高寒草甸不同功能型植物根系寿命对土壤氮素有效性的响应

The Tibetan Plateau covers an area of more than 2.4 million km2. The ecosystems play essential roles in regional and national ecological safety, water resource conservation, and also in economic and social functions. Whereas, the region is fragile and under severe impacts by global change and human activities. .Alpine meadow is a main vegetation type on the plateau. Soil organic carbon density, root biomass and root/shoot ratio are very high, as compared with other ecosystems. Besides, soil organic nitrogen mineralized slowly and available nitrogen is low due to low temperature. Therefore, root longevity, which closely connected with nitrogen uptake capacity, is one factor controlling community succession, biomass production and carbon budget. However, few studies have been carried on root turnover in this ecosystem..Based on the facts that soil nitrogen is of low availability and air nitrogen deposition is increasing, we are to study the responses of plant traits to nitrogen addition on levels of plant species, functional type, and community, focusing on root lifespan..Mono-culture of 16 species, including various functional types will be conducted to elucidate response patterns of root longevity at species, and functional type levels. In the second experiment, four different rates of KNO3 will be applied to alpine meadow and root longevity will be monitored in situ by minirhizotron methods, in order to clarify short term response of lifespan at community and species levels. The third experiment will take use of a long-term nitrogen deposition simulating experiment, which began from May, 2007. Ingrowth bags will be buried in the plots and taken out after 7-8 years of continuously nitrogen addition. .Data on plant aboveground and belowground traits, as well as community parameters will be collected. Soil physical and chemical properties are to be measured. These data will be synthetically analyzed to answer the following two major questions: are there different response patterns among species, functional types, and among the three biological levels to nitrogen addition? Which is better in predicting community succession, using functional type-based or trait-based method

高寒草甸生态系统根系生物量大，根系寿命等性状在群落演替、物质生产和碳收支等方面有着重要作用，但至今缺乏深入研究。本项目基于高寒草甸土壤有效氮不足和氮沉降逐渐增加的事实，在物种、功能型和群落三个层次研究以根系寿命为核心的植物性状对氮有效性升高的响应。.项目通过对包含不同功能型的16个物种的单种栽培试验，探讨在没有其它植物作用时，物种、功能型根系寿命对氮添加的响应模式；通过群落施肥试验，利用微根管原位连续动态观测等方法研究不同深度根系数量、比例和寿命的变化；利用长期施肥试验平台，研究连续施用氮肥7~8年后根系寿命等性状的变化规律。结合植物地上、地下性状观测、群落物种组成与结构观测及环境要素观测，从机理上解析物种、功能型和群落尺度根系寿命对土壤氮有效性变化的响应规律，理清以功能型划分方式和以植物性状为基础预测群落对氮素添加响应的可行性，为理解高寒生态系统结构和功能对全球变化的响应与反馈提供试验基

N是植物生长需求量最多的矿质元素，也是很多自然生态系统的限制因子。根系吸收、运输和同化N素会消耗大量的C，当土壤N有效性变化时，会通过改变向地上部分的N供应，进而影响光合生产能力，反过来改变植物投入到根系中的C量，从而对根系生产和周转产生影响。目前关于土壤N的有效性对细根寿命的影响仍然存在很大的不确定性，对其影响机制的认识也不尽相同。此外，根系形态和构型也可能随着土壤N有效性的改变而改变。近年来，由于区域经济的发展和大气远距离传输，青藏高原东部地区大气氮沉降已接近全国平均水平，并呈现逐年增加的趋势。氮素供应的增加对氮限制的高寒草甸生态系统地下过程的影响急需深入研究。.本研究依托中国科学院海北高寒草甸生态系统定位研究站的模拟氮沉降实验平台，利用其中施用了(NH4)2SO4和KNO3两种氮素形态的样地，每种氮素形态的施用样地内包括4个施氮水平：对照（0）、低氮（10 kg N hm-2 yr-1）、中氮（20 kg N hm-2 yr-1）、高氮（40 kg N hm-2 yr-1），分别模拟未来大气沉降增加1倍、2倍和4倍的情景，以矮嵩草草甸植被为研究对象，采用内生长袋法，结合原状土壤取样，研究根系的分布、寿命和构型对长期氮素添加的响应。研究发现：（1）低氮处理下，使根系都倾向于更深层的土壤中生长，KNO3氮素引起此效应较(NH4)2SO4氮素更为明显；中氮和高氮处理下，不同氮素类型对根系在在土壤中分布格局的影响规律不完全一致。（2）对于直径< 2 mm的细根而言，低氮和中氮处理下，无论是(NH4)2SO4氮素还是KNO3氮素的中低剂量的施入都没有明显改变根系在不同深度土层中的分布比重，而高剂量的(NH4)2SO4氮素和KNO3氮素施入使细根在离地面10 cm土层中的分布比重减少到47.32 %和52.77 %，表明高剂量的氮肥施入可能引起细根向土壤深层分布。（3）高浓度的大气氮沉降将明显增加细根生产力，而低浓度大气氮沉降下，细根生产力的提高受到氮素类型的影响；（4）大气氮沉降对于细根直径的影响不仅取决于氮素类型，还与氮沉降浓度密切相关。（5）大气氮沉降对于细根比根长的影响受到氮素类型和氮沉降浓度双重影响，而影响的方向可能完全相反。本研究为阐明氮素缺乏的高寒草甸地下根系系统对土壤氮素有效性持续增加的响应机制提供了实验数据，为预测高寒草甸群落对全球变化的响应和反馈提供