Response of soil nutrients and stoichiometry to elevated nitrogen deposition in alpine grassland on the Qinghai-Tibetan Plateau

Nitrogen deposition is notable in China, even in the high altitude region of Qinghai-Tibet Plateau (QTP). To identify the effects of nitrogen deposition on soil nutrient and ecological stoichiometry of the QTP's alpine grasslands, we treated three major types of grasslands (alpine meadow, alpine steppe, and cultivated grassland) with 6 different nitrogen addition levels: 0, 8, 24, 40, 56, 72 kg N ha(-1)yr(-1) through the years of 2015 to 2016. We sampled the soils at two depths, 0-10 and 10-20 cm in the peak-growing season (July) of each year. Results showed that: (i) soil total carbon and phosphorus are relatively stable under nitrogen deposition. (ii) Soil total nitrogen increases under low nitrogen addition rates while decreases under higher rates, and soil available nitrogen increases under nitrogen deposition in surface soil while decreasing in deeper soil. (iii) C:N, C:P, N:P had no significant difference between different nitrogen addition levels while there were significant differences between the two years under nitrogen addition. We speculate that nitrogen addition could alter soil nutrient conditions but there were certain limit of soil nutrient under N deposition.