large open space in a forest.28.What is the capital of Italy?A. VeniceB. MilanC. FlorenceD. Rome答案:D29.The __________ is an important trade route.30.Energy from the sun is called ______ energy.31.My favorite holiday is ________ (万圣节) for the costumes.32. A volcanic eruption can release ash, gas, and ______ into the atmosphere.33.The ______ is what keeps us grounded on Earth.34.I love exploring new ________ (地方).35.Which of these is a common pet?A. LionB. GoldfishC. ElephantD. Giraffe答案:B36.The __________ is a large desert located in North Africa. (撒哈拉沙漠)37.I enjoy _____ (reading/writing) stories.38. A _______ is a reaction that involves the release of light.39.Plants help to prevent ______ (土壤侵蚀) by holding the soil in place.40.What is the name of the famous mouse created by Walt Disney?A. Mickey MouseB. Donald DuckC. GoofyD. Pluto41.She is _____ (coloring/drawing) a picture.42.The __________ (希腊神话) includes many gods and heroes.43.We had a _______ (愉快的) picnic yesterday.44. A __________ (溶胶) is a colloidal mixture with solid particles dispersed in a liquid.45.It is _____ to swim in the ocean. (fun)46.My uncle loves to __________ (探险) in nature.47.The first Olympic Games took place in _______.48.Chemical energy is stored in the _____ of molecules.49.What is the capital of Honduras?A. TegucigalpaB. San Pedro SulaC. La CeibaD. Choluteca答案:A50.The ______ (气候) affects plant growth.51. A ______ (蜘蛛) spins webs to catch insects.52.The process of combining elements to form compounds is called ______.53.小猴子) loves to eat fruits. The ___54.The __________ can indicate the presence of ancient life forms in rock.55.We should keep ______ (环境清洁) for plants to thrive.56.The invention of ________ has transformed daily life.57. A reaction that occurs spontaneously is said to be ______.58.What is the chemical symbol for gold?A. AuB. AgC. PbD. Fe答案:A59.The element with the symbol O is __________.60.The dog is _____ at the ball. (looking)61.I see a _____ on the table. (book)62.What do we call the study of the Earth's atmosphere?A. MeteorologyB. GeologyC. OceanographyD. Astronomy答案:A Meteorology63.My friend enjoys __________ (旅行).64.The __________ (历史的解读) invites discussion.65.We eat _____ (breakfast/lunch) at AM.66.What is the capital city of the Netherlands?A. AmsterdamB. RotterdamC. UtrechtD. The Hague答案:A67.The _____ (老虎) is a powerful predator in the wild.68.What is the capital of Thailand?A. BangkokB. Chiang MaiC. PhuketD. Pattaya答案:A69.We have a big ___. (garden)70.Which country is known for the Eiffel Tower?A. ItalyB. FranceC. SpainD. Germany答案:B71.We can find _______ in the forest (我们可以在森林里找到_______).72.I have many ______ cousins. (我有很多______表兄弟姐妹。
小学上册英语第一单元期末试卷(有答案)英语试题一、综合题(本题有100小题，每小题1分，共100分.每小题不选、错误，均不给分)1.__________ are substances that cannot be broken down into simpler substances.2.We have a picnic _____ the park. (at)3.Elephants have large ______.4.What is the main ingredient in sushi?A. RiceB. FishC. SeaweedD. Vegetables答案:A5.Photosynthesis converts sunlight into ______ energy.6.I have a toy _______ that rolls and spins everywhere I go.7.The ancient Romans created a system of _____.8.The trees in the _______ provide shade and a place to relax.9.Many plants need ______ (水) to survive.10.The ________ (热带雨林) is rich in biodiversity.11. A simple circuit includes a power source, a load, and ______ (wires).12. A __________ is a common garden pest that can destroy crops.13.The _______ (The Women's Suffrage Movement) fought for women's voting rights.14.My __________ (玩具名) can __________ (动词) in many ways.15.The __________ is an important trade route.16.The __________ (历史的见证者) recount significant moments.17.I like to ride my ______ (horse).18.I saw a _______ (小松鼠) in the park.19.Do you like _____ (鱼) in your aquarium?20.Dial telescopes use mirrors to gather ______.21. A _____ (狼) is a pack animal and very social.22.I listen to _____ (音乐) every day.23.We are going to the ___. (concert) tonight.24.What is 10 - 6?A. 5B. 4C. 6D. 3答案:B 425.I enjoy _______ (运动) after school.26.The parrot can be very ________________ (吵闹).27. A dolphin jumps gracefully out of the _______ and plays.28.My ________ (玩具名称) is very unique.29.I often ask for ________ (名词) for my birthday so I can buy new toys.30.The ________ is celebrated for its independence from Britain.31. Pyramid of Giza was built as a __________. (陵墓) The Grea32.physical map) shows terrain features like mountains and rivers. The ____33.My aunt lives in . (我阿姨住在。
小学下册英语第4单元真题[含答案]考试时间：100分钟（总分:140）A卷一、综合题(共计100题共100分)1. 听力题：I want to _______ (travel) the world.2. 听力填空题：I enjoy participating in school clubs. They provide a platform for us to explore interests outsid e of academics. I’m currently involved in __________, which is a lot of fun!3. 听力题：The __________ is a region known for its wildlife conservation.4. 填空题：I saw a _______ (蟋蟀) in the grass.5. 选择题：What is the name of the fairy in "Peter Pan"?A. Tinker BellB. CinderellaC. Snow WhiteD. Ariel答案: A6. 听力题：The _____ (信封) is addressed.7. 填空题：A ____(feedback mechanism) collects opinions and suggestions.8. 听力题：A ________ is a large area of land with a specific climate.9. 听力题：The chemical symbol for aluminum is _______.10. ssance era brought a revival of ________ (艺术和科学). 填空题：The Rena11. 选择题：What do we call a story that is not true?a. Factb. Fictionc. Legendd. History答案:b12. 填空题：My favorite fruit is ________ (桃子) in the summer.13. 填空题：The tarantula can live up to _______ (几十年).14. 听力题：The chemical property of reacting with acid is called ______.15. 填空题：I love to play outside with my sports ____. (玩具名称)16. 