Nutrient Additions Regulate Height Growth Rate but Not Biomass Growth Rate of Alpine Plants Through the Contrasting Effect of Total and Available Nitrogen

Plant growth, a fundamental biological process that underpins terrestrial ecosystem function, is susceptible to nutrient availability. Despite extensive research on lowland ecosystems, the responses of alpine plant growth to nutrient addition remain poorly understood, particularly given the heightened sensitivity of alpine ecosystems to global change. To investigate the effects of nitrogen (N) and phosphorus (P) additions on the growth rates of alpine plants and the underlying mechanisms of how these nutrient additions influence plant growth rates, we conducted an experiment in an alpine grassland on the Qinghai–Tibet Plateau, targeting 14 common plant species. Growth rates were measured using biomass and height, with plant height and soil physicochemical properties recorded biweekly during the growing season. We assessed the effects of nitrogen and phosphorus additions on growth rates, their seasonal dynamics, and their relationships with soil physicochemical properties. Results showed that phosphorus addition and combined nitrogen-phosphorus additions significantly increased the relative growth rate based on height (RGR_H). In contrast, nutrient additions had no significant effect on the relative growth rate based on biomass (RGR_B). RGR_H decreased from June and early July to August, exhibiting species-specific responses to nutrient additions. Additionally, RGR_H was significantly influenced by the interaction of nitrogen and phosphorus additions, species, and seasonal dynamics (<i>p</i> < 0.05). Soil available N, available P, and moisture were significantly positively correlated with RGR_H (<i>p</i> < 0.05), while soil temperature (ST), total nitrogen (TN), and soil organic carbon (SOC) exhibited significant negative correlations (<i>p</i> < 0.05). Nutrient additions altered the hierarchy, as well as the direct and indirect factors that influence RGR_H, revealing the opposing regulatory effects of total and available nitrogen. These findings highlight the critical roles of nitrogen and phosphorus, suggesting phosphorus is a potential limiting factor for plant growth in this alpine region. This study offers a comprehensive analysis of how nitrogen and phosphorus additions affect alpine plant growth rates and clarifies the underlying mechanisms in these sensitive ecosystems.