长期高氮沉降对常绿阔叶林土壤微生物多样性及有机碳稳定性的影响机制

Soil carbon and nitrogen cycles 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 organic carbon in forest ecosystems are, to a great extent, controlled and limited by nitrogen cycling. The atmospheric reactive nitrogen deposition has been increasing in China since 1980s, which resulted in a series of serious ecological and environmental problems. Therefore, the great attention has been drawn to the effects of the increasing nitrogen deposition. Although nitrogen deposition was considered to improve forest net primary productivity and carbon sequestration potential, there was very controversial on its effects on carbon storage and carbon sink in forest soils. Based on our previous study results, there existed singnificant differences in soil carbon cycling in subtropical evergreen broad-leaved forest over age sequence. However, it is not clear what controls this differences. 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 similation experiment, using chronosequence study. This project is designed (1) to determine the effects of increasing nitrogen deposition on soil microbial diversity and soil organic carbon stability; (2) to reveal the interactions between soil microbial diversity and organic carbon stability; and (3) to elucidate the responsive mechanisms of organic carbon stability to the long-term increased 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 in soil 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.

土壤碳、氮循环不仅影响有机碳储存、养分转化、森林生产力，而且与全球变化存在着复杂的交互作用。土壤碳储量和碳循环在很大程度上受到氮循环的影响和限制。自1980年代以来，我国大气活性氮沉降量不断增高，引发了严重生态环境问题，已引起高度关注。已有研究显示氮沉降能提高森林净生产力和碳汇潜力，而氮沉降对森林土壤碳储量影响却存在很大争议。基于前期研究结果，亚热带林不同林龄土壤碳循环对氮沉降的响应存在显著差异，其形成机制不明确。为此，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，利用林龄梯度，深入探讨持续高氮沉降对亚热带林土壤微生物多样性和土壤有机碳稳定性的影响，揭示土壤微生物多样性与有机碳稳定性之间的作用关系，阐明土壤有机碳稳定性对持续高氮沉降的响应机制，为亚热带林可持续经营、提高森林增汇潜力提供理论依据。

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，深入探讨持续高氮沉降对亚热带林土壤微生物多样性和土壤有机碳稳定性的影响。主要研究结果如下：. （1）为期10年的监测结果显示，试验区大气年氮沉降量为10.73–13.46 kg/hm2/yr；长期氮添加提高了林分胸径生长量，不同林分提高7.3%–13.9%；氮磷添加林分的凋落物量为6.37–9.41 Mg/hm2，整体上高于对照林分（5.82–7.90 Mg/hm2）。高氮+磷处理显著提高林分NPP（林分生长量和凋落物产量）；磷添加能够缓解持续高氮添加对林分NPP的抑制效应。. （2）长期高氮添加显著降低了土壤pH，呈现一定的酸化现象；氮磷添加的林分土壤pH降低不甚明显；氮添加一定程度上提高林分SOC，但不显著（p>0.05）；氮磷添加仅提高了老龄林的SOC含量。氮添加增加了成熟林和老龄林土壤团聚体有机质的分解程度，而低C/N新鲜有机质在中龄林团聚体中富集。. （3）持续氮添加降低中林龄土壤细菌丰度，而成熟林增加、老龄林变化不明显；不同林龄林分土壤真菌丰富度无明显差异；成熟林的碳分解基因丰度显著小于中龄林和老龄林，而固碳基因丰度在成熟林最高，中龄林和成熟林显著高于老龄林。. 研究结果表明亚热带常绿阔叶林不同林龄阶段土壤有机碳稳定性对长期氮添加的响应机制存在一定差异，丰富了土壤有机碳稳定性机制。