Leaf photosynthetic responses to nitrogen and phosphorus additions of dominant species in farm-withdrawn grassland in the loess hilly-gully region

In order to clarify the photosynthetics characteristics and difference in response to nitrogen and phosphorus additions, three dominant species（Bothriochloa ischaemum, Stipa bungeana and Lespedeza davurica） of farm-withdrawn grassland communities in the loess hilly-gully region were selected. A split-plot experiment including three N(0, 50, and 100 kg N hm-2 a-1) and three P(0, 40, and 80 kg P2O5 hm-2 a-1) treatments was conducted to investigate the effect of N and P addition on net photosynthetic rate, transpiration rate, water use efficiency, stomatal limitation, initial fluorescence, photosystem Ⅱ maximal efficiency, photosystem Ⅱ potential activity, leaf N content, leaf P content, photosynthetic nitrogen use efficiency and photosynthetic phosphorus use efficiency of the three dominant species. Results showed that net photosynthetic rate, transpiration rate, water use efficiency, stomatal limitation, photosystem Ⅱ maximal efficiency and photosystem Ⅱ potential activity under 50 and 100 kg N hm-2 a-1 treatments were significantly higher than those under no N addition treatment of three dominant species. Nitrogen addition significantly increased photosynthetic nitrogen use efficiency and photosynthetic phosphorus use efficiency of B. ischaemum and L. davurica. The photosynthetic phosphorus use efficiency of S. bungeana. net photosynthetic rate, water use efficiency, stomatal limitation and photosynthetic phosphorus use efficiency under 40 kg P2O5 hm-2 a-1 and 80 kg P2O5 hm-2 a-1treatments were significantly higher than those of L. davurica under no P addition treatment. Under 50 kg N hm-2 a-1treatment, compared with no P application, net photosynthetic rate of L. davurica and photosynthetic nitrogen use efficiency of the three dominant species increased significantly, and photosynthetic phosphorus use efficiency of B. ischaemum decreased significantly at 40 kg P2O5 hm-2 a-1 and 80 kg P2O5hm-2 a-1 treatments; while under 100 kg N hm-2 a-1treatment, compared with no P application, net photosynthetic rate and photosynthetic nitrogen use efficiency of L. davurica increased significantly, photosynthetic phosphorus use efficiency of B. ischaemum and photosynthetic nitrogen use efficiency of S. bungeana only increased significantly at 40 kg P2O5 hm-2 a-1 treatments. There were positive relationships between net photosynthetic rate with photosystem Ⅱ maximal photochemical efficiency and leaf N contents of the three dominant species, and positive relationships bet