人类活动的不断加剧，导致全球气候及碳、氮生物地球化学循环发生了根本变化。地下部碳、氮循环不仅影响土壤有机质形成与储存、养分转化、森林生产力，而且与全球变化存在复杂的交互作用。森林在全球碳平衡中具有举足轻重的作用，森林土壤碳储量和碳循环在很大程度上受氮循环的影响和限制。因此碳、氮生物地球化学循环耦合机制的研究受到广泛关注。本项目拟采取长期定位观测与模拟试验相结合的试验方法，利用林分的年龄梯度，系统探讨亚热带常绿阔叶林地下系统碳、氮通量的变异特征及其耦合关系，揭示地下碳氮通量之间的生态计量关系，阐明常绿阔叶林地下碳氮耦合循环关键过程对氮沉降增加的响应机制，为亚热带林可持续经营、提高森林增汇潜力提供理论依据。

Global climate and biogeochemical cycles of carbon and nitrogen have been greatly altered due to the increasing of anthropogenic activities. Belowground cycles of carbon and nitrogen not only affect the formation and storage of soil organic matter, nutrient transformation and forest productivity, but also have a complicated linkage to global change. The storage and cycling of soil carbon in forest ecosystems are, to a great extent, controlled and limited by nitrogen cycling. Forest ecosystems play a significant role in global C budgets. Therefore, the great attention has been drawn to the mechanisms of coupled biogeochemical cycling of carbon and nitrogen. In the proposed research program, we are going to use an approach concentrating on the fixed observation, in which we combine long-term fixed measurement with general measurement, using chronosequence study. This project is designed (1) to determine the changes of carbon and nitrogen fluxes of belowground system and their coupled relations; (2) to reveal the ecological stroichiometry between carbon and nitrogen fluxes in belowground system; and (3) to elucidate the responsive mechanisms of the key processes of coupled cycling of carbon and nitrogen to the increasing of nitrogen deposition in subtropical evergreen broad-leaved forest. The results of the proposed study can provide some realizable methods to enhence potential of carbon sequestration and to conduct sustainable management of the subtropical evergreen broad-leaved forest. Moreover, the results can lay certain foundations for further understanding of the long-term response of forest ecosystems to global change.