整树水分利用对林冠模拟N沉降的响应及间接水力驱动的研究

Antropogenic activities have already generated and continue to increase the load of atmospheric nitrogen deposition on terrestrial ecosystems. The increase of nitrogen source reduces N-limition of tree growth, which can drive and enhance the carbon intake of forest. It means that the alternation of vegetation water use based on CO2 / H2O coupling would influence the regional water balance. So far, most studies on simulated atmospheric N depostion have mainly applied understory nitrogen addition. Such treatment ignores the fact that the nitrogen should be absorbed by the leaves and is, because of being directly added into the soil, soon incorporated into soil microbes, resulting in weakening the responses of trees to nitrogen. Additionally, nitrogen addition induces shift of carbon allocation that causes adaptive derivation of hydraulic architecture. It would indirectly affect the relation of stomatal conductance with environmental factors, which plays a crucial role in carbon intake and vapour exchange of forest. Accordingly, it is proposed that the adaptable change of hydraulic architecture drives the variation of stomata and indirectly affects the responses of tree water use to nitrogen deposition. We, for the first by means of simulated canopy N deposition in a deciduous broadleaf forest in Xin Yang, Henan Province, investigate the effects of nitrogen deposition on the whole-tree water use of dominant tree species and the related hydraulic driving. The results will contribute to revealing the interaction mechanism of forest C, N and H2O cycles and providing experimental supports for assessing and predicting the regional water balance in the future.

人类活动已经而且还将加重陆地生态系统承受大气N沉降的荷载，N源的增加缓解树木生长的N限制，驱动和提高森林的碳吸收，意味着基于CO2/H2O耦合的植被水分利用发生变化而影响区域水循环。至今，有关N沉降试验无一例外地采用林下施N的方式，排除了N在冠层进行交换和被叶片吸收的过程，加上土壤微生物对N的迅速同化，施加N的营养效应滞后和被弱化，以此分析森林对N沉降的功能响应掩盖了部分的真实规律。此外，N增加促使C分配的转换引发水力结构的适应性调整，间接影响决定森林C吸存和水汽交换强度的气孔导度与环境因子的关系。据此，本项目提出水力结构的调整驱动气孔变化，间接影响树木水分利用响应N源变化的观点，首次依托林冠模拟N沉降实验平台，探讨N沉降对河南信阳鸡公山落叶阔叶林优势树种整树水分利用的效应和水力驱动。研究结果有助于揭示森林C、N、H2O循环的交互作用机理，为评估和预测未来区域水量平衡提供可靠的实验支撑。

N沉降驱动和提高森林的碳吸收,基于CO2/H2O耦合的植被水分利用发生变化而影响区域水循环。本项目提出水力结构的调整驱动气孔变化，间接影响树木水分利用响应N沉降变化的观点。我们通过对河南信阳鸡公山落叶阔叶林优势树种树干液流、树干和叶片功能参数以及环境因子的监测，探讨N沉降对整树水分利用的效应和水力驱动。研究结果显示：（1）林下和林冠N添加对树木蒸腾耗水的影响不同，林下施氮降低了蒸腾速率，林冠施氮对不同树种的影响不一致，但是也没有增加蒸腾速率；（2）N沉降处理下，落叶阔叶林优势树种水分利用对水气压匮缺响应与蒸腾速率的变化相似；（3）环孔材和散孔材树种对干旱的响应存在差异，环孔材的耗水降低，其中夜间液流在响应干旱胁迫时发挥了作用。以上研究结果丰富了N沉降对森林生态系统水循环影响的研究，为碳、氮、水循环交互作用机理提供理论支持。