填空题：The ________ has a long tail and is very playful.17. 选择题：What do we call the process of taking care of plants?A. GardeningB. CookingC. DrawingD. Writing答案:A18. 填空题：I love to read ______ (小说) that transport me to different worlds.19. 选择题：What is the sound of a sheep?A. MeowB. BarkC. BaaD. Moo答案:CWe should _______ (尊重) each other.21. 填空题：The ancient Romans used _______ for their plumbing. (水管)22. 听力填空题：I think music brings people together. My favorite song is __________.23. 填空题：I love watching birds build their _________. (巢)24. 填空题：We celebrate _______ (生日) with cake.25. 选择题：What is the capital of Seychelles?A. VictoriaB. MahéC. PraslinD. La Digue答案: A26. 选择题：What do you call the time period when dinosaurs lived?A. CretaceousB. JurassicC. TriassicD. All of the above27. 选择题：What do we call a large amount of snow that falls quickly?A. ShowerB. BlizzardC. DrizzleD. Storm28. 填空题：________ (植物资源有效利用) supports health.29. 听力题：The sun is shining ________.30. 填空题：The goat loves to _______ (攀爬).A _______ is a chemical reaction that occurs between an acid and a base.32. 听力题：The train is _______ (fast).33. 选择题：What is the name of the famous bridge in San Francisco?A. Golden Gate BridgeB. Brooklyn BridgeC. London BridgeD. Sydney Harbour Bridge答案:A34. 选择题：What do you call a baby elephant?A. CalfB. FoalC. CubD. Kid答案: A35. 选择题：What do you call the season when leaves fall from trees?A. SpringB. SummerC. WinterD. Fall答案:D36. 填空题：The ________ is a popular pet among children.37. 填空题：A tortoise can live for over ______ (一百年).38. 听力题：My brother is very ________.39. 填空题：I like to draw ______ (漫画人物) and create my own ______ (故事).40. 填空题：A ________ (园艺设计) reflects personal style.She has a beautiful ________.42. 填空题：__________ (化学创新) leads to advancements in technology and medicine.43. 填空题：I have a great _____ (老师).44. 听力题：__________ are used in the production of glass.45. 选择题：What is the largest mammal on land?A. GiraffeB. ElephantsC. HippoD. Rhino46. ens have __________ (美丽的风景) that attract visitors. 填空题：Some gar47. 选择题：What is the capital of France?A. BerlinB. MadridC. RomeD. Paris48. 填空题：I enjoy exploring new ______ (地方), especially historical sites.49. 听力题：The _______ of a pendulum is the time it takes to swing back and forth.50. 听力题：I can _____ (dance/sing) very well.51. 听力题：My friend is a ______. He enjoys creating music.52. 选择题：What is the name of the famous painting of a woman with a mysterious smile?A. The ScreamB. The Birth of VenusC. The Girl with a Pearl EarringD. The Mona Lisa答案:D53. 听力题：A ______ has a symbiotic relationship with flowers.54. 填空题：The _____ (annual) plants only live for one season.55. 选择题：What do we celebrate on July 4th in the United States?A. ThanksgivingB. Independence DayC. ChristmasD. Halloween56. 填空题：A __________ (化学反应器) is a vessel where chemical reactions occur.57. 听力题：My ______ loves to engage in discussions.58. 选择题：Which animal can fly?A. CatB. FishC. BirdD. Dog答案: C59. 听力题：The chemical symbol for gold is ______.60. 听力题：__________ are used in the production of cosmetics.61. 选择题：What do we call the process of making new cells?A. Cell divisionB. MitosisC. MeiosisD. Fertilization答案:A62. 填空题：I like to use my toy ________ (玩具名称) for creative projects.63. 填空题：A sunny day is great for flying a __________. (风筝)64. 选择题：What is the capital of Turkmenistan?A. AshgabatB. TashkentC. BishkekD. Dushanbe答案: A. Ashgabat65. 选择题：What do we call a large area of flat land?A. PlateauB. PlainC. ValleyD. Hill66. 听力题：The chemical formula for sodium acetate is ______.67. 听力题：The dog is ________ in the grass.68. 填空题：I found a _______ (小蝴蝶) resting on a flower.69. 听力填空题：I enjoy cooking healthy meals to fuel my __________.70. 选择题：What do we call the natural satellite that orbits the Earth?A. SunB. MoonC. StarD. Planet答案:B71. 听力题：Light-years measure the ______ that light travels in one year.72. 听力题：My dad is ______ (working) from home today.73. 选择题：Which of these is a vegetable?A. BananaB. CarrotC. AppleD. Grape答案:B74. 填空题：My uncle is a __________ (艺术家).75. 填空题：The capital of Norway is ________ (奥斯陆).76. 选择题：What sound does a pig make?A. OinkB. RoarC. NeighD. Baa77. 填空题：The ________ was a major event that changed the course of history.78. 听力题：We are going to the ___. (beach) this summer.79. 填空题：My uncle is a ____.80. 填空题：The cake is ________ (美味).81. 填空题：_____ (compost) enriches the soil.82. 选择题：What is the opposite of hot?A. ColdB. WarmC. CoolD. Boiling答案:A83. 听力题：A _______ is formed when two or more elements combine chemically.84. smart growth) focuses on sustainable urban development. 填空题：The ____85. 听力题：The ice cream is ______ on a hot day. (refreshing)86. 选择题：What is the main ingredient in a Caesar salad?A. LettuceB. SpinachC. KaleD. Cabbage答案:A87. 填空题：The __________ (历史的多样性视角) enrich discussions.88. 选择题：What do you call the imaginary line that divides the Earth into the Northern and Southern Hemispheres?A. EquatorB. Prime MeridianC. Tropic of CancerD. International Date Line答案:A89. 听力题：The _____ (草坪) is freshly mowed.90. 填空题：The ancient Egyptians practiced _____ as part of their rituals.91. 听力题：A chemical that donates protons in a reaction is an ______.92. 听力题：My cousin is a talented ____ (guitarist).93. 选择题：What do you call a person who works in a hospital?A. PatientB. NurseC. DoctorD. Both B and C答案: D94. 选择题：What is the main ingredient in bread?A. SugarB. FlourC. RiceD. Corn95. 填空题：I went ________ (购物) with my mom.96. 听力题：My friend plays the ____ (trombone) in the band.97. 听力题：The ______ is a talented vocalist.98. 填空题：A goat can eat ______ (草) and climb steep hills.99. 填空题：The orca is also known as the killer ________________ (鲸).100. 填空题：I enjoy exploring new adventures with my toy ________ (玩具名称).。
