1. IntroductionThe Three Rivers Source Region, loca in alpine grasslands (Wright et al., 2001). The application of nitrogen fertilizers has become one of the most common methods for restoring degraded grasslands (Du et al., 2020; Zong et al., 2019). In experiments involving nitrogen addition in alpine grasslands, researchers often choose either single inorganic nitrogen or organic nitrogen sources. Soil inorganic nitrogen mainly consists of ammonium (NH4+) and nitrate (NO3-), which plant roots can directly absorb and utilize from the soil (Zhong et al., 2015). Organic nitrogen, primarily in the form of urea, serves to supplement the nutritional elements essential for plant growth in soil. However, during its conversion into nitrogen that plants can absorb, there is a potential for some nitrogen loss (Zhang et al., 2019).The addition of ammonium and nitrate nitrogen can greatly alleviate nutrient limitation in degraded alpine grasslands, increase plant species richness, enhance soil nutrients, and boost mycorrhizal fungal richness (Shi et al., 2024). The addition of urea quickly improved nitrogen turnover potential (Zhang et al., 2019) and microbial biomass (Li et al., 2020). Furthermore, it can enhance the abundance of decomposing bacteria and fungi, thereby facilitating nitrogen cycling in grassland ecosystems (Liao et al., 2024). With increasing duration of urea and ammonium nitrate addition, nitrogen fertilizers have negative effects on decreasing soil pH, leading to soil acidification, transition from alkaline cations (Ca2+, Mg2+, K+) to non–alkaline cations (Mn2+, Al3+), and causing toxicity from manganese and aluminum ions (Tian & Niu, 2015). Moreover, long–term nitrogen addition can reduce bacterial diversity and fungal community diversity in soil (Chen et al., 2020; Dai et al., 2018). Therefore, the input of different forms of nitrogen and the timing of nitrogen addition can affect plant uptake, alter plant richness, and thereby modify soil nutrient regulation and biodiversity. However, since soil is the primary nutrient source for terrestrial plants, soil properties such as pH and nitrogen availability, influenced by various nitrogen compounds inputs, can have different impacts on plant traits and growth. Most studies focus on evaluating the restoration effectiveness of alpine grassland (Gao et al., 2019; Zhang et al., 2022), but little is known about how nitrogen addition affects the biodiversity and ecosystem multifunctionality of grasslands. Understanding the relationship between biodiversity and ecosystems is particularly important (Naeem et al., 2012). Biodiversity plays a crucial role in maintaining ecosystem stability and health, preventing plant extinction, and providing ecological functions (Bai et al., 2004; 2020; Yang et al., 2021). Especially in terms of plant diversity, as primary producers, plants play a crucial role in the most important conversion from inorganic to organic matter in ecosystem material cycling. Plant diversity is closely related to the stability, productivity, resilience, and capacity to adapt to global climate change of ecosystems (O’Brien et al., 2022; Urban et al., 2016).Biodiversity serves as a buffer for ecosystems, preserving a large number of species, and high biodiversity can greatly mitigate impacts such as climate change and overgrazing, especially in ecologically fragile and sensitive areas (Trew & Maclean, 2021). Ecosystem multifunctionality refers to the attribute of ecosystems simultaneously providing multiple functions, including production, material cycling, and carbon sequestration (Manning et al., 2018; Xu et al., 2016). Ecosystem multifunctionality is an important method for comprehensively understanding and evaluating the health of an ecosystem, especially for sensitive and fragile areas that require protection (Jing & He, 2021). The relationship between biodiversity and ecosystem multifunctionality is very close, with higher biodiversity generally increasing the ability of ecosystems to simultaneously deliver multiple functions (Mori et al., 2023). Meanwhile, ecosystems with higher biodiversity have a mitigating effect on the multifunctionality of disturbed ecosystems (Zhang et al., 2022). The relationship between biodiversity and multifunctionality is influenced by both biotic and abiotic factors (van der Plas, 2019). For example, the relationship between diversity and multifunctionality is affected by the complexity of trophic levels; the more complex the trophic levels, the closer the relationship between the two (Jiao et al., 2022). Climate factors are also important influencing factors of the relationship between the two; for example, in arid ecosystems, increased precipitation tightens the relationship between biodiversity and multifunctionality (Zhai et al., 2024).Therefore, considering the context of nitrogen supplementation for the restoration of slightly degraded alpine grasslands, an examination of the interplay between diversity and ecosystem multifunctionality becomes imperative. This exploration can deepen our comprehension of ecosystem value and functions, facilitating a comprehensive evaluation of ecosystem health and thereby offering insights of the restoration and preservation of alpine ecosystems and their surrounding environment (Tian et al., 2022). To address these objectives, we conducted study in slightly degraded alpine grasslands located in Maqin County, Qinghai. Through the combination of rapidly absorbable inorganic nitrogen (nitrate nitrogen and ammonium nitrogen) with more persistent organic nitrogen (urea), we established various treatment conditions. Our primary aim was to scrutinize the impacts of both short–term and long–term nitrogen supplementation on vegetation and ecosystem functionalities, thereby furnishing theoretical underpinnings and empirical evidence for the restoration of degraded alpine grasslands. We formulated the following hypotheses: (1) The number of plant species show a downward trend under long–term nitrogen addition; (2) The relationship between species diversity and ecosystem multifunctionality changes with the timing of nitrogen addition; (3) Species gain rates alter the number of species and thus affect ecosystem multifunctionality under short–term nitrogen addition (Figure 1).

