Distributions and controlling factors of soil total nitrogen and nitrogen fractions along an altitude gradient in Qinghai-Tibet Plateau.

Purpose: Understanding spatial variations in soil nutrients and its drivers is essential for sustainable nutrient management and related environmental assessment in terrestrial ecosystems. Here, we investigated the distribution characteristics and controlling factors of soil total nitrogen (N) and N fractions along an altitude gradient in an alpine meadow of the Qinghai-Tibet Plateau. Materials and methods: Six experimental sites at altitudes 4400, 4500, 4650, 4800, 4950, and 5200 m were established on southern slope of Nyainqentanglha Mountain since 2005. Total N and N fractions were quantified and their relationships with climate, vegetation, and edaphic factors were analyzed by Pearson correlation, redundancy analysis, and structural equation modeling. Results and discussion: With increasing altitude, the contents of total N, alkali-hydrolyzable N, inorganic N fractions (ammonium N and nitrate N), and organic N fractions (acid-hydrolyzable and acid-insoluble organic N) gradually increased, reached their maximums at altitude 4800 or 4950 m, and then decreased. Soil organic carbon explained 90.5% of the variations in TN and N fractions, and soil moisture, pH, mineralogy, texture, and N hydrolyzing enzyme activity explained 8.80% of the variations in TN and N fractions. Mean annual temperature and precipitation exhibited direct and positive impact on soil organic carbon, which had positive effect on inorganic N fractions and plant aboveground biomass in sequence. Conclusions: Soil organic carbon is the most dominant factors driving the variations in TN and N fractions along the altitude gradient. Climate change can impact alpine grassland productivity by regulating soil inorganic N distribution. [ABSTRACT FROM AUTHOR]