我国东北两个典型次生林大气沉降氮的去向和周转：15N示踪

With globalization of nitrogen (N) deposition, quantifying the distribution and turnover of the atmospherically deposited N in plant-soil systems is a key step for evaluating nitrogen deposition contribution to carbon sequestration in forest ecosystem and the fates to the environments. However, we cannot reach these goals by traditional nitrogen addition experiments. Carbon sequestration potential in forests is expected to play a critically important role in compensating anthropogenic CO2 emissions in China, but research on to what extent forest productivity and carbon sequestration has been improved by N deposition and expected to be in the future is still limited. In the proposed project, we will perform a field 15N tracer experiment in two typical temperate forests in northeastern China, to trace the distribution of the deposited N in plants and soils, and turnover among different soil N pools (fine root, microorganism, ammonium, nitrate, dissolved organic N and amino acids) by relatively frequent sampling following applications of 15N tracer. The two studied forests will be a natural secondary forest and a larch plantation in Qingyuan county, Liaoning Province. The forests are about 45 years old, being adjacent to each other. Three replicated plots will be set in each forest, with each plot size 20 m by 20 m. 0.25 kg N/ha.yr Ammonium nitrate labled in ammonium will be applied on one half of each plot and 0.25 kg N/ha.yr Ammonium nitrate labled in nitrate on the other half. Plants and soils will be sampled one and two years after 15N labeling, to quantify the fates of labeled N in plants and soils. Additional soils will be collected to trace labeled N transfers among soil N pools following labeling. The results of this study are expected to reveal how the deposited N will distribute and transform within plants and soils, and to quantify the contribution of N deposition on carbon sequestration in typical temperate forests in northeastern China. These results can also be helpful to reveal the mechanisms limiting plants to absorb and utilize the deposited N.

随着氮沉降全球化和我国氮沉降形势越来越严峻，需要准确量化沉降氮在植物和土壤中的分配和周转，从而评估氮沉降对森林生态系统碳吸存贡献和预测氮沉降的最终去向。然而，通过传统的外加氮模拟氮沉降实验难以实现这些目标。我国森林固碳潜力巨大，但对于氮沉降在多大程度上提高了我国森林生产力和碳吸存却知之甚少。为此，本研究拟选取我国东北典型天然次生林和落叶松人工林为研究对象，运用野外生态系统水平上的15N示踪技术，研究大气沉降的氮在森林植物和土壤中的分配，跟踪标记的15N在土壤不同的氮库（细根、微生物、铵态氮、硝态氮、可溶性有机氮(DON)、氨基酸）之间的周转。本研究的结果将可以揭示大气沉降氮在我国东北森林植物和土壤分配和周转规律，在评价氮沉降对我国东北森林生态系统碳吸存的贡献、揭示限制植物吸收利用沉降氮的机制和认识氮沉降对森林氮循环的影响和环境效应等具有重要的理论价值。

持续增加的氮沉降对森林的影响是多方面的，其结果对全球环境变化的影响可能是正反馈，也可能是负反馈。准确量化沉降氮在系统中的去向和分配，是评估氮沉降对森林结构和功能影响的关键；明确影响沉降氮滞留和再分配的因素，是评估和预测氮沉降对全球森林综合效应的理论基础。本研究选取我国东北2个样点的落叶松人工林和次生林，亚热带的1个南方铁杉林等4个森林为研究对象。运用野外生态系统水平上的15N示踪技术，追踪沉降氮（NH4+ 和NO3-）在植物和土壤等组分中的去向分配；探讨两种形态沉降氮在不同氮循环状态、不同外加氮水平和不同气候带森林滞留的主要影响因素。结果表明，15N标记1年后，自然氮沉降背景下，东北落叶松人工林和次生混交林总回收率变化范围为47-64%，低于世界范围内森林已有的研究平均值（75%）。落叶松人工林向混交次生林演替（辽宁清原），地表凋落物分解速率加快，氮有效性增加降低了NO3-的总回收率。同样，外加氮（100 kg N ha-1year-1）处理也降低了落叶松人工林（黑龙江帽儿山）对NO3-的总回收率。以上两种情景下，NH4+总回收率的变化都很小。在4个森林中，NH4+都更易被凋落物层和矿质土壤吸收固持，NO3- 更易被植物吸收利用。在标记后短时间内（≤1年），地表凋落物都是两种形态沉降氮最大的去向库。随着凋落物分解，矿质土壤和植物成为主要的沉降氮去向库（3年）。亚热带南方铁杉林地表凋落物存量低和分解速率快，在标记1年后，矿质土壤和植物分别为NH4+和NO3-的最大去向库。矿质土壤中有机碳氮含量和乔木树种组成 (AM和ECM)是分别影响各自氮库对沉降氮固持能力的重要因素。本项目首次系统量化了沉降氮在我国多个典型森林的去向；阐明了沉降氮滞留和再分配的主要影响因素；获得了大量生物量和碳氮储量等第一手数据；探索了15N标记技术在古树保育过程的应用。培养博士生2名，博士后1名。已完成1篇论文稿，即将投稿，正在撰写1篇论文稿。