Abstract: An investigation on soil soluble nitrogen and its influence factors was carried out in different vegetation types (alpine steppe, alpine meadow, grassland, forest land, desert, and saline land) along different altitudes (2500-5500 m) on the Tibetan Plateau, China. The results showed that total soluble nitrogen (TSN) in the soils appeared at high levels, and the major form of soil soluble nitrogen was soluble organic nitrogen (SON), accounting for 50% of TSN. In addition, the proportion of NO₃^--N to soluble inorganic nitrogen (SIN) was higher (60%) than that of NH₄⁺-N. Variance analysis showed that the difference of soil soluble nitrogen was significant at different altitudes, and TSN and SON in alpine steppe and alpine meadow were significantly higher than in the other vegetation types. Soil pH was negatively correlated with 〖JP2〗the different forms of soil soluble nitrogen, suggesting that soil pH was an important factor impacting soil soluble nitrogen. Stepwise regression analysis showed that air relative humidity and temperature in growing season were the most significant factors impacting soil NH₄⁺-N and NO₃^--N, respectively.

Key words: mechanistic model of light-response, light-use efficiency, CO₂ concentration., wateruse efficiency, soybean