关于抗老基金的英语作文The Fountain of Youth: Investing in an Anti-Aging FundAs we grow older, the relentless march of time takes its toll on our bodies and minds. Wrinkles deepen, hair grays, and energy levels decline. The prospect of growing old can be daunting, filled with concerns about declining health, loss of independence, and the specter of mortality. However, what if there was a way to slow down the aging process and maintain our youthful vigor well into our golden years? This is the promise of an anti-aging fund – a financial investment that aims to unlock the secrets of longevity and provide individuals with the resources to age gracefully.The concept of an anti-aging fund is built on the rapidly advancing field of rejuvenation biotechnology. Scientists around the world are making groundbreaking discoveries in areas such as stem cell therapy, gene editing, and cellular reprogramming, all with the goal of reversing the biological signs of aging. By investing in companies and research initiatives at the forefront of this field, an anti-aging fund seeks to capitalize on the potential breakthroughs that coulddramatically extend the human lifespan.One of the key components of an anti-aging fund is its focus on cutting-edge medical treatments and therapies. This might include investments in pharmaceutical companies developing novel drugs to combat age-related diseases, or biotech startups exploring innovative approaches to cellular rejuvenation. The fund could also allocate resources to research institutions conducting clinical trials on longevity-enhancing interventions, from calorie-restricted diets to nanorobotic therapies.Beyond medical advancements, an anti-aging fund may also invest in technologies that enhance human performance and delay the aging process. This could include wearable devices that monitor vital signs and provide personalized health recommendations, or smart home systems that optimize living environments for improved sleep, nutrition, and physical activity. The fund might even explore investments in space exploration, as the unique conditions of microgravity and cosmic radiation may yield insights into the fundamental mechanisms of aging.Of course, the prospect of an anti-aging fund is not without its critics and skeptics. Some argue that the pursuit of extended lifespan is unnatural and goes against the inherent order of the universe. Others raise ethical concerns about the potential for suchtechnologies to exacerbate social inequalities, as only the wealthy may have access to the most cutting-edge rejuvenation treatments.Despite these challenges, the proponents of anti-aging funds believe that the potential benefits far outweigh the risks. By investing in the science of longevity, they argue, we can not only improve the quality of life for individuals but also alleviate the societal burden of an aging population. Reduced healthcare costs, increased economic productivity, and a more vibrant and engaged older generation –these are just some of the potential upsides of successful anti-aging interventions.Moreover, the advocates of these funds contend that the pursuit of extended lifespan is not merely about personal vanity or the desire to cheat death. Rather, it is about empowering individuals to live fuller, more fulfilling lives, free from the constraints of age-related decline. Imagine a world where the elderly are not relegated to the sidelines, but instead remain active, engaged, and contributing members of their communities well into their later years. This, they argue, is the true promise of an anti-aging fund.Of course, investing in an anti-aging fund is not without its risks. The field of rejuvenation biotechnology is still in its infancy, and the path to commercialization for many of these technologies is uncertain. There is also the potential for unexpected side effects or unintendedconsequences, as with any emerging medical intervention. Investors in an anti-aging fund must be prepared to weather the ups and downs of this volatile and rapidly evolving market.Nevertheless, for those with a long-term vision and a willingness to embrace the unknown, an anti-aging fund may represent a unique opportunity to invest in the future of human health and longevity. By supporting the cutting-edge research and development that could unlock the secrets of aging, these investors are not only securing their own financial future, but also contributing to a broader societal transformation that could have profound implications for generations to come.In conclusion, the concept of an anti-aging fund is a fascinating and complex proposition that speaks to our deepest human desires – the desire to live longer, healthier, and more fulfilling lives. While the road ahead may be uncertain, the potential rewards of this investment are nothing short of transformative. As we continue to push the boundaries of what is possible in the realm of rejuvenation biotechnology, the anti-aging fund may well emerge as a critical catalyst in our quest for the fountain of youth.。
朗斯生物医学美容教材（编写纲要）绪论随着当今世界先进的生物科技的研究与发展，综观国际医学美容的动态，二十一世纪医学美容必将是一个生物生化科技美容的时代。
从生理生化角度来看，皮肤的美容修复和更新是一个复杂的生理生化代谢过程，不可能由任何一种生物因子独立完成。
人们一直在生物学领域探求这些活性物质，直至八十年代中期，细胞生长因子等被首次发现，并获诺贝尔奖。
在美国、法国、日本、德国、瑞典等国相继应用到医学美容及化妆品中。
伴随着各类生物因子研究不断的深入，发现由于皮肤结构的多重性，在不同的皮肤结构层次中，有需索不同因子的受体结合位点，由此得出结论皮肤的修复和更新，是有多种生物因子在不同的结构层次中协同作用完成的。
自八十年代后期，日本美容研究会就与法国VCIB公司及瑞典的LBR生物医学研究中心合作开发研究，关于《多种生物因子对皮肤多层次多位点的协同作用机理》的科研项目，并取得了专利，应用到医学美容中，形成可一个全新的概念体系。
