Growth And Photo-physiology Of Dominant Species In Responses To Nitrogen And Phosphorus Addition In Farm-withdrawn Grassland Community At The Loess Hilly-gully Region

To add nitrogen（N）and phosphorus（P）is one of the main measures for grassland management.It is of great significance to reveal the effects of nitrogen and phosphorus addition on species diversity and stability of grassland communities,after investigating the physiological and growth responses of dominant species in response to different nitrogen and phosphorus additions.It can also provide scientific basis for the management and regulation of fertilization during the restoration of the farming-withdrawn grassland.In order to clarify the photosynthetics characteristics and difference in response to N and P additions,three dominant species（Bothriochloa ischaemum,Stipa bungeana and Lespedeza davurica）of farm-withdrawn grassland communities in the loess hilly-gully region were selected.A split-plot experiment including three N(0,50,and 100 kg N·hm^-2·a^-1)and three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,and80 kg P₂O₅·hm^-2·a^-1)treatments was conducted,and photosynthetic gas parameters,fluorescence parameters,leaf functional traits and aboveground biomass of the three species were measured and calculated.The main results are as follows:（1）The net photosynthetic rate（P_n）,transpiration rate（T_r）,water use efficiency（WUE_i）,stomatal limitation（Ls）,photosystemⅡmaximal efficiency（PSⅡ）,SPAD values and photosystemⅡpotential activity under 50 and 100 kg N·hm^-2·a^-1 treatments were significantly higher than those under no fertilzation treatment of three dominant species.Nitrogen addition significantly increased photosynthetic nitrogen use efficiency（PNUE）and photosynthetic phosphorus use efficiency（PPUE）of B.ischaemum and L.davurica.The P_n,WUE_i,Ls and PPUE under 40 and 80 kg P₂O₅·hm^-2·a^-1treatments were significantly higher than those of L.davurica under no P addition treatment.The SPAD values of three dominant species increased significantly under P addition.There were positive relationships between net photosynthetic rate with photosystemⅡmaximal photochemical efficiency and leaf N contents of the three dominant species,and positive relationships between net photosynthetic rate with photosystemⅡpotential activity and leaf P contents of S.bungeana and L.davurica were also found.Under 50 kg N·hm^-2·a^-1treatment,compared with no P application,P_n of L.davurica and PNUE of the three dominant species increased significantly,and PPUE of B.ischaemum decreased significantly at 40 kg P₂O₅ hm^-2 a^-1 and 80 kg P₂O₅·hm^-2·a^-1treatments;while under 100 kg N hm^-2 a^-1treatment,compared with no P application,P_n and PNUE of L.davurica increased significantly,PPUE of B.ischaemum and PNUE of S.bungeana only increased significantly at 40 kg P₂O₅·hm^-2·a^-1 treatments.（2）Under 50 kg N·hm^-2·a^-1 treatments the specific leaf area of B.ischaemum significantly increased by 13.3%than those under no N addition treatment,Under 50 and100 kg N·hm^-2·a^-1 treatments the leaf dry matter content of B.ischaemum significantly by8.5%and 3.1%decrease than those under no N addition treatment.The specific leaf area of S.bungeana increased by 80.9%and 6.7%,and the dry matter content of the leaves decreased by 9.2%and 4.4%.The specific leaf area of L.davurica increased by 11.3%and16.1%,and the dry matter content of the leaves decreased by 10.5%and 16.4%.Under 50and 100 kg N hm^-2 a^-1 treatments the leaf nitrogen content of B.ischaemum and S.bungeana increased significantly by 28.7%、27.9%and 121.7%、212.4%than those under no N addition treatment,the leaf phosphorus content of L.davurica increased by 4.8%and 36.4%,and the leaf phosphorus content of S.bungeana decreased by 47.5%and 16.8%.The leaf nitrogen content of B.ischaemum and S.bungeana and specific leaf area and leaf nitrogen content of three dominant species under 40 and 80 kg P₂O₅·hm^-2·a^-1 treatments were significantly higher than those of under no P addition treatment.Phosphorus addition significantly decreased leaf dry matter content of B.ischaemum and L.davurica.,phosphorus addition significantly decreased leaf phosphorus content of L.davurica.Under50 and 100 kg N·hm^-2·a^-1 treatments,leaf phosphorus content of B.ischaemum and S.bungeana,while specific leaf area of increased significantly and leaf dry matter decrease significantly of L.davurica after P application.（3）Under 50 and 100 kg N·hm^-2·a^-1 treatments the aboveground biomass of B.ischaemum was 1.26 and 1.68 times than those under no N addition treatment.S.bungeana only increased significantly under the treatment of 50 kg N·hm^-2·a^-1.Under 40 and 80 kg P₂O₅·hm^-2·a^-1 treatments the aboveground biomass of B.ischaemum was 24.7 and 21.4 times than those under no P addition treatment.Compared with the addition of nitrogen or phosphorus alone,the grassland community under the combination of nitrogen and phosphorus has higher aboveground biomass.In summary,the treatments of 50 kg N·hm^-2·a^-1 and 80 kg P₂O₅·hm^-2·a^-1 can improve the photosynthetic capacity and photosynthetic nutrient utilization efficiency of the three dominant species and the three dominant species grassland communities maintain the best combination of higher ground biomass.