【作者】 赵洁；

【导师】 程积民；

【作者基本信息】 西北农林科技大学 ， 草学， 2017， 硕士

【摘要】 随着气候变化的影响不断加剧,氮沉降增加带来的生态学效应正逐渐成为学者们的研究热点,而目前关于氮沉降速率较低的黄土区研究较少,且处于氮素限制的黄土区天然草地生态系统可能对氮富集的响应更加敏感。为数不多的研究也集中于分析单一的草地管理方式下(封育或放牧)草地生态系统对氮素添加的响应。因此,本研究以黄土区长期(30年)封育和连续放牧草地为对象,通过设置不同氮素添加水平,分析和比较封育和放牧草地土壤养分含量、地上生物量、叶功能性状、群落组成、物种多样性以及物种多样性与地上生物量的关系随氮素水平的变化及异同,探索两种典型的草地管理方式下群落功能和结构对氮素添加的响应并试图解释其背后的机制,得出如下结果:(1)氮素添加显著提高了封育草地的土壤硝态氮、铵态氮含量和放牧草地的土壤硝态氮含量。但与封育草地相比,放牧显著降低了土壤硝态氮含量,铵态氮、全氮和速效磷含量。(2)氮素添加显著提高了封育和放牧草地群落地上生物量,并分别在N5和N10水平下达到最大值。氮素添加对封育草地各功能群生物量无显著影响,但显著提高了放牧草地中可食禾草类占地上总生物量的比重,同时显著降低了非禾本科可食草类占地上总生物量的比重,并出现了封育草地中未出现的豆科草类。与封育草地相比,放牧和施肥提高了可食性草类的比例。(3)随着氮素水平增加,封育草地中的优势物种尤其是比叶面积较小的物种通过吸收土壤有效氮提高叶氮含量,进而提高比叶面积以获取更多的光资源,造成物种格局更替,改变群落组成及物种多样性;放牧草地中的物种则保持比叶面积基本不变甚至减小比叶面积以保存内部资源,维持物种多样性。(4)氮素添加降低了黄土区天然草地物种多样性,但放牧草地物种多样性整体高于封育草地物种多样性。由以上结果可得出主要结论如下:(1)随着氮素水平增加,封育草地与放牧草地的物种采取不同的资源获取与生存策略以响应生境的变化,进而维持物种多样性。(2)氮素添加降低了黄土区天然草地物种多样性,主要是由凋落物积累与物种对地下养分的竞争转为对地上光资源的竞争所引起。(3)放牧掩盖了氮素添加对物种多样性的负效应,一定程度上减缓了物种多样性下降速率。更多还原

【Abstract】 With the increasing influence of climate change,the ecological effects of increasing nitrogen deposition is becoming a research hotspot to scholars.However,the research on the loess plateau region with limited nitrogen which has lower nitrogen deposition rate is still less,and the loess plateau region may be more sensitive to nitrogen enrichment.In addition one of these studies so far focused on the response of grassland ecosystem to nitrogen addition just with a single management(fencing or grazing).So,in this study we added nitrogen in a series levels to simulate nitrogen deposition,explored the effects of nitrogen addition on soil nutrient content,community aboveground biomass,leaf functional traits,community composition,species diversity,and the relationship between species diversity and aboveground biomass in long-term(30 years)fenced and grazed grassland on the loess plateau region.We also analyzed the differences and similarities in responses of grassland and tried to explain the underlying ecological mechanisms of these responses under the two typical grassland managements.The main results are as follows:(1)Nitrogen addition significantly enhanced soil nitric nitrogen,ammonium nitrogen in fenced grassland and nitric nitrogen in grazed grassland.While grazing obviously decreased soil nitrogen,ammonium nitrogen,total nitrogen and available phosphorus compared to fenced grassland.(2)Nitrogen addition elevated the community aboveground biomass significantly both in fenced and grazed grassland,and the maximum emerged respectively at the N5 and N10 levels.Nitrogen addition had no significant effect on functional group biomass in fenced grassland,but it improved the proportion of grass species group biomass and reduced the proportion of forbs species group biomass in the total biomass markedly.Compared to the fenced grassland,grazing and fertilization improved the proportion of edible grass.(3)With the increase of nitrogen addition levels,dominant species in fenced grassland with smaller SLA(specific leaf area)acquired more light by absorbing available nitrogen in the soil to increase Nmass(leaf nitrogen content),changed the species pattern,community composition and species diversity.The grazed grassland kept the species diversity through conserving internal resource in plant by keeping SLA unchanged and even decreased.(4)Nitrogen addition reduced species diversity of grassland ecosystem in loess plateau region,but species diversity was bigger in grazed grassland overall.According to these results,we draw the following conclusions:(1)Species in fenced and grazed grassland took different ways in acquiring resources and survival strategies to response the changes in the habitat and keep species diversity.(2)Nitrogen addition reduced species diversity of grassland ecosystem in loess plateau region,the main reasons are accumulation of litter biomass and transformation of competition from underground nutrients to light on the surface which resulted in changing of community species.(3)Compared to the fenced and fertilized grassland,grazing counteracted the negative effects of nitrogen addition to species diversity,slowed down the rate of descent to some extent.更多还原