Abstract: Nitrogen (N) is one of the most important nutrient elements controlling plant structure and function and maintaining ecosystem stability. The differences in exogenous N availability and form have significant effect on plant growth and physiological traits. Due to global climate change and intensified human activities, the mobility and deposition of N into terrestrial ecosystems increase rapidly and the form of N deposition is also changing, which seriously damages the normal growth of plants and the balance and stability of ecosystem, and has become a hot topic. We summarized the effects of different N addition levels and forms on plant growth, photosynthesis, nutrient absorption, and metabolic enzyme activity. We concluded that: (1) proper N input could promote plant growth, photosynthesis, and nutrient absorption capacity, but has inhibiting effect on plants once exceeding the threshold of plant tolerance; (2) due to the difference in plant preference for N form absorption, the effects of ammonium or nitrate N on plant growth, photosynthesis, nutrient absorption, and metabolic enzyme activity are different, and the appropriate ratio of ammonium and nitrate N has a more significant promotion effect on most plants than the addition of a single N form. Four important research directions should be considered in future: (1) carrying out large scale longterm positioning observation of manipulative experiments; (2) exploring the microscopic mechanism of N form affecting plants using molecular biology techniques; (3) focusing on the effect of soil rhizosphere environment on root N uptake; (4) comprehensively analyzing the interaction between N addition and other environmental factors and accurately assessing typical plant population dynamics and community structure changes. Our study provides theoretical reference for exploring the mechanism of plant growth and physiology and enabling sustainability of plant production systems under exogenous N addition.

Key words: N input, N availability, N form, plant growth, plant physiology and ecology