Increasing nitrogen (N) deposition has become a threat in many terrestrial ecosystems, and the effects of N deposition have been widely studied. Nitrogen deposition can increase plant biomass, decrease richness of plant species, and alter soil microbial composition and activity. Biocrusts, which consist of communities of bacteria (cyanobacteria), fungi, algae, lichens and mosses, are important for the functioning of desert ecosystems. Biocrusts can stabilize sand surfaces, greatly increase the threshold friction velocity. Biocrusts can also fix N₂ and serve as the major source of N for plants and microbes in desert ecosystems. Both N limitation and N excess can result in stress environment for biological activity, especially for physiological responses. However, how biocrusts respond to N physiologically is unclear. In 2010, sixty 2 × 2 m plots were established at the study site, with six plots in each of 10 blocks. Five concentrations of N were applied to each block of the plots in addition to a control without added N. Six simulated N deposition treatments, i.e. 0(N0), 0.3(N0.3), 0.5(N0.5), 1.0(N1), 1.5(N1.5) and 3.0 (N3) g N m^-2 a^-1, were applied on the biocrust plots in the center of the Gurbantunggut desert. The N was added annually in March after the snow thaw and again in October before the first snowfall. NH₄NO₃ and NH₄Cl were mixed at a ratio of 2 : 1 NH₄⁺ : NO₃^-, which approximated the composition of N deposition in the nearby city.The growth and physiological indicators of algal, lichen and moss crusts were determined after three year' exposure to N addition. Chlorophyll (a+b) contents, actual photochemical efficiency YII, soluble sugar concentration of the three biocrust types and moss individual biomass increased and then decreased with the enhancements of N, with peak value at different N applied rates. Carotenoid content of algal and lichen crusts were not significantly affected by N additions (P > 0.05). However, low N additions (N 0.3-0.5) had positive effects on the carotenoid contents. High N additions (N3) significantly decreased maximum photochemical efficiency Fv/Fm. N addition non-significantly increased chl a/b of lichen crusts at N1.5 treatments and significantly decreased at N1 treatments compared to N0, while no significant effects of N addition were found in algal crusts. Chlorophyll/carotenoid of algal crusts increased and then decreased with increasing N addition. Lichen and moss crusts did not show significant effects of N addition on the chlorophyll/carotenoid. The highest value of soluble sugar content of moss occurred at N0.3 treatments, while the algal and lichen crusts occurred at N1.5 treatments. N addition had no significant effect on the proline contents of cyanobacterial and moss crusts. Lichen crusts had relatively lower proline contents in response to higher N addition rates, with a significant decrease being observed in response to N1.5 and N3 relative to N0.3. The soluble protein of algal and lichen crusts were not significantly affected by N addition, but moss soluble protein increased and then decreased after a gradient of N addition. Among the three biocrusts, moss were the most sensitive to N treatments, followed by algal and lichen crusts. Our results suggest that low levels of N addition do not significantly affect biocrust performance, but high N pollution negatively affects the growth. Therefore, Chlorophyll and its fluorescence of biocrusts, could be used for the assessment of high N pollution.