Lanace国际生物医学美容机构，采用现代化生物高科技方法制造的生物医学美容产品，是全国唯一一家多因子产品，于1999年5月上市，领导了潮流。
朗斯产品应用许多医药领域的饿高新技术，如纳米技术、微乳化工艺脂肪技术等，使产品能够进入细胞内部，更快更安全，更彻底地解决问题。
朗斯产品立足高科技的生物制品，以损伤修复长效护理为主要功效。
目前上市的主要有三大类：Biomax生物组织细胞高能修复因子系列。
Genamax基因诊断治疗系列（含外用和内用）。
Slimax基因减肥美体系列，以后还将陆续开发研制新产品。
为使朗斯产品更广泛地应用到美容院，发挥其最大的效能，我们会同有关专家编写这套教材，由《朗斯生物医学美容概论》、《朗斯生物医学美容品》、《朗斯生物医学美容操作技法》三部分组成，由浅入深，适合多层次的美容加盟者学习。
本教材力求通俗易懂，突出实用性，真正成为美容师的朋友，相信通过阅读和学习，您会成为一名生物医学美容队伍的佼佼者。
Micro-foundations of urban agglomeration economiesGilles Duranton∗‡London School of EconomicsDiego Puga∗§University of Toronto11June2003Abstract:This handbook chapter studies the theoretical micro-foundations of urban agglomeration economies.We distinguish threetypes of micro-foundations,based on sharing,matching,and learningmechanisms.For each of these three categories,we develop one ormore core models in detail and discuss the literature in relation to thosemodels.This allows us to give a precise characterisation of some of themain theoretical underpinnings of urban agglomeration economies,todiscuss modelling issues that arise when working with these tools,andto compare different sources of agglomeration economies in terms ofthe aggregate urban outcomes they produce as well as in terms of theirnormative implications.Key words:cities,agglomeration,increasing returns,micro-foundationsjel classification:r12,r13,r32∗This is a draft of a chapter written for eventual publication in the Handbook of Regional and Urban Economics,Volume 4,edited by J.Vernon Henderson and Jacques-François Thisse,to be published by North-Holland.We are grateful to the editors,to Johannes Bröker,Masa Fujita,Mike Peters,Frédéric Robert-Nicoud,and to the participants at the the2002 narsa meetings for comments and suggestions.‡Department of Geography and Environment,London School of Economics,Houghton Street,London wc2a2ae, United Kingdom(e-mail:g.duranton@;website:/~duranton).Also affiliated with the Centre for Economic Policy Research,and the Centre for Economic Performance at the London School of Economics.§Department of Economics,University of Toronto,150Saint George Street,Toronto,Ontario m5s3g7,Canada(e-mail: d.puga@utoronto.ca;website:http://dpuga.economics.utoronto.ca).Also affiliated with the Canadian Institute for Advanced Research,the Centre for Economic Policy Research,and the National Bureau of Economic Research.Funding from the Social Sciences and Humanities Research Council of Canada is gratefully acknowledged.1.IntroductionOnly1.9%of the land area of the United States was built-up or paved by1992.Yet,despite the wide availability of open space,almost all recent development is less than one kilometre away from earlier development.Not only does the proximity of earlier development matter,but so does its density.Places where about one-half of the land in the immediate vicinity is already built-up seem to be most attractive for new development(Burchfield,Overman,Puga,and Turner,2003).One cannot make sense of this sort of numbers,of the extent to which people cluster together in cities and towns,without considering some form of agglomeration economies or localised aggregate increasing returns.While space is not homogenous,it is futile to try to justify the marked unevenness of development solely on the basis of space being naturally heterogeneous:the land on which Chicago has been built,for instance,is not all that different from other places on the shore of Lake Michigan that have been more sparsely developed(see Cronon,1991).And,once we abstract from the heterogeneity of the underlying space,without indivisibilities or increasing returns,any competitive equilibrium in the presence of transport costs will feature only fully autarchic locations (this result,due to Starrett,1978,is known as the spatial impossibility theorem).1People in each of these locations,like Robinson Crusoe,will produce all goods at a small scale for self-consumption. Re-stated,without some form of increasing returns we cannot reconcile cities with trade.While increasing returns are essential to understand why there are cities,it is hard to think of any single activity or facility subject to large-enough indivisibilities to justify the existence of cities. Thus,one of the main challenges for urban economists is to uncover mechanisms by which small-scale indivisibilities(or any other small-scale non-convexities)aggregate up to localised aggregate increasing returns capable of sustaining cities.We can then regard cities as the outcome of a trade-off between agglomeration economies or localised aggregate increasing returns and the costs of urban congestion.This is the object of this chapter:to study mechanisms that provide the microeconomic found-ations of urban agglomeration economies.We focus on the theoretical underpinnings of urban agglomeration economies,while the chapter by Rosenthal and Strange(2004)in this volume discusses the corresponding empirical evidence.By studying the micro-foundations of urban agglomeration economies we are looking inside the black box that justifies the very existence of cities.We regard this as one of the fundamental quests in urban economics for three main reasons.First,it is only by studying what gives rise to urban agglomeration economies—rather than merely stating that they exist—that we gain any real insight into why there are cities.Second,alternative micro-foundations cannot be regarded as interchangeable contents for the black box.The micro-foundations of urban agglomeration economies interact with other building blocks of urban models in ways that we cannot recognise unless they are explicitly stated.For instance,the composition