1. Biodiversity and ecosystem multifunctionality are currently hot topics in ecological research. However, little is known about the role of multitrophic diversity in regulating various ecosystem functions, which limits our ability to predict the impact of biodiversity loss on human well–being and ecosystem multifunctionality. 2. In this study, multitrophic diversity was divided into three categories: plant, animal, and microbial communities (i.e., plant diversity, rodent diversity, bacterial and fungal diversity). Also, 15 ecosystem functions were divided into four categories–water conservation, soil fertility, nutrient cycling and transformation, and community production–to evaluate the significance of biotic and abiotic variables in maintaining ecosystem multifunctionality. 3. Results indicated that species diversity at multiple trophic levels had a greater positive impact on ecosystem multifunctionality than species diversity at a single trophic level. Notably, the specific nature of this relationship depended on the niche breadths of plants, indicating that plants were key indicators linking above and below ground trophic levels. Abiotic factors such as altitude and pH directly acted on ecosystem multifunctionality and could explain changes in ecosystem functions. 4. Overall, our study offers valuable insights into the critical role of multitrophic species diversity in preserving ecosystem multifunctionality within alpine grassland communities, as well as strong support for the importance of biodiversity protection.

We conducted a comprehensive investigation of the interrelationships among the species diversity, productivity, community structure, and soil nutrients of vegetation communities of an alpine meadow ecosystem on the eastern Qinghai–Tibet Plateau. We performed biodiversity manipulation experiments to examine the effects of removing plant functional groups (Gramineae, Cyperaceae, Legumes, and other Forbs) for 3 and 10 years at a research station in Haibei. Interannual variation in the species richness and above- and belowground biomass of the community gradually decreased over time. Species richness and productivity were positively correlated, and this correlation became increasingly significant over time. Removal of the plant functional groups resulted in fewer Gramineae species within the community. However, soil total nitrogen, phosphorus, organic matter, and moisture contents increased significantly in the Legume removal treatment. The removal of other Forbs led to the lowest negative cohesion values, suggesting that this community may have had difficulty recovering its previous equilibrium state within a short period of time. The effects of species removal on the ecosystem were likely influenced by the species structure and composition within the community. Changes in the number of Gramineae species indicated that they were more sensitive and less resistant to plant functional group removal. Legume removal may also have indirectly caused distinct community responses through starvation and compensation effects. In summary, species loss at the community level led to extensive species niche shifts, which caused community resource redistribution and significant changes in community structure.

In the Alpine Meadow ecosystems of eastern Qinghai-Tibet Plateau, the interrelations among the species diversity of different vegetation communities, productivity, community structure as well as soil nutrients were thoroughly researched through running biodiversity manipulation experiment to explore the species survey consequences of 3 and 10 years of plant functional groups (Gramineae, Cyperaceae, Legumes, and other Forbs) removal at Haibei station. The results demonstrated that the interannual variation of the remaining species richness, above-ground and below-ground biomass of the community gradually presented a tendency to decrease as the removal time increased, and there was a positive relationship between species richness and productivity, and the correlation became increasingly significant. The removal behavior reduced the number of Gramineae within the remaining community. The content of soil total nitrogen, phosphorus, organic matter and moisture content of Legumes loss treatment increased significantly. The treatment that removal Forb had the lowest negative cohesion values, revealing that it is difficulty for this community to recover to the previous equilibrium state in a short time. In our study, all affects of species removal on ecosystem may be related to variance in the structure and composition of species in community. Meanwhile, changes in the number of Gramineae indicated that Gramineae are more sensitive and less resistant to removal behavior. Furthermore, the specific performance of Legumes removal indirectly indicates that the loss of diverse plant function groups prompted distinct replies to the starvation and compensation effects. In a nutshell, species loss at the community level leads to shifts in the niche of each species, inducing a redistribution of community resources and leading to significant changes in community structure.