南亚热带常绿阔叶林土壤有机碳对林冠氮添加的响应

Nitrogen (N) deposition is one of important issues of global change. Up to date, numerous research concerning on the responses of forest ecosystems to N deposition have been carried out through the direct understory addition of N (UAN). However, the treatment of UAN migh overstate or overesitmate the effects of realistic N deposition on forest ecosystems due to the inorging of some important processes taking place through the forest canopies (e.g. absorption, retention, conversion of NOx to NH4, etc.) before the N reached to the forest floor. Based on the lasting increase of N deposition occurred in south China, we will select, in the present study, sub-tropical evergreen broad-leaved forest, which is extreme of importance for balancing the terrestrial ecosystem carbon (C) in China, to study the responses of soil organic carbon (SOC) to the elevated N deposition through a canopy addition of nitrogen (CAN) controlled experiment. The controlled experiment in this study is designed to more realistically simulate N deposition from the atmosphere. By comparing CAN with UAN, and CK approaches, we studied the temporal (1-5 years) and spatial (0-80 cm depth) responses of the variation of inputs, the dynamic and the sensitvity of SOC's fractions according to physical and chemical classification (Particulate soil organic carbon, Active organic carbon and None-labile organic carbon, respectively). Physic-chemical, stabel C and N isotopical analysis and nuclear magnetic resonance spectrum (NMR) analytic procedures will be jointly invovled in this study. Results in the study will be expected to better capture the responses of the dynamic and the stability of SOC in evergreen broad-leaved forest facing future evaledte N dopotion. This kind of natural field control experiment is significantly and scientifically important for advancing our understanding on the carbon sequestation, carbon dynamic variation and it's stability in subtropical evergreen broad-leaved forest at the background of increased N deposition in China.

N沉降增加是全球变化的代表性问题之一。森林生态系统对N沉降增加的响应研究多是通过林下直接喷施N实验开展，忽略了发生在林冠层的许多重要生态过程，可能高估森林生态系统对N沉降的响应程度。本项目立足我国南亚热带地区N沉降不断上升的现实，以在我国陆地生态系统碳平衡中占有重要地位的南亚热带常绿阔叶林为对象，依托"林冠模拟N沉降"野外控制实验样地平台，设计林冠N添加、林下N添加和对照实验处理，通过物理分组和化学分组，采用理化分析、C，N同位素技术和核磁共振光谱方法，揭示林冠N沉降对土壤有机碳输入、不同组分有机碳动态变化的影响及不同活性土壤有机碳响应的敏感性，阐明南亚热带常绿阔叶林土壤有机碳总量、形态特征和稳定性对N沉降在时间(1-5 年)和空间（0-80cm）上的响应规律、变化机理，为未来评判我国N沉降增加背景下南亚热带森林土壤的固碳功能、碳库动态和稳定水平的真实响应规律提供野外研究的实证和科学依据。

森林生态系统对N沉降增加的响应结果多是基于林下N添加实验获得，此类实验忽略了林冠生态过程，其结果可能与现实存在偏差。.本项目以南亚热带常绿阔叶林为对象，依托"林冠模拟N沉降"野外控制实验平台，设计了林冠、林下N添加和空白处理，研究了2类N添加方式对森林土壤有机碳（SOC）输入、组分动态变化、周转和稳定性的影响及其机理。.结果如下：.1）SOC组分对林冠、林下N添加的响应存在差异性，且该差异性主要发生在湿季最表层土壤(0-5cm)；.2）短期N添加仅显著提高了新鲜凋落叶C含量，未提高林下灌木叶片C含量，基于单一林下施N的实验结果可能会高估大气N沉降对植物C积累影响；.3）林冠与林下N添加并未显著改变植物N可利用性，但与光合有关的生理过程(如水分利用效率)对不同N添加方式的响应存在差别；.4）5年林冠、林下N添加未显著影响“植物-土壤”系统植硅体和植硅体碳含量，可能N并非影响植硅体产生和积累的主要因子，更长期N添加实验观测，能获得相对真实的数据支撑；.5）林冠、林下N添加对惰性SOC稳定性影响差异显著，深层土壤烷基碳和芳香碳的显著变化表明，深层土壤SOC稳定性对不同N添加方式更敏感；.6）SOC输入、周转对林冠、林下N添加响应不一致。林冠N添加减少新碳输入，延缓老碳分解；林下N添加提高新碳输入，加快老碳分解；.7）林冠、林下N添加对SOC含量动态变化影响不一致。低浓度林冠N添加，SOC含量变化趋势不明显；高浓度林冠N添加，SOC含量有下降趋势；林下高N添加下，SOC含量呈上升趋势; 在10-20 cm及以下土层，SOC含量无明显变化趋势。.本项目结果证实，仅基于林下N添加实验的结果不足以评判森林土壤碳输入、积累对大气N沉降真实响应规律。林冠N添加模拟大气N沉降的实验结果可能更接近亚热带森林土壤碳输入、周转和稳定性影响的真实性。更长期的林冠、林下N添加实验可为森林土壤SOC对大气N沉降的响应规律提高更多数据支撑。

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