我国东北森林典型树种氮吸收偏好及其对氮沉降的响应

Atmospheric nitrogen (N) deposition is rapidly increasing worldwide, and the increasing rate in China is among the fastest. Increased N deposition will inevitably alter the N cycle and carbon (C) balance of forest ecosystems. Continuous N inputs will drive the N cycle to be “more open”, and soil N supply will shift from ammonium dominant to nitrate dominant. Nitrogen deposition may promote forest productivity and thereby C sequestration when plant growth is initially limited by N supply. Species composition is one of the key factors influencing ecosystem N retention and forest productivity. Therefore, it is of great importance to determine the N use strategy and C sequestration capacity of different plant species within forest ecosystems, under the context of increased N deposition (and resultant N saturation) at both local scale and regional scale. However, so far limited information on N uptake preference and its response to N deposition is available for tree species in forests in northeastern China. In this proposed project, we will perform a 15N tracer and N addition experiment to determine N preference of four typical forest species in northeastern China, including two broadleaf species, Quercus mongolica and Juglans mandshurica, typical in natural secondary forests, and two pine species, Pinus koraiensis and Larix keampferi, typical in coniferous plantations. The objectives of the proposed project are to test if the study species have species-specific N preference during N acquisition, under the conditions of with and without N competition among plant species, and how the study species will change N uptake behavior under increased N inputs. The results of this project are expected to be valuable to both forest ecology study and forest management, from the perspectives of ecosystem N retention and C sequestration evaluation under N deposition.

我国持续上升的氮沉降将深刻影响森林生态系统氮循环和碳固定。森林物种组成是决定森林生产力和碳固定的关键因素。因此，了解和确定森林主要树种氮利用偏好以及生长固碳潜力，进而筛选更适合氮沉降背景下碳固定的树种具有重要的理论研究价值和森林经营管理方面的指导意义。有关我国东北森林主要植物是否具有氮吸收偏好性，将如何适应氮沉降引起的氮饱和等方面的研究尚无报道。为此，本项目拟采用新的15N同位素标记技术和模拟氮沉降相结合的方法来研究我国东北天然次生林和人工林中4个典型树种（蒙古栎、胡桃楸、红松和落叶松）在有和无种间氮竞争条件下的氮吸收偏好，及其对模拟氮沉降所致氮饱和的响应。研究结果将揭示我国东北森林植物氮吸收偏好性，为评价和预测氮沉降对森林生态系统氮循环和碳固定的影响提供基础。

我国持续上升的氮沉降将深刻影响森林生态系统氮循环和碳固定。森林物种组成是决定森林生产力和碳固定的关键因素。因此，了解和确定森林主要树种氮利用偏好以及生长固碳潜力，进而筛选更适合氮沉降背景下碳固定的树种具有重要的理论研究价值和森林经营管理方面的指导意义。有关我国东北森林主要植物是否具有氮吸收偏好性，将如何适应氮沉降引起的氮饱和等方面的研究尚无报道。为此，本项目拟采用新的15N同位素标记技术和模拟氮沉降相结合的方法来研究我国东北天然次生林和人工林中4个典型树种（蒙古栎、胡桃楸、红松和落叶松）在有和无种间氮竞争条件下的氮吸收偏好，及其对模拟氮沉降所致氮饱和的响应。经过3年的野外观测结合15N标记实验研究，发现在我国东北典型森林——清原落叶松人工林和天然次生林，（1）无机氮沉降量已达18.1kgNha-1yr-1，呈现明显的季节趋势，与降雨同期，沉降氮形式以NH4+-N为主；（2）经溪水氮流失达7.6 kgNha-1yr-1，与降雨同期，流失氮形式以NO3--N为主；（3）野外15N标记实验表明，所有优势乔木及灌木树种偏好吸收NO3--N，将所有树种进一步区分为外生菌根树种（ECM）和内生菌根树种（AM），发现内生菌根树种具有更快的氮吸收速率；（4）盆栽15N标记实验表明红松叶片优先吸收NO3--N，但是根系则更多积累有机氮（Glycine），红松吸收转移不同形态氮的机制待进一步研究。以上研究结果揭示我国东北森林植物氮吸收具有偏好性，以吸收NO3--N为主，并且内生菌根树种相对外生菌根树种具有更快的氮吸收速率，在未来氮沉降加剧的背景下，有可能逐渐占优势而导致森林植物多样性及群落组成的改变。以上结果为评价和预测氮沉降对森林生态系统氮循环和碳固定的影响提供了理论基础。项目执行期间，发表相关SCI论文1篇于Forest Ecology and Management上，完成了研究任务和计划。