杨树和杉木人工林细根生长动态对氮沉降的响应机制

Nitrogen deposition has been one of the hot topics in ecological science. Human interference on the global nitrogen cycle has gone far beyond the impact of other major biogeochemical cycles on Earth. The physiological and ecological response processes of fine root in forest ecosystem to the continuous nitrogen growth and rapid nitrogen cycle caused by nitrogen deposition has been put more attention by ecologists. In this project, Poplar and Chinese fir have been used as the experimental species. By simulated nitrogen deposition method (gradient nitrogen fertilization increased), Response patterns to the fine root production, mortality, lifespan and turnover in three plantations to increased soil available N content are studied, and then the response mechanism of fine root dynamics to nitrogen deposition in plantation are investigated by combining with the fine root physiological and morphological structure (such as the cortex and vascular diameter, length, specific root length, branching level), chemical composition (eg, N, TNC, lignin, cellulose), mycorrhizal infection, stand age-structure and other characteristics. This study will help us to understand clearly the feedback mechanisms between underground physiological and ecological processes (especially the root effect) in forest and the increasing nitrogen deposition, and there will have important theoretical significance and application value for scientifically managing and improving productivity in plantation under the conditions of global change.

目前，N 沉降已成为国际生态学领域的热点问题之一，人类对全球氮循环的干扰已经远远超过对其它主要生物地球化学循环的影响。而森林细根的生理生态过程如何响应由氮沉降而引起的持续氮增长和快速氮循环，已成为生态学家关注的焦点。本项目拟选取杨树和杉木人工林生态系统，通过模拟氮沉降方法（即多梯度施肥增氮试验）来研究树木细根生长量、死亡量、寿命及周转对土壤有效N含量增加的响应模式，并结合细根生理及形态结构（如皮层和维管束，直径、长度、比根长、分枝等级）、化学组成（如N，TNC，木质素，纤维素）、菌根侵染、林龄结构等特征，探讨人工林细根生长动态对氮沉降的响应机制。这将有利于更清晰地了解森林地下生理生态过程（尤其是根系效应）与持续氮沉降之间的反馈机制，对于实现全球变化条件下人工林的科学管理，提高森林生产力具有重要理论意义和应用价值。

森林细根的生理生态过程如何响应由氮沉降而引起的持续氮增长和快速氮循环，已成为生态学家关注的焦点。本项目选取杨树和水杉人工林生态系统，通过模拟氮沉降方法（即多梯度施肥增氮试验）来研究树木细根生长量、死亡量、寿命及周转对土壤有效N含量增加的响应机制。研究结果表明：（1）外源氮输入增加小径级（0~0.5 mm）的根系生物量，特别是幼龄林；林龄、土层、径级以及施氮浓度四个因素的综合效应能够解释细根生物量66.3%的变异，其中林龄、土层、径级三个因素各自对细根生物量的影响极显著，而增氮处理影响并不显著。（2）氮添加对细根生产无显著影响，但对细根死亡影响显著，高氮（N4）导致细根死亡量显著增加；细根周转随氮浓度增加呈现先减小后增加的趋势，但差异不显著。（3）随林龄增加，细根寿命延长；总体上低氮（N1或N2）处理有助于延长细根寿命，而高氮处理（N3或N4）会降低细根累积存活率，缩短细根寿命；通常小直径、低级根序、土壤表层根系和春夏季根群细根受氮处理的影响效果较为明显。（4）N沉降对细根碳水化合物的影响主要表现为淀粉含量和纤维素含量变化，二者在低氮（N1）时含量最高；随施N浓度增加，细根中全N含量逐渐增大，C/N逐渐减小。（5）细根直径、皮层厚度和维管组织直径或初生木质部直径均随施N浓度升高而减小，在N1或N2处理时达最大值；比根长随施N浓度升高而减小；细根直径变化对模拟N沉降的响应，在解剖结构上主要受皮层厚度变化和初生木质部或维管组织直径的变化的影响。研究结果有利于更清晰地了解森林地下生理生态过程（尤其是根系效应）与持续氮沉降之间的反馈机制，对于实现全球变化条件下人工林的科学管理，提高森林生产力具有重要理论意义和应用价值。