Effects Of Ectomycorrhizal Fungi On Nitrogen Characteristics Of Quercus Variabilis-soil Under Different Nitrogen Addition Levels

The increasing atmospheric nitrogen（N）deposition leads to the imbalance of forest soil nutrients,soil acidification,and the decrease of biodiversity,which seriously affects the structure and function of forest ecosystems in China,Ectomycorrhizal fungi（EMF）is one of the key factors to regulate nutrient cycling in forest ecosystem,which plays an important role in maintaining forest health and ecosystem function stability.In this study,Quercus variabilis,a major tree species in subtropical deciduous broadleaved forest of China,was selected as the research object.Through indoor pot control experiment,four nitrogen levels were artificially simulated with different nitrogen deposition levels:N0(0g N m^-2 a^-1),N1(10 N m^-2 a^-1),N2(40 g N m^-2 a^-1),N3(80 N m^-2 a^-1),at the same time,inoculated three different mycelium types EMF at each nitrogen application level:Suillus Grevillea（Sg）,Pisolithus Tinctorius（Pt）,Tuber sp.（T）and Ck;Using ¹⁵N stable isotope tracer technique to research different nitrogen levels on the Quercus variabilis-EMF symbiotic relationship,exploring the effects of EMF on soil chemical properties,growth and physiological characteristics of Quercus variabilis seedlings under different nitrogen addition levels,elucidating the role and internal mechanism of EMF in regulating Quercus variabilis-soil nitrogen transport under different nitrogen application levels.In order to deepen the understanding of soil nitrogen cycle under the background of global change,and provide scientific basis for evaluating the comprehensive environmental impact of nitrogen deposition and sustainable development of forest ecosystem.The main research results are as follows:1.Different nitrogen addition levels significantly changed the colonization rates of Quercus variabilis EMF in three inoculation treatments（P<0.05）.With the increase of nitrogen addition rate,EMF colonization rate of Pt and Sg bacteria increased firstly and then decreased,and reached the highest at N1 level,which increased by 11.24%and 13.68%compared with control（N0）,respectively.The EMF colonization rate of T inoculation decreased significantly（P<0.05）,and was the highest at N0,and decreased by 2.25%,9.74%and 13.93%with the increase of nitrogen addition rate,respectively.2.Under different nitrogen addition levels,the three EMF inoculation treatments had different degrees of influence on soil chemical properties.With the increase of nitrogen addition rate,soil p H,total carbon and available phosphorus inoculated with Sg bacteria were significantly decreased（P<0.05）,while Pt and T bacteria treatments had no significant changes（P>0.05）.Soil total phosphorus inoculated with Pt was significantly increased（P<0.05）,while soil total phosphorus inoculated with Sg and T had no significant change（P<0.05）.The total nitrogen content of soil inoculated with Sg bacteria increased first and then decreased,and the highest value was N2,which was significantly increased by 15.35%compared with N0,while the total nitrogen content of soil inoculated with Pt and T bacteria did not change significantly（P>0.05）.There was no significant change of soil ammonium nitrogen in each inoculation treatment（P>0.05）.Soil nitrate nitrogen content in each inoculation treatment was significantly increased（P<0.01）.Soil microbial nitrogen content inoculated with T and Sg bacteria was significantly increased（P<0.05）,while Pt bacteria had no significant change（P>0.05）.3.The nitrogen content in leaves of Quercus variabilis increased firstly and then decreased,and reached the highest in N1,which was significantly increased by 75.58%compared with N0（P<0.05）.The nitrogen content in stem of Quercus variabilis inoculated with T and Sg bacteria increased first and then decreased,and was the highest in N1（P<0.05）,while Pt bacteria had no significant effect（P>0.05）.Root nitrogen content of Quercus variabilis inoculated with Pt and Sg bacteria was significantly decreased（P<0.05）,while root nitrogen content of Quercus variabilis inoculated with T bacteria was significantly increased（P<0.05）.The abundance of¹⁵N in leaf and stem of Quercus variabilis was significantly decreased（P<0.05）.The abundance of ¹⁵N of Pt and T decreased significantly with the increase of nitrogen addition level（P<0.05）,while the abundance of ¹⁵N of Sg bacteria increased firstly and then decreased,and the abundance was the highest at N1 level,which was significantly increased by 63.80%compared with that at N0 level（P<0.05）.The phosphorus content of Quercus variabilis leaves was not significantly affected（P>0.05）.However,phosphorus content in stem of Quercus variabilis inoculated with T,Sg and Pt bacteria increased,decreased and had no significant change,respectively.The root phosphorus content of Pt and Sg bacteria was significantly decreased（P<0.05）,while T bacteria had no significant change（P>0.05）.With the increase of nitrogen addition level,photosynthetic parameters,seedling height and ground diameter growth,and carbohydrate accumulation in leaf and stem of Quercus variabilis were firstly increased and then decreased,and reached the highest at N1 level,and were significantly inhibited afterwards（P<0.05）.The root carbon content and root dry weight of Quercus variabilis under different inoculation treatments were significantly inhibited（P<0.05）.