Effect Of Nitrogen Application And Drought Stess On Ecophysiological Characteristics Of Quercus Mongolica Seedlings

Quercus mongolica is an important tree species in northern China,and its application in afforestation has been increasing in recent years.However,it often encounters drought stress in afforestation practice,which affects its survival and growth,especially in arid and barren areas.Studies have shown that nitrogen can alleviate the drought stress of plants.In this experiment,1-year-old Q.mongolica seedlings were taken as the research object to study the effects of nitrogen fertilizer and drought stress on their physiological and ecological characteristics,and to explore whether nitrogen fertilizer can alleviate drought stress on Q.mongolica seedlings,so as to provide a basis for overcoming the effects of drought stress on Q.mongolica afforestation.The main results of this study are as follows:（1）Proper nitrogen application can improve the growth of seedling height and root collar diameter of Q.mongolica seedlings under drought stress.Under the condition of 60%field water-holding capacity（60%FC）,N₂ treatment(applying 104 mg·kg^-1 urea)has the largest increment of seedling height and root collar diameter,while under the conditions of 40%FC and 20%FC,N₁ treatment(applying 52 mg·kg^-1 urea)has the highest increment of seedling height and root collar diameter.（2）Appropriate nitrogen application can increase the relative water content of Q.mongolica seedlings leaves under drought stress,and improve the water retention capacity of leaves.Under the conditions of 40%FC and 20%FC,the relative water content of N₁treatment reached the maximum values of 89.09%and 86.66%respectively.（3）Appropriate nitrogen application can improve the antioxidant enzyme activity of Q.mongolica seedlings under drought stress,and improve the ability of scavenging reactive oxygen species in leaves and reduce the degree of membrane peroxidation.Under the condition of 40%FC,the activities of CAT,SOD and POD in leaves of Q.mongolica seedlings treated with N₁ reached the highest value at the same time.Under the condition of20%FC,the activities of CAT and POD in leaves treated with N₂ reached the highest value,and the activities of SOD in leaves treated with N₁ reached the highest value.Under mild and moderate drought stress,the MDA content in leaves of Q.mongolica seedlings treated with N₂ reached the lowest value of 0.079μ·mol^-1 and 0.108μ·mol^-1,respectively,and under severe drought stress,the MDA content of N₁ treatment reached the lowest value of 0.159μ·mol^-1,under severe drought stress.Under mild and moderate drought stress,the relative conductivity of leaves of Q.mongolica seedlings treated with N₂ reached the lowest values of74.49%and 64.21%.（4）The overall photosynthetic rate decreased with the increase of drought,but only N₁treatment increased the photosynthetic rate.The trend of stomatal conductance,transpiration rate and water use efficiency is basically the same.Under different drought conditions,N₁treatment increased stomatal conductance,transpiration rate and water use efficiency,but stomatal conductance decreased,and N₂ treatment mostly decreased.The concentration of intercellular carbon dioxide generally decreased with the increase of drought,but the seedlings of Q.mongolica treated with N₂ increased with the increase of drought.Under mild and moderate drought conditions,the maximum photochemical efficiency of N₂ treatment is higher than that of other treatments,and under severe drought conditions,the maximum photochemical efficiency of N₁ treatment is higher than that of other treatments.Under the conditions of 60%FC and 20%FC,the total chlorophyll content of N₂ treatment reached the highest value,and under the condition of 40%FC,the total chlorophyll content of N₁treatment reached the highest value.（5）Appropriate nitrogen application increased the content of osmotic adjustment substances in leaves of Q.mongolica seedlings under drought stress.Combining the changes of the contents of soluble protein,soluble sugar and proline,it was found that N₂ treatment under mild drought could increase the contents of osmotic adjustment substances more efficiently and enhance the water-holding capacity of Q.mongolica,and N₁ treatment had a better effect under moderate and severe drought conditions.（6）Appropriate nitrogen application increased the accumulation of nutrients in leaves of Q.mongolica seedlings under drought stress,promoted the utilization of phosphorus（P）and increased the biomass accumulation.Under the conditions of 60%FC and 40%FC,the carbon（C）content of N₂ treatment reached the highest value,and that of N₁ treatment reached the highest value under the condition of 20%FC.The nitrogen（n）content of N₂ treatment is higher than that of other treatments only when it is 20%FC.P content was lower than other treatments except N₁ treatment at 40%FC.In a word,this study shows that applying appropriate amount of nitrogen can alleviate the inhibition of drought stress on the growth and development of Q.mongolica seedlings,but it can’t change the declining trend of growth,metabolism and nutrient accumulation.Excessive nitrogen application will aggravate the inhibition of drought stress on Q.mongolica seedlings.