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

我们对营养可获得性、供水和林下竞争对北方森林林分生产力的交互影响知之甚少。在北方森林中，竞争激烈的林下物种，如加拿大蓝豆草，种类繁多，生长旺盛，而N往往是最有限的养分。在这项研究中，我将使用田间和温室两种方法来研究土壤N的动态，林下和树种之间的竞争，以及它们如何受到资源(N和水)添加的影响。其目的是：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)探索土壤资源有效性(或竞争)与树木生态生理过程(气孔导度、光合作用、稳定碳同位素分馏和植物N吸收)之间的联系；3)研究竞争和资源有效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草地和白云杉营养中的作用。我将研究氮矿化的净速率和总速率、净硝化速率和总硝化速率、异养硝化速率、可溶有机氮的产生速率、微生物固定N的速率、乔木和林下物种对N的吸收以及森林生态系统的净初级生产力。将使用稳定的N和C同位素(15N和13C)技术、生态生理(例如气体交换)测量和矢量分析来研究N竞争和资源限制的机制。本研究的结果将有助于我们更好地了解控制竞争林下植被的森林生产力的因素，设计适应不断变化的降水模式的森林管理策略，并提供北方地区森林生产力上限的信息。这项研究将促进土壤-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.