长白山苔原带凋落物生态化学计量特征及其对模拟氮沉降的响应

长白山苔原是我国乃至欧亚大陆东部独有的高山苔原，根据前人调查植被以灌木苔原为主要类型。在全球变暖背景下，近30年来，长白山岳桦林下的草本植物侵入苔原带，原生灌木苔原分化为灌木苔原、灌草苔原和草本苔原，形成了灌木、灌草混合和草本3种不同类型的凋落物，凋落物数量和质量发生显著改变。与此同时长白山苔原氮沉降量也在逐年增加，导致了土壤中氮的累积，势必影响凋落物的分解。凋落物作为连接植物和土壤的纽带，其分解过程中碳（C）、氮（N）、磷（P）等化学组分和化学计量比的变化直接和间接影响着土壤养分有效性和植物养分利用策略。为揭示氮沉降增加对长白山苔原带不同类型凋落物化学组分及生态化学计量特征早期变化的影响，开展了为期8个月的模拟氮沉降室内凋落物分解实验。在苔原带采集灌木优势种牛皮杜鹃和草本优势种小叶章的凋落物带回实验室，模拟灌木牛皮杜鹃群落、灌草混合的牛皮杜鹃-小叶章群落和草本小叶章群落的3种不同类型凋落物，设置三个施氮处理：对照（CK，0 g N m^-2 a^-1）、低氮（LN，10 g N m^-2 a^-1）、高氮（HN，20 g N m^-2 a^-1）。研究表明：（1）不施氮处理时，3种凋落物的C、P均呈释放状态，木质素（Li）呈先累积再略有降解趋势；牛皮杜鹃凋落物的N元素富集而其余两种凋落物N元素呈释放状态；灌草混合和草本凋落物比原生的灌木凋落物C和N元素释放快、Li累积少；而灌木凋落物的P释放略快于灌草和草本凋落物。3种植被类型凋落物的C/N、C/P、Li/N大小表现为：牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物>小叶章凋落物；N/P表现为：小叶章凋落物>牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物。（2）氮沉降促进3种类型凋落物分解过程中C、N和P化学组分的释放，且氮浓度越高促进作用越显著。在牛皮杜鹃凋落物分解过程中，氮素添加到达某一阈值后，其C/N、C/P、N/P、Li/N的降幅最大，后续若再增加氮素，其对化学计量比的影响均会减弱；本实验中的氮素添加量增加促进了小叶章凋落物的C/N、Li/N下降。（3）草本植物入侵引起凋落物类型的变化带来凋落物分解加快，将导致长白山苔原带养分循环的变化；氮沉降增加对小叶章凋落物化学组分的释放及C/N、Li/N的下降更为促进，小叶章凋落物内难分解化合物减少，分解受到促进。高氮沉降加快了小叶章凋落物与土壤、草本植物之间的养分循环。因此，随着未来苔原带氮沉降量的增加，将更有利于小叶章在与牛皮杜鹃的竞争中获胜，使苔原带呈现草甸化趋势。

Response of chemical composition and ecological stoichiometric characteristics of three types of litter to simulated nitrogen deposition in the Changbai Mountain tundra

The Changbai Mountain tundra is the only typical alpine tundra of eastern China and eastern Eurasia. In the recent 30 years, herbaceous plants have invaded from low altitude to the alpine tundra, which transformed shrub tundra to shrub-herb-mixed tundra and herb tundra, causing significant changes to the litter components. To understand the response of chemical composition and ecological stoichiometric characteristics of three types of litter to simulated nitrogen deposition in the Changbai Mountain tundra, an artificial simulated litter decomposition experiment was designed and conducted. Rhododendron aureum (native dominant shrub species) litter, Deyeuxia angustifolia (invasion herb species) litter and Rhododendron aureum-Deyeuxia angustifolia-mixed litter collected from the Changbai alpine tundra were used to set up nitrogen addition experiment at three levels under laboratory condition:control group (CK:without nitrogen addition), low N level group (LN:10 g N m^-2 a^-1)), and high N level group (HN:20 g N m^-2 a^-1)). Then we collected litter after decomposition and measured the content of C, N, P and lignin (Li), aiming to study the differences of chemical composition and ecological stoichiometric characteristics of the litter with different components and nitrogen addition levels. The results showed that:(1) Without nitrogen addition, the content of C and P in the litter reduced significantly while the content of Li increased at the beginning and then reduced; the content of N in the R. aureum litter increased significantly while the content of N in the R. aureum litter and the mixed litter reduced. C/N, C/P, Li/N of the litter of three types decreased by R. aureum>R. aureum-D. angustifolia-mixed>D. angustifolia; N/P of the litter of three types decreased by D. angustifolia>R. aureum> R. aureum-D. angustifolia-mixed. (2) The simulated nitrogen deposition promoted the release of nutrients during decomposition process and the promoting effects of high nitrogen addition turned out to be more significant. C/N, C/P, N/P, Li/N of the R. aureum litter decreased rapidly at a certain nitrogen addition threshold then the effects on ecological stoichiometric characteristics would be weakened if more nitrogen was added. In our study, high nitrogen addition promoted the decline of C/N and Li/N of the D. angustifolia litter. (3) Change of litter components caused by herbaceous species invasion would promote the decomposition of litter and affect the nutrient cycle mechanisms of Changbai Mountain alpine tundra. As nitrogen deposition increased, the contents of C, N, P, Li and C/N, Li/N of the D. angustifolia litter decreased further and refractory compounds decreased. Nitrogen deposition accelerated nutrients cycle between the D. angustifolia litter, soils and herbaceous plants. Thus, we suggest that the increase of nitrogen deposition will be more favorable to the win of D. angustifolia in the competition with R. aureum, which will impulse meadowization trend in the Changbai Mountain alpine tundra.