Responses Of Tree Growth And Soil Respiration To Long-term Nitrogen Additions In Boreal Forests

As two dynamic processes of carbon(C)cycle,forest tree growth and soil respiration play the roles of C sequestration and C output respectively.How tree growth and soil respiration respond to atmospheric nitrogen(N)deposition has a crucial impact on global climate change.Different forest ecological communities may respond differently to nitrogen deposition.Therefore,simulating nitrogen deposition to study the changes of forest tree biomass and soil respiration has become a hot issue in recent years.In order to predict the response of northern forest to nitrogen deposition,a simulated nitrogen deposition experiment was carried out in