生态系统净初级生产养分限制的模式是现代生态学关注的重要问题。养分的可利用性是草原生态系统生产力动态变化的关键决定因素, 但土壤养分可利用性与整个生态系统中养分限制之间的关系尚不清楚。该研究通过在藏北降水梯度上4种类型高寒草地(从东到西依次是高寒草甸、高寒草甸草原、高寒草原和高寒荒漠草原)设置氮磷养分添加试验, 系统研究氮磷养分添加对不同类型高寒草地的影响, 并探讨降水梯度上高寒草地的氮磷限制模式。结果表明: (1)氮磷添加对不同高寒草地的影响存在差异: 氮添加显著提高了高寒草甸和高寒草甸草原地上生产力, 而对高寒草原和高寒荒漠草原无影响; 单独磷添加对4种高寒草地均无显著影响, 而氮磷添加对4种高寒草地地上生产力均有促进作用。(2)通过计算氮磷共同限制指数发现: 随着降水量减少, 高寒草地氮限制指数从1.18逐渐降低到0.52-0.64, 养分限制模式从氮限制过渡到氮磷共同限制; 磷限制指数在高寒草甸草原和高寒草原为负值, 说明单独磷添加对高寒草甸的生产力有负向作用, 高寒草甸主要受氮限制; 高寒草甸草原介于氮限制与氮磷限制之间, 受到氮磷共同限制, 单独磷添加有负向作用; 高寒荒漠草原受到氮磷共同限制。研究表明, 高寒草地氮磷限制模式存在环境梯度上的递变规律, 随着降水量减少, 高寒草地养分限制模式从氮限制逐渐过渡到氮磷共同限制。由此推断, 未来气候变化条件下氮沉降增加对不同类型高寒草地的影响可能存在差异。同时, 利用养分添加恢复不同类型退化高寒草地时也应将氮磷限制模式的差异考虑进去。

Aims Understanding the mode of nutrient limitation on ecosystem net primary production is an important issue of modern ecology. Nutrient availability is a key determinant of ecosystem dynamics, but the relationship between soil resource availability and ecosystem nutrient limitation is still unclear.
Methods A series of nitrogen and phosphorus nutrient addition experiments were set up in four types of alpine grasslands (alpine meadow, alpine meadow-steppe, alpine steppe and alpine desert-steppe) along the precipitation gradient on the Northern Xizang, to systematically study the effects of nitrogen and phosphorus addition on different types of alpine grasslands, and to explore the nitrogen and phosphorus limitation models of different alpine grasslands.
Important findings The results showed that: (1) The effects of nitrogen and phosphorus addition on different alpine grasslands varied. Nitrogen addition significantly increased the aboveground biomass of alpine meadows and alpine meadow grasslands, but had no effect on alpine meadows and alpine desert grasslands. The addition of phosphorus alone had no significant effect on the four alpine grasslands, while the addition of nitrogen and phosphorus had a promoting effect on the aboveground biomass of the four alpine grasslands. (2) With the decrease of precipitation, the nitrogen limitation index of the alpine grasslands gradually decreased from 1.18 to 0.52-0.64, and the nutrient limitation mode transitioned from nitrogen limitation to co-limitation by nitrogen and phosphorus; the phosphorus limitation index was negative in the alpine meadow-steppe and alpine steppe, indicating that phosphorus addition alone has side effects on these two grassland types. These results suggest that alpine meadow is mainly limited by nitrogen availability, and phosphorus addition alone has side effects; the alpine meadow-steppe is between the nitrogen limit and the joint nitrogen and phosphorus limitation, and phosphorus addition alone also has side effects; the alpine steppe is limited by both nitrogen and phosphorus availability, and the addition of phosphorus has side effects; the alpine desert-steppe is jointly limited by nitrogen and phosphorus availability. These results show that nutrient limitation mode transits from nitrogen limitation to nitrogen and phosphorus co-limitation with the decrease of precipitation. This study implies that the impacts of increasing nitrogen deposition under future climate change on different types of alpine grasslands may be different. Additionally, the differences in nitrogen and phosphorus limitation mode should also be taken into consideration when nutrient addition is used to restore different types of degraded alpine grasslands.