Study On The Mechanism Of Gross Nitrogen Release And Immobilization During Roots Litter Decomposition Of Pinus Koraiensis In Changbai Mountain

Roots litter is the main source of nutrients in forest ecosystem.When the nutrients of roots litter were released to the soil environment,the fate and distribution of nitrogen will be directly related to the distribution and cycle of nitrogen in the forest ecosystem.Previous studies on nitrogen release during roots litter decomposition paid more attention to the net nitrogen release of roots litter,which was the result of the combined effect of gross nitrogen release and gross nitrogen immobilization,which could not reflect the real situation of nitrogen release from roots litter.The existing litter decomposition experiments showed that the nitrogen in the litter increased,decreased and remain unchanged after adding nitrogen,which was the role of nitrogen release and nitrogen immobilization.At present,the effect of nitrogen addition on gross nitrogen release and gross nitrogen immobilization in roots litter and its regulatory mechanism are not clear.The influencing factors of nitrogen release during roots litter decomposition include roots litter quality,environment and microorganisms.After nutrients were released into the soil environment,there are few studies on the fate and distribution of nitrogen.However,it has important ecological significance for the nutrient cycle of the ecosystem.This study took the protected broad-leaved Korean pine forest in Changbai Mountain as the sample plot,¹⁵N labeled roots litter of broad-leaved Korean pine in Changbai Mountain as the research object,and carried out the roots litter decomposition experiment in the form of litter decomposition bag for 3.25 years under the treatment of in situ nitrogen addition(0kg N·ha-¹·yr^-1/50 kg N·ha^-1·yr^-1),in order to solve the following problems:（1）the characteristics of mass loss in the process of roots litter decomposition and its response to nitrogen addition;（2）the effect of nitrogen addition on the release and immobilization of gross nitrogen during roots litter decomposition and its influencing factors;（3）the fate and distribution characteristics of nitrogen in soil after roots litter decomposition.The main results are as follows:（1）During the 2.5-year roots litter decomposition period,the mass loss was slow,that is the mass loss was less than 50%,and the addition of nitrogen did not significantly accelerate the roots litter decomposition.The results showed that the loss of leaching material,the higher lignin content in the later stage of decomposition delayed the process of decomposition.Nitrogen immobilization during roots litter decomposition mainly occurred in the early stage of decomposition（0-0.5 years）,which was related to the lack of microbial carbon in the early stage of decomposition and rapid colonization to roots litter.During the whole decomposition period,the relationship between gross carbon release and gross nitrogen release ratio was not1:1,and the gross nitrogen release ratio was more than 2 times of the gross carbon release ratio.We speculate that uptake of more nitrogen by microorganisms.（2）In the process of roots litter decomposition,the release of gross nitrogen was related to the nitrogen content and C/N ratio of roots litter.The results showed that cellulose may be the main energy source in the later stage of fine root decomposition.The chemical properties of initial litter was the influencing factors of decomposition,lignin and hemicellulose increased significantly during the whole roots litter decomposition,which may be the reason for the decrease of decomposition rate and nitrogen release.（3）Nitrogen immobilization mainly occurred in the early stage of decomposition（0-0.5years）in the process of roots litter decomposition.The gross nitrogen release and immobilization have different preference for metal elements,which affected the nitrogen balance of roots litter by regulating the enzyme activity of microorganisms.The refractory substances and metal elements manganese（Mn）,calcium（Ca）and magnesium（Mn）in roots litter can be used as indicators to predict the decomposition rate.In the early stage of roots litter decomposition,P（Phosphorus）and Mg were negatively correlated with the immobilization rate of gross nitrogen.（4）The influencing factors of gross nitrogen release and gross nitrogen immobilization during roots litter decomposition,combined with principal component analysis table and multiple regression model,the higher relative explanation in the first principal component was roots litter C/N,soil moisture,soil gross nitrogen and the concentration of metal elements without Mn and Ca.In the second principal component,the relative explanation of roots litter mass,lignin,cellulose and nitrate concentration in soil was higher.（5）During the decomposition period,nitrogen was mainly stored in roots litter,and ¹⁵N entering the soil environment mainly existed in organic form.Nitrogen addition did not affect the gross ¹⁵N recovery,and the ¹⁵N recovery rate labeled by roots litter reached a stable level in2.25 years.The fate of ¹⁵N labeled by roots litter in soil was mainly in the form of organic nitrogen,followed by nitrate nitrogen and ammonium nitrogen.The experimental results showed that the addition of nitrogen had no significant effect on the fate of nitrogen decomposed by roots litter.