不同类型高寒草地群落结构与生产对施氮的响应及其敏感性

大气氮沉降增加被认为是目前重要的环境问题,会引起生物多样性的丧失和生态系统稳定性的降低。但作为草地改良的管理措施,养分添加被广泛应用于退化草地的恢复。但由于不同类型草地所处气候与群落组成的差异,对氮输入的响应可能不同。通过在藏北高原高寒草甸与高寒草甸草原设定长期氮添加梯度试验(对照, 25, 50, 100, 200 kg N hm~(-2) a~(-1)),来探讨氮输入对生物多样性与生产的影响,并估算不同类型高寒草地的氮饱和阈值。施氮对高寒草甸物种多样性指数无影响,而随着施氮量的提高高寒草甸草原植物物种数和多样性指数均逐渐降低。开始施肥前两年,随着施氮量提高高寒草甸地上生物量呈现逐渐增加趋势,随着施肥时间的延长地上生物量呈现先增加后降低的趋势。在高寒草甸草原随着施氮量提高地上生物量均呈现先增加后降低的趋势。随着施氮量提高,开始施氮前三年高寒草甸禾草植物地上生物量逐渐提高;随着施氮时间的延长,禾草和豆科植物地上生物量呈现先增加后降低的趋势。高寒草甸莎草植物地上生物量由施氮开始时的逐渐增加转变为先增加后降低趋势,最后变为逐渐降低的趋势,这说明施氮不利于莎草植物的生长。施氮只在施肥第四年显著提高杂草植物地上生物量。高寒草甸草原呈现不同的规律,开始施氮前三年随着施氮量提高,禾草植物地上生物量呈现先增加后降低的趋势;随着施氮时间的延长,禾草地上生物量逐渐提高。莎草和杂草植物地上生物量呈现先增加后降低趋势。利用对氮输入响应最敏感的植物功能群禾草生物量估算的高寒草甸和高寒草甸草原的氮饱和阈值分别是109.5、125.8 kg N hm~(-2) a~(-1),这说明高寒草甸氮敏感性显著高于高寒草甸草原。由此可见,未来氮沉降增加会对不同类型高寒草地产生不同的影响,在不同类型高寒草地进行施肥恢复时也应将氮饱和阈值的差异考虑在内。

Differential responses of community structure and production, and the sensitivities of different alpine grasslands to nitrogen addition

Increasing Nitrogen (N) deposition is widely considered to be an environmental problem that leads to biodiversity loss and reduced ecosystem resilience. However, N fertilization is also an important management measure that enhances community primary production and coverage, thereby promoting the restoration of degraded grasslands. Currently, empirical evaluations of these contrasting impacts are scarce. In this study, we evaluated the dual effects of N enrichment on biodiversity and ecosystem functioning at different organizational levels (i.e., plant species, functional groups, and community) by adding N at 0, 25, 50, 100, and 200 kg N hm^-2 a^-1 to two types of alpine grasslands in the northern Tibetan Plateau:alpine meadow and alpine meadow-steppe. We also estimated the N saturation thresholds of the different alpine grassland types. Nitrogen addition had no effect on species diversity in the alpine meadow type. On the other hand, plant species number and diversity decreased gradually with increasing N addition rate in the alpine meadow-steppe. The aboveground biomass increased gradually with increasing N addition rate in the first two years of fertilization in the alpine meadow. With extended fertilization time (after 2015), the aboveground biomass first increased and then decreased in the alpine meadow-steppe. Additionally, in the alpine meadow-steppe, the aboveground biomass first showed an increasing trend followed by a decreasing trend under increasing N addition rates. At the start of fertilization (before 2015), grass aboveground biomass in the alpine meadow increased with increasing N addition rate, and the grass and legumes aboveground biomass increased with the extension of N addition time (after 2016). Sedge aboveground biomass changed from a gradual increase at the beginning of N addition (2014) to an initial increase followed by a decrease (2015), and finally to a gradual decreasing trend (2017) in the alpine meadow, indicating that N addition is not conducive to sedge growth. Nitrogen addition only significantly increased the aboveground biomass of other forbs in 2016. The alpine meadow-steppe showed different biomass patterns. In the first three years of N addition (before 2015), the grass aboveground biomass initially increased and then decreased with increasing N addition rate. Under an extended N addition period (after 2016), grass biomass gradually increased. The aboveground biomass of sedges and forbs initially increased and then decreased. The N saturation thresholds of the alpine meadow and the alpine meadow-steppe, as estimated by the responses of grasses, were 109.5 and 125.8 kg N hm^-2 a^-1, respectively. The sensitivity of the alpine meadow was significantly higher than that of the alpine meadow-steppe. This shows that increased N deposition in the future will have different effects on different types of alpine grasslands. These differences in N saturation thresholds should also be taken into account when fertilization is carried out aiming to recover different types of alpine grasslands.