Physiological Nitrogen Uptake and Utilisation Responses in Two Native Plants from the Qinghai-Tibet Plateau under Different Water and Fertiliser Conditions

Drought and poor soil quality are the main characteristics of extreme environments in arctic–alpine areas. Understanding how herbaceous plants in alpine grasslands maintain the normal supply and utilisation of nutrients under different rainfall conditions is key to maintaining population stability. In the present study, the native plants Poa crymophila and Stipa purpurea of the Qinghai–Tibet Plateau were used to conduct a controlled experiment involving water and fertiliser to analyse their physiological responses in terms of nutrient uptake and utilisation. The results showed that decreased soil moisture increased proline and non-structural carbohydrates in P. crymophila, mainly accumulating in the leaves and stems. Nitrogen (N) addition promoted proline accumulation, whereas nonstructural carbohydrate content decreased. However, the proline and non-structural carbohydrate contents of S. purpurea were less affected by water and fertiliser. Additionally, drought restricted rhizospheric and non-rhizospheric alkaline-hydrolysed N release, increased rapidly available phosphorus (RAP) content in rhizospheric soil, limited root growth, and reduced surface area, root length, and root volume. Both aboveground and underground N fertiliser utilisation rates decreased. Under well-hydrated conditions (WH), high N levels increased rhizospheric alkaline-hydrolysed N and urease activity while inhibiting RAP and activity of alkaline phosphatase contents, thereby limiting root growth and reducing N fertiliser utilisation. The results indicate that both plant species have relatively low overall nutrient requirements that are limited mainly by water availability. The addition of low amounts of fertiliser is beneficial for nutrient release and utilisation, improving their adaptability to arctic–alpine environments and their suitability and superiority in the community. This study has significant implications for nutrient management and ecological restoration measures in arctic–alpine grasslands.