Abstract:Precipitation and nitrogen deposition are two of the most investigated factors of global climate change which also are the controlling factors of desert ecosystems. This study examined the combined effects of precipitation and nitrogen deposition on arid ecosystems in order to provide insight into the responses of ecosystems to global climate change. We selected plantation of Haloxylon ammodendronin the southern margin of Junggar Basin. Two precipitation conditionsnatural precipitation (W ₀) and 30% increased precipitation (W ₁) and three levels of nitrogen depositionsnatural nitrogen deposition (N ₀), 30 kg(N)·hm ^-2·a ^-1(N ₁) and 60 kg(N)·hm ^-2·a ^-1(N ₂) increased natural nitrogen deposition were randomly added to simulate nitrogen deposition and precipitation for two years to explored the effects of precipitation, nitrogen deposition and the combined effects on diurnal change in photosynthetic and physio-ecological traits of H. ammodendron. The results showed that precipitation, nitrogen deposition and the combined effects had direct significant correlation with diurnal change in net photosynthetic rate of H. ammodendron. Based on the variation in net photosynthetic rate, intercellular CO ₂concentration and stomatal limitation of H. ammodendron, "midday depression" of photosynthesis was mainly caused by non-stomatal factors. Moreover, with increased nitrogen deposition, there were decreasing physiological indexes (MDA content, POD activity, CAT activity, SOD activity, soluble protein content and soluble sugar content) under W ₀precipitation condition. However, proline content initially decreased before eventually increasing. There were initial increases followed by decreases in physiological indexes (MDA content, POD activity, CAT activity, SOD activity, soluble protein content, proline content and soluble sugar content) under W ₁precipitation condition. With the exception of W ₁N ₁, proline content was lower under nitrogen deposition increase treatments than under the natural deposition treatment. Also MDA content, antioxidant enzymes activities, soluble protein content and soluble sugar content of H. ammodendronwere also significantly lower under the other water/nitrogen treatments than under the control regimes. The results from the comprehensive analysis indicated that precipitation, nitrogen deposition and the combined effects benefited the growth of H. ammodendron, but the strength of the combined effect depended on the ratio between the nitrogen deposition and precipitation.