of cities typically emerges as a consequence of the scope of different sources of agglomeration economies and their interaction with other aspects of individual behaviour.Third,different micro-foundations have very different welfare and policy implications.If we begin building an urban model by postulating an aggregate1See Ottaviano and Thisse(2004)in this volume for a detailed discussion of Starrett’s(1978)theorem.production function with increasing returns,we can only take this function as given.If instead we derive this aggregate production function fromfirst principles,we may see that its efficiency can be improved upon.The means for achieving such an improvement will depend on the specifics of individual behaviour and technology.Thus,while different assumptions regarding individual behaviour and technology may support similar aggregate outcomes,the normative implications of alternative micro-foundations can differ substantially.Urban agglomeration economies are commonly classified into those arising from labour market interactions,from linkages between intermediate-andfinal-goods suppliers,and from knowledge spill-overs,loosely following the three main examples provided by Marshall(1890)in his dis-cussion of the sources of agglomeration economies.2While this may be a sensible starting point for an empirical appraisal,we do not regard this as a particularly useful basis for a taxonomy of theoretical mechanisms.Consider,for instance,a model in which agglomeration facilitates the matching betweenfirms and inputs.These inputs may be labelled workers,intermediates,or ideas.Depending on the label chosen,a matching model of urban agglomeration economies could be presented as a formalisation of either one of Marshall’s three basic sources of agglomeration economies even though it captures a single mechanism.Since the focus of this chapter is on theory, we want to distinguish theories by the mechanism driving them rather than by the labels tagged to model components in particular papers.With this objective in mind,we distinguish three types of micro-foundations,based on sharing,matching,and learning mechanisms.3Our discussion of micro-foundations of urban agglomeration economies based on sharing mechanisms deals with sharing indivisible facilities,sharing the gains from the wider variety of input suppliers that can be sustained by a largerfinal-goods industry,sharing the gains from the narrower specialisation that can be sustained with larger production,and sharing risks.In discussing micro-foundations based on matching,we study mechanisms by which agglomeration improves either the expected quality of matches or the probability of matching,and alleviates hold-up problems.Finally,when we look at micro-foundations based on learning we discuss mechanisms based on the generation,the diffusion,and the accumulation of knowledge.4 For each of the three main categories of this taxonomy,sharing,matching,and learning,we develop one or more core models in detail and discuss the literature in relation to those models. That allows us to give a precise characterisation of some of the main theoretical underpinnings of urban agglomeration economies,to illustrate some important modelling issues that arise when2Marshall is arguably the best known and most influent of the early analysts of agglomeration.However Smith (1776)can be credited with thefirst analysis of the benefits from agglomeration,albeit with a more narrow argument relying on the division of labour.Thünen(1826)pursued and extended his work.He also proposed original arguments building on the interaction betweenfixed costs and transport costs not far from some of those developed below as highlighted by Fujita(2000).3Marshall(1890,iv.x.3)successively discusses knowledge spill-overs,linkages between input suppliers andfinal producers,and labour market interactions.However,his discussion of each of these sources of agglomeration economies highlights a different mechanism.Spill-overs are discussed in relation to the acquisition of skills by workers and their learning about new technologies.The discussion of linkages explicitly mentions the benefits of sharing intermediate suppliers producing under increasing returns.Finally,thefist part of his labour market argument points at a matching mechanism.4In this chapter,we discuss only models in which both the demand and supply of factors(mainly labour)are endogenous.In particular,we do not discuss the strategic location literature which takes the location of consumers as given.This literature is discussed in depth by Gabszewicz and Thisse(1992).working with these tools,and to compare different sources of agglomeration economies in terms of the aggregate urban outcomes they produce as well as in terms of their normative implications.2.Sharing2.1Sharing indivisible goods and facilitiesTo justify the existence of cities,perhaps the simplest argument is to invoke the existence of indivisibilities in the provision of certain goods or facilities.Consider a simple example:an ice hockey rink.This is an expensive facility with substantialfixed costs:it needs to be of regulated dimensions,have a sophisticated refrigeration system to produce and maintain the ice,a Zamboni to resurface it,etc.Few individuals,if any,would hold a rink for themselves.And while having a community of1,000people share a rink is feasible,building a rink for each of those people at 1/1,000th of the usual scale is not.An ice hockey rink is therefore an indivisible facility that can be shared by many users.It is also an excludable good,in the sense that