Abstract:

The Tibetan Plateau is characterized by high elevation, low precipitation, low temperatures, and fragile ecosystems. The alpine forests of the Tibetan Plateau constitute 12.1% of the total plateau area, making it a representative ecosystem in the region. Alpine forests are characterized by high elevation, low precipitation and low rate of nitrogen and sulfur biochemical reaction. Therefore, they have a lower critical load for nitrogen and sulfur deposition than tropical and subtropical forests, and they are more sensitive to increasing atmospheric nitrogen and sulfur deposition. The increasing nitrogen and sulfur deposition may pose negative influences on soil and surface water chemistry in alpine forests, causing damages to ecological balance and health of the forest ecosystem. Focusing on the alpine forests of the Tibetan Plateau located in regions affected by anthropogenic disturbances (Datong and Huangyuan), this study aimed to analyze the distribution characteristics of nitrogen and sulfur between throughfall, surface soil water, and surface water. The findings of this research can serve as a regional example for developing comprehensive air pollution control policies and ecological protection policies in the Tibetan Plateau. The pH values of throughfall, surface soil water, and surface water in both Datong and Huangyuan alpine forests fell within the range of 6 to 8. Calcium ions are the main base cations found in throughfall, surface soil water, and surface water, while sulfate and nitrate are the main anions. The sulfur deposition in the Huangyuan alpine forest site (0.48 keq∙hm⁻²∙a⁻¹) exceeded the least critical load for alpine forests (0.22 keq∙hm⁻²∙a⁻¹). The nitrogen deposition in the anthropogenic disturbed Tibetan Plateau alpine forests (0.22 keq∙hm⁻²∙a⁻¹ for Datong and 0.30 keq∙hm⁻²∙a⁻¹ for Huangyuan) exceeded the average nitrogen deposition of the Tibetan Plateau (0.21 keq∙hm⁻²∙a⁻¹). Still, it remained below the least critical load for the alpine forests (0.71 keq∙hm⁻²∙a⁻¹). Consequently, nitrogen deposition is less of a concern for the region’s forests than sulfur deposition. The nitrogen and sulfur flux in both alpine forests’ throughfall were higher than that in surface soil water (nitrogen: 0.16 keq∙hm⁻²∙a⁻¹, sulfur: 0.21 keq∙hm⁻²∙a⁻¹) and surface water (nitrogen: 8.37×10⁻³ keq∙hm⁻²∙a⁻¹, sulfur: 0.13 keq∙hm⁻²∙a⁻¹). As a result, alpine forests are manifested as the nitrogen and sulfur sinks in the litter layer and alpine forest small watershed. Although little nitrogen and sulfur leach in the alpine forests, increasing nitrogen and sulfur deposition will increase nitrogen and sulfur flux in surface soil and surface water. As a result, the rising emissions of nitrogen and sulfur pollution pose an increasing ecological threat to the alpine forests in the Tibetan Plateau.

Key words: Tibetan Plateau, nitrogen deposition, sulfur deposition, nitrogen flux, sulfur flux, alpine forest

摘要：

青藏高原海拔高、降水少、气温低、生态系统脆弱，其中高寒森林面积占青藏高原总面积的12.1%，是该地区具有代表性的生态系统。高寒森林气温低、海拔高、氮硫生物化学反应速率低，导致其氮硫沉降临界负荷低于热带和亚热带森林，对大气氮硫沉降增加敏感。增加的氮硫沉降可能对高寒森林土壤和地表水化学产生负面影响，进而破坏森林生态系统的生态平衡和健康。选取人为扰动地区的青藏高原高寒森林（大通和湟源站点）为研究对象，分析氮硫在穿透水、表层土壤水和地表水中的分布特征，为中国制定大气污染综合防治政策和青藏高原生态保护政策提供区域实例。大通和湟源高寒森林的穿透水、表层土壤水和地表水pH均位于6－8之间。Ca²⁺是穿透水、表层土壤水和地表水的主要盐基阳离子，SO₄²⁻和NO₃⁻是主要阴离子。湟源高寒森林站点硫沉降量（0.48 keq∙hm⁻²∙a⁻¹）高于高寒森林最小硫沉降临界负荷0.22 keq∙hm⁻²∙a⁻¹。两处高寒森林的氮沉降量（大通为0.22 keq∙hm⁻²∙a⁻¹，湟源为0.30 keq∙hm⁻²∙a⁻¹）均超过青藏高原平均氮沉降量0.21 keq∙hm⁻²∙a⁻¹，但低于高寒森林最小氮沉降临界负荷0.71 keq∙hm⁻²∙a⁻¹。因此，硫沉降对该地区高寒森林的潜在威胁大于氮沉降。高寒森林的穿透水氮硫通量均高于表层土壤水（氮0.16 keq∙hm⁻²∙a⁻¹、硫0.21 keq∙hm⁻²∙a⁻¹）和地表水氮硫通量（氮8.37×10⁻³ keq∙hm⁻²∙a⁻¹、硫0.13 keq∙hm⁻²∙a⁻¹），因此高寒森林枯落物层和高寒森林小流域均表现为氮汇和硫汇。尽管高寒森林地表水几乎没有氮硫输出，但是氮硫沉降的增加会导致表层土壤水和地表水中氮硫通量的升高，因此，持续增加的氮、硫污染物排放量加剧了对青藏高原高寒森林的生态威胁。

关键词: 青藏高原, 氮沉降, 硫沉降, 氮通量, 硫通量, 高寒森林