Abstract: In order to provide theoretical reference for the protection and recovery of Ammopiptanthus mongolicus population, the diurnal and seasonal variation patterns of the photosynthetic characteristics in A. mongolicus were investigated by measuring the foliar gas exchange in the field.The results showed that the photosynthesis of A. mongolicus showed significant seasonal variations. In general, the photosynthesis was higher during spring and summer compared with that in fall and winter. The net photosynthetic rate (P_n) in A. mongolicus decreased during 10:00-13:00 from March to September, but its underlying mechanisms were seasonally specific. The results of multiple regression analysis showed that the major physiological factors affecting P_n in A. mongolicus were stomatal conductance, intercellular CO₂ concentration, and stomatal limitation to CO₂. Meanwhile, the environmental factors affecting P_n were air relative humidity, air CO₂ concentration and photosynthetic photon flux density. In conclusion, A. mongolicus grew rapidly during spring and summer, and the main limiting factor of its photosynthesis was the droughtinduced stomatal limitation. It was suggested that the restoration of A. mongolicus population should be conducted during spring and summer, and the protection of A. mongolicus population should be emphasized during drought season and winter.

Key words: nitrogen application level, litter decomposition, water treatment, desert grassland