Nitrogen cycling and tree nutrition in boreal forests: interactions with understory vegetation and soil water availability

我们对北方森林中养分有效性、供水和林下竞争对林分生产力的相互作用了解甚少。在北方森林中，竞争林下物种（如加拿大蓝节草）丰富且生长旺盛，而且N通常是最具限制性的养分。在这项研究中，我将使用基于田间和温室的方法来研究土壤N动态，林下植物和树种之间的竞争，以及它们如何受到资源（N和水）添加的影响。目的是：1)研究林下植被和水分有效性如何相互作用影响土壤养分动态和树木营养；2)探讨土壤资源有效性（或竞争）与树木生理生态过程（气孔导度、光合作用、稳定碳同位素分馏和植物氮吸收）之间的联系；3)研究竞争和资源可得性对土壤微生物种群功能和多样性的影响；4)更好地了解有机氮在加拿大大叶草和白云杉营养中的作用。我将研究净氮矿化率和总氮矿化率、净氮矿化率和总氮矿化率、异养硝化率、可溶性有机氮产量、微生物氮固定化率、乔木和林下物种对氮的吸收以及森林生态系统的净初级生产力。稳定的N和C同位素（15N和13C）技术、生态生理学（如气体交换）测量和矢量分析将用于研究N竞争和资源限制的机制。研究结果将有助于我们更好地了解竞争林下植被孳生地森林生产力的控制因素，设计适应降水模式变化的森林管理策略，并提供有关北方地区森林生产力上限的信息。本研究将推动土壤-植物关系、资源限制、竞争和北方植被动态等生态学理论的发展。

We have a poor understanding of the interactive effects of nutrient availability, water supply, and understory competition on stand productivity in boreal forests where competitive understory species such as Canada bluejoint grass are abundant and grow vigorously and where N is often the most limiting nutrient. In this study, I will use both field- and greenhouse-based approaches to study soil N dynamics, competition between understory and tree species, and how they are affected by resource (N and water) additions. The objectives are: 1) to investigate how understory vegetation and water availability interact to affect soil nutrient dynamics and tree nutrition; 2) to explore the linkage between soil resource availability (or competition) and tree ecophysiological processes (stomatal conductance, photosynthesis, stable C isotope fractionation, and plant N uptake); 3) to study the effects of competition and resource availability on the function and diversity of soil microbial populations; and 4) to gain a better understanding of the role of organic N in Canada bulejoint grass and white spruce nutrition. I will study net and gross rates of N mineralization, net and gross rates of nitrification, rate of heterotrophic nitrification, rates of soluble organic N production, N immobilization by microbes, N uptake by both tree and understory species, and the net primary productivity of the forest ecosystem. The stable N and C isotope (15N and 13C) techniques, ecophysiological (e.g., gas exchange) measurements, and vector analysis will be used to investigate the mechanisms for N competition and resource limitations. A greenhouse experiment will be conducted to test the hypothesis that Canada bluejoint grass can take up simple organic forms of N. Results from this study will help us better understand the factors controlling forest productivity in sites infested with competitive understory vegetation, design forest management strategies to adopt to the changing precipitation patterns, and provide information on the upper limits of forest productivity in the boreal region. The proposed research will advance ecological theories in soil-plant relations, resource limitation, competition, and boreal vegetation dynamics.