use of the rink can be limited to members of a club or a community.At the same time,as the size of the community using the rink grows,the facility will be subject to increasing crowding.Crowding will take two forms.First,there will be capacity constraints when too many people simultaneously try to use the facility.Second,and more interesting in an urban context,crowding will also occur because the facility needs to be located somewhere and,as the size of the community of users grows,some of those users will be located too far away from the facility.5The problems associated with the provision of this type of facilities werefirst highlighted by Buchanan(1965).They are the subject of a voluminous literature referred to as club theory(or theory of local public goods when the spatial dimension is explicitly taken into account).The main focus of this very large literature is on equilibrium concepts(competitive,free mobility,Nash,core) and policy instruments.These issues are well beyond the scope of this chapter and are thoroughly reviewed in Scotchmer(2002).Here we just describe briefly how one large indivisibility could provide a very simple formal motive for the existence of cities.Consider then a shared indivisible facility.Once the largefixed cost associated with this fa-cility has been incurred,it provides an essential good to consumers at a constant marginal cost. However,to enjoy this good consumers must commute between their residence and the facility. We can immediately see that there is a trade-off between the gains from sharing thefixed cost of the facility among a larger number of consumers and the costs of increasingly crowding the land around the facility(e.g.,because of road congestion,small lot sizes,etc.).We may think of a city as the equilibrium outcome of such trade-off.In this context,cities would be no more than spatial clubs organised to share a common local public good or facility.65Note that this example is representative of a wide class of shared facilities that are excludable and subject to indivisibilities and crowding.These range from parks,museums,opera houses,and schools,to airports,train stations, and even power plants.6We do not worry here about thefinancing of the shared facility.Let us simply note that under competitive facility provisionfinanced by local capitalisation in the land market,the equilibrium is efficient.This result is known as the Henry George Theorem(Flatters,Henderson,and Mieszkowski,1974,Stiglitz,1977,Arnott and Stiglitz,1979)and is discussed at length in Fujita(1989)and in Fujita and Thisse(2002).An equivalent result applies to the case of a factory-town discussed below(Serck-Hanssen,1969,Starrett,1974,Vickrey,1977).This‘large indivisibility’argument motivates urban increasing returns by directly assuming increasing returns at the aggregate rge indivisibilities in the provision of some public good are just one possible motivation for this.A common alternative is to assume large indivisibilities in some production activity.This corresponds to the idea of a factory-town,where largefixed costs create internal increasing returns in a production activity that employs the workforce of an entire city whose size is bounded by crowding.There is in fact a long tradition of modelling cities as the outcome of large indivisibilities in production(Koopmans,1957,Mills,1967,Mirrlees, 1972).And since they constitute such a simple modelling device,factory-towns are still used as the simplest possible prototype cities to study a variety of issues,includingfiscal decentralisation (Henderson and Abdel-Rahman,1991),urban production patterns(Abdel-Rahman and Fujita, 1993),and economic growth in a system of cities(Duranton,2000).However,it is fair to say that factory-towns are empirically the exception rather than the rule in most countries.Finally,it has been suggested that this type of large indivisibilities could apply to the existence of market places(Wang,1990,Berliant and Wang,1993,Wang,1993,Berliant and Konishi,2000, Konishi,2000).7Indeed,economic historians(e.g.,Bairoch,1988)have long recognised the crucial role played by cities in market exchange.However,the hypothesis of large indivisibilities in marketplaces is once again at best a small part of the puzzle of why cities exist.To summarise,given Starrett’s(1978)result that without some form of increasing returns we cannot explain agglomeration within a homogenous area,the easiest route to take in justifying the existence of cities is to assume increasing returns at the city level by means of a large indivisibility. While large indivisibilities are useful modelling devices when the main object of interest is not the foundations of urban agglomeration economies,they side-step the issue of what gives rise to increasing returns at the level of cities.Cities facilitate sharing many indivisible public goods, production facilities,and marketplaces.However,it would be unrealistic to justify cities on the basis of a single activity subject to extremely large indivisibilities.The challenge in urban modelling is to propose mechanisms whereby different activities subject to small non-convexities gather in the same location to form a city.Stated differently,micro-founded models of cities need to reconcile plausible city-level increasing returns with non-degenerate market structures.2.2Sharing the gains from varietyIn this section wefirst derive an aggregate production function that exhibits aggregate increasing returns due to input sharing despite constant returns to scale in perfectly-competitivefinal produc-7These papers typically