Abstract: Indigenous vegetation in the foreland of river oases at the extremely arid southern margin of the Taklimakan Desert in Xinjiang,NW China,is dominated by a few perennial phreatophytes,primarily [WTBX]Alhagi sparsifolia[WTBZ] Shap.,[WTBX]Tamarix ramosissima[WTBZ] Ledeb. and [WTBX]Karelinia caspica[WTBZ] Less. Water is well known as a limiting factor for plant growth in deserts,availability of nutrients has also been determined as a critical factor limiting plant growth in arid regions. Water source for plants is groundwater at the southern fringe of the Taklimakan Desert. So nutrients are the most important resource for plant growth in the same groundwater depth. When the soil N content or the nitrogen validity are low,plants with the ability to fix N2 might have an advantage and be capable of taking in more N than plants without this ability. Leaf N content correlates strongly with photosynthesis as most of the leaf N is allocated to photosynthesis. So some people considered that the nitrogen fixing plants might have higher Pmax than the nitrogen nonfixing ones living in the same habitat. However,in our previous research,we found that nitrogen nonfixing species had higher photosynthetica capacity. In order to answer this question,nitrogen fixing species A. sparsifolia and nitrogen nonfixing species K. caspica and T. ramosissima were compared in their typical habitat at the southern fringe of the Taklimakan Desert. The diurnal variations of net photosynthetic rate,indexs of light and CO2 to net photosynthetic rate (Pn),Chl content,leaf nitrogen content of nitrogen fixing and nonfixing species were investigated in June,July and August 2008. The results showed that from 08∶00 to 20∶00 the diurnal variations of Pn showed double peak curve both nitrogen fixing and nonfixing species in June and July. In August,the curve changed into a single peak. The order of average photosynthetic rate of three months: K. caspica＞T. ramosissima＞A. sparsifolia. It also showed that nitrogen nonfixing species had higher Pn than nitrogen fixing species. The maximum net photosynthetic rate (P max),apparent quantum requirement (AQY), CO2 saturation point ,dark respiration rate (Rday) of nitrogen nonfixing species were higher than nitrogen fixing species,the difference was significant (P0.05). The nitrogen fixing species had significantly higher light compensation point (LCP),light saturation point (LSP),CO2 compensation point (CCP),Chl content and leaf nitrogen content than nitrogen nonfixing species (P<0.05). Through correlative analysis,we found that there was no obviously relationship between nship between Pmax and Chl in K. caspica and T. ramosissima. Pmax of T. ramosissima was negatively correlated with leaf nitrogen content. But Pmax of K. caspica was positively correlated with leaf nitrogen content. In despite of higher leaf nitrogen content in nitrogen fixing species,nitrogen nonfixing species had higher photosynthetic capacity. The higher leaf nitrogen content not always bring higher photosynthetic rate in nitrogen fixing and nonfixing species.

Key words: photosynthetic characteristics, Chlorophyll content, leaf nitrogen content, nitrogen fixing and nonfixing species, Taklimakan Desert