Abstract: Global warming and atmospheric nitrogen (N) deposition are two of the most important global change drivers. In grassland ecosystem, increasing atmospheric temperature and nitrogen deposition are expected to modify the carbon sequestration; however, how warming and nitrogen addition affect ecosystem carbon exchange in the given conditions is unclear. The objective of this study was to assess the effects of global warming and N addition on C exchange of the desert steppe. A random split plot experiment (2×2 factors) in a desert steppe with dominant species of Stipa breviflora was conducted to mimic the climate warming using the infrared heaters and enrichment of nitrogen deposition by fertilizer addition. The gross ecosystem productivity (GEP), ecosystem respiration (ER) and net ecosystem exchange (NEE) were measured in different conditions. The integrated impact factors on ecosystem carbon exchange were analyzed during 2013 and 2014. The results showed that: (1) Warming significantly increased soil temperature and soil moisture about 0.70 ℃ (P<0.001) and 7.58% (P<0.001), respectively. (2) Warming, N addition and their interaction significantly increased the GEP and ER (P<0.05), but they had no significant effect on NEE (P>0.05). (3) In 2013, GEP peaked in early August, but ER reached peak during late August and early September, so NEE changed with GEP and ER. GEP, ER and NEE all peaked during late August and early September in 2014. (4) ER and GEP increased with the increasing air temperature and precipitation. Furthermore, soil temperature and soil moisture were also two of the most important factors affecting ecosystem carbon exchange.

Key words: mangrove, Avicennia marina, cadmium, sodium, potassium, magnesium.