consider a smallfinite number of connected regions with differing endowments.Because of Ricardian comparative advantage,some marketplaces emerge and they are labelled cities.Wang(1990)establishes the existence and optimality of a competitive equilibrium with one endogenous marketplace in a pure exchange economy with exogenous consumer location.Berliant and Wang(1993)allow for endogenous location of consumers in a three region economy.Wang(1993)also allows for endogenous location in a two region economy with immobile goods. Berliant and Konishi(2000)revisit this problem in a production economy.Allowing for multiple marketplaces and differences in transport costs and marketplace set-up costs,they establish some existence and efficiency results.Finally, Konishi(2000)shows how asymmetries in transport costs can lead to the formation of hub-cities where workers employed in the transport sector agglomerate.The large indivisibilities assumed in these papers presumably reflect not so muchfixed costs of market infrastructure but other considerations,such as the advantages of centralised quality assurance(see Cronon,1991,and the chapter by Kim and Margo,2004,in this volume for a discussion of how this sort of consideration helped Chicago become the main metropolis of the American Midwest).tion.This is based on Ethier’s (1982)production-side version of Dixit and Stiglitz (1977).Aggregate increasing returns arise here from the productive advantages of sharing a wider variety of differentiated intermediate inputs produced by a monopolistically-competitive industry àla Chamberlin (1933).We then embed this model in an urban framework following Abdel-Rahman and Fujita (1990).This allows us to derive equilibrium city sizes resulting from a trade-off between aggregate increasing returns and congestion costs as well as a basic result on urban specialisation due to Henderson (1974).2.2.1From firm-level to aggregate increasing returnsThere are m sectors,super-indexed by j =1,...,m .In each sector,perfectly competitive firms produce goods for final consumption under constant returns to scale.Final producers use inter-mediate inputs,which are specific to each sector and enter into plants’technology with a constant elasticity of substitution (1+ j )/ j ,where j >0.Thus,aggregate final production in sector j is given byY j = n j 0 x j (h ) 11+ j d h 1+ j ,(1)where x j (h )denotes the aggregate amount of intermediate h used and n j is the ‘number’(mass)of intermediate inputs produced in equilibrium,to be endogenously determined.As in Ethier (1982),intermediate inputs are produced by monopolistically competitive firms àla Dixit and Stiglitz (1977).Each intermediate producer’s technology is described by the production functionx j (h )=βj l j (h )−αj ,(2)where l j (h )denotes the firm’s labour input,βj is the marginal productivity of labour,and αj is a fixed cost in sector j .Thus,there are increasing returns to scale in the production of each variety of intermediates.8This and the fact that there is an unlimited range of intermediate varieties that could be produced and no economies of scope imply that each intermediate firm produces just one variety and that no variety is produced by more than one firm.Let us denote by q j (h )the price of sector j intermediate variety h .The minimisation of final production costs n j 0q j (h )x j (h )d h subject to the technological constraint of equation (1)yields the following conditional intermediate input demand:x j (h )= q j (h ) −1+ j j Y jn j 0[q j (h )]−1 j d h 1+ .(3)It is immediate from (3)that each intermediate firm faces an elasticity of demand with respect to its own price of −(1+ j )/ j .Hence,the profit-maximising price for each intermediate is a fixed relative markup over marginal cost:q j =1+ j βj w j ,(4)8Obviously,technology could also be described in terms of the labour required to produce any level of output,l j (h )=(x j (h )−αj )/βj ,as it is often done following Dixit and Stiglitz (1977).where w j denotes the wage in sector j.Note that we have dropped index h since all variables take identical values for all intermediate suppliers in the same sector.There is free entry and exit of intermediate suppliers.This drives their maximised profits to zero:q j x j−w j l j=ing equations(2)and(4)to expand this expression and solving for x j shows that the only level of output by an intermediate producer consistent with zero profits isx j=αjj.(5)This,together with(2),implies that each intermediate producer hires l j=αj(1+ j)/(βj j) workers.Hence,the equilibrium number of intermediate producers in sector j isn j=L jl j=βj jαj(1+ j)L j,(6)where L j denotes total labour supply in intermediate sector j.By choice of units of intermediate output,we can setβj=(1+ j)(αj/ j) j/(1+ j).Substituting (6)and(5)into(1)yields aggregate production in sector j asY j=n jx j11+ j1+ j=L j1+ j.