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).