Effect of climate warming and nitrogen deposition on net photosynthetic rate of alpine plants on Qinghai-Tibet Plateau

Simulated warming and nitrogen deposition experiments were conducted to examine the responses of the net photosynthetic rate of key dominant plants in four types of alpine grasslands, including alpine meadow, alpine steppe, alpine desert, and artificial grassland on Qinghai-Tibetan Plateau by using Li-6400. There were seven treatments: control CK, 0 kg·(ha·a)-1, low nitrogen Ⅰ NⅠ, 8 kg·(ha·a)-1, low nitrogen Ⅱ NⅡ, 24 kg·(ha·a)-1, medium nitrogen NⅢ,40 kg·(ha·a)-1, high nitrogen Ⅰ NⅣ, 56 kg·(ha·a)-1, high nitrogen Ⅱ NⅤ, 72 kg·(ha·a)-1, warming W, 0 kg·(ha·a)-1, and the interaction of warming and nitrogen addition W-NⅠ, 8 kg·(ha·a)-1. The results showed that 1) the net photosynthetic rate of Dracocephalum heterophyllum and Stipa capillata increased initially and then decreased with the increasing gradients of warming and nitrogen addition. With increasing nitrogen deposition, the net photosynthetic rate of Artemisia scoparia, Silene aprica, Kobresia humilis, Poa crymophila, and P. attenuate in the alpine meadow increased markedly. 2) The net photosynthetic rate of Thermopsis lanceolate, Iris lactea var. chinensis, and Potentilla multifida increased with nitrogen deposition, while that of K. humilis, Plantago depressa, A. scoparia, K. capillifolia, P. crymophila, Pedicularis kansuensis, Elymus nutans, and Dracocephalum heterophyllum decreased with nitrogen deposition. 3) The net photosynthetic rate of T. lanceolate, P. crymophila, and Aster tataricus in the artificial grassland decreased initially and increased thereafter with the increase of nitrogen deposition. 4) The net photosynthetic rate of Blysmus sinocompressus in soil-salinization grassland decreased, while that of Ceratoides compacta in alpine desert and Leymus secalinus in the transitional zone between soil-salinization grassland and desert increased with the experimental warming.