(7)This obviously exhibits aggregate increasing returns to scale at the sector level.The reason is that an increase in the labour input of sector j must be associated with more intermediate producers,as can be seen from(6);and,by(1),final producers become more productive when they have access to a wider range of varieties.Re-stated,an increase infinal production by virtue of sharing a wider variety of intermediate suppliers requires a less-than-proportional increase in primary factors.92.2.2Urban structureNext,following Abdel-Rahman and Fujita(1990),let us place in an urban context the production structure we have just described.Consider an economy with a continuum of potential locations for cities,sub-indexed by i.109Papageorgiou and Thisse(1985)propose an alternative approach in which agglomeration also relies on sharing the gains from variety.Their approach,which builds on a shopping framework,highlights well the importance of interactions betweenfirms and households.While the shopping behaviour of individuals is exogenously imposed rather than derived from a well-specified preference structure,it has later been shown to be consistent with random utility maximisation(Anderson,de Palma,and Thisse,1992).10Here we follow Abdel-Rahman and Fujita(1990)in embedding the production structure described above in a system of cities.In earlier papers,Abdel-Rahman(1988),Fujita(1988),and Rivera-Batiz(1988)embed similar production structures in an urban framework with a single city.The details of these pioneering papers differ slightly from those of our presentation.Instead of having the monopolistically competitive sector supplying differentiated intermediates, Abdel-Rahman(1988)has this sector producing differentiatedfinal goods.They are aggregated with a constant elasticity of substitution into a sub-utility function,which enters as an argument into a Cobb-Douglas utility function together with land and a traded good produced under constant return.Also,our specification of the urban structure is a bit simpler than his,as we impose afixed size for residences.Rivera-Batiz(1988)considers two sources of agglomeration economies,by assuming gains from variety both at the level of intermediates(like here)and offinal goods.Finally,Fujita (1988)uses a different specification for the gains from variety,which bears resemblance with the entropy functions used in information theory.Another difference is that in his model bothfirms and households compete for urban land, whereas we only consider a residential land market.Also,he assumes that goods are imperfectly mobile within the city, which allows for non-monocentric urban structures.Let us model the internal spatial structure of each city in a very simple fashion.Production in each city takes place at a single point,defined as the Central Business District(cbd).11Surrounding each city’s cbd,there is a line with residences of unit length.Residents commute from their residence to the cbd and back at a cost.In practice,commuting costs include both the direct monetary cost of travelling and the opportunity cost of the time spent on the journey(Small,1992). However,let us simplify by assuming that the only cost of commuting is the opportunity cost of time.12Specifically,each worker loses in commuting a fraction of her unit of working time equal to2τtimes the distance travelled in going to the cbd and back home(τ>0).Each worker chooses her place of residence so as to maximise utility given her income and the land rent function in the city.Because offixed lot size,this is equivalent to choosing residence so as to maximise net income.Thus,a worker in sector j in city i maximises w j i(1−4τs)−R i(s)with respect to s,where s is the distance to the cbd and R i(s)is the differential land rent in city i for a residence located at distance s from the cbd.The possibility of arbitrage across residential locations both within and across sectors ensures that at the residential equilibrium the sum of commuting cost and land rent expenditures is the same for all residents with the same wage;that workers sort themselves according to their wage, with higher-paid workers(who have a higher opportunity cost of commuting time)living closer to the cbd;that the city is symmetric and the city edges are at a distance N i/2of the cbd(where N i is total population in city i);and that the land rent function(i.e.,the upper envelope of all bid-rent functions,Alonso,1964)is continuous,convex,and piece-wise linear.13Without loss of generality, the rent at the city edges is normalised to zero.Integrating land rent over the city yields total landrent:R i= Ni/2−N i/2R i(s)d s.(8)The net amount of labour available to sector j at the cbd of city i,L j i,is equal to the number of workers employed in that sector minus their commuting time.Summing across sectors immedi-ately implies the following expression relating labour supply net of commuting costs,∑m j=1L j i,to population,N i:m∑j=1L ji=N i(1−τN i).(9)Finally,workers can move at no cost across sectors as well as across cities.Income from land rents is equally distributed across all local residents.11We take the existence of a cbd as given.For contributions that derive this endogenously,see Borukhov and Hochman(1977),Fujita and Ogawa(1982),and the additional references provided in subsection4.2.2.12This allows us to solve for equilibrium city sizes without having to track output prices in a multiple-sector setting. In section3.1we explore the opposite simplification of having only a monetary cost of commuting.13Models with a richer internal urban structure often consider endogenous differences across residences in terms of their size and land intensity(see Brueckner,1987,for a review).Then,residences closer to the cbd are not only more expensive but also smaller and more land intensive.Matters are further complicated when one considers durable housing and the possibility of redevelopment(see Brueckner,2000,for a review).There is also a large literature analysing the sorting of residents across neighbourhoods.It considers how income affects the valuation of land,that of leisure foregone in commuting,and that of access to amenities,all of which contribute to determining residential location(see Beckmann,1969;Brueckner,1987;and Brueckner,Thisse,and Zenou,1999).。
