| Phyllostachys edulis is an important economic and ecological bamboo species in southern China.Extremely low phosphorus content in soil and high nitrogen deposition in its main distribution region have affected the growth and development of P.edulis to a great extent.In this study,we assessed the effects of soil phosphorus content and nitrogen deposition on 1-year-old P.edulis seedlings.Three levels of soil phosphorus（1.25 mg·kg-1,10.76 mg·kg-1,21.39 mg·kg-1）and three rates of nitrogen deposition（0 kg·ha-1·a-1,30 kg·ha-1·a-1,60 kg·ha-1·a-1）were implemented,resulting in nine treatments as total for this study.We analyzed the effects of nitrogen deposition on soil physical and chemical properties and seedling growth,photosynthetic physiology,nutrient uptake and utilization,therefor to provide a theoretical basis and technical support for the cultivation and management of P.edulis seedlings under the background of atmospheric nitrogen deposition.The main results are as follows:（1）After one year commencing the experiment,the content of available P in soils of Medium and High phosphorus treatments was 16.1%and 36.5%larger than that of Low phosphorus treatment.Compared with Low phosphorus treatment,the pH values and exchangeable Mg contents for Medium and High phosphorus treatments were also increased,while the contents of soil N,K and exchangeable Al decreased significantly.The effects of nitrogen deposition on soil physical and chemical properties were carried with phosphorus levels.Under Low phosphorus level,the contents of N and K in soil increased with the increase of nitrogen deposition rate;under Medium and High phosphorus levels,the contents of N,P,K and exchangeable Mg decreased significantly with the increase of nitrogen deposition rate.The results suggest that the short of phosphorus may be the main restriction factor for P.edulis seedlings to absorb nutrients from soil under Low phosphorus level,whereas under Medium and High phosphorus level,which may supply sufficient P for seedling growth,the application of nitrogen may act a main role in processes for seedling to uptake nutrients from soil.（2）Under Low phosphorus level,the mass of P.edulis seedlings was extremely small,whereas when the soil phosphorus content was leveled up,seedling height increased for 5.5 to 6.5fold,the tiller number increased for 1.7 to 1.9 fold and the mass increased for 35.1 to 38.0 fold,the root to shoot mass ratio decreased for 68.871.5%.These indicate that deficient phosphorus application in soils has significant inhibiting effects on seedling growth,and while the seedlings may tend to allocate more mass to root,developing a relatively large root system,to enhance the absorption of the limited phosphorus in soils.Nitrogen deposition was feeble to promote seedling growth under Low phosphorus level,while significantly accelerated seedling growth under Medium and High phosphorus levels and decreased the root to shoot mass ratio.This indicates that N application has no compensatory effect on the influences caused by deficient phosphorus stress;however,once the stress condition of low phosphorus was relived,nitrogen has much more important effects on seedling growth.（3）Compared with Medium phosphorus treatment,the content of photosynthetic pigments,P,K and Mg in leaves of seedlings in High phosphorus treatment increased for 16.8%,34.1%,24.1%and 11.4%,respectively;while N/P decreased for 34.2%.Nitrogen deposition consistently increased the photosynthetic content,light use efficiency,N and Mg content,and N/P in leaves of P.edulis seedling,and also enhanced CO2 fixation by increasing stomatal conductance,thereby enhancing photosynthetic rate.The results suggest that under conditions in this study,increase of soil phosphorus and nitrogen deposition have positive effects on P.edulis seedling photosynthetic and nutrient physiology,which may have implications for subsequent seedling cultivation with the N and P application to control seedling development.（4）Compared with Medium phosphorus treatment,the nitrogen and phosphorus utilization efficiency of seedlings in High phosphorus treatment increased for 5.6%and 8.6%,respectively;the acid phosphatase activity of roots and leaves decreased by 31.7%and 30.9%,respectively.Nitrogen deposition increased phosphorus utilization efficiency and decreased nitrogen utilization efficiency under either Medium and High phosphorus levels,and also increased the acid phosphatase activity in leaves,but decreased the acid phosphatase activity of roots.In addition,the acid phosphatase activity of roots was negatively correlated with phosphorus utilization efficiency,and positively correlated with nitrogen utilization efficiency.This indicates that N and P interacted to affect the uptake and utilization processes for either nutrient element in plant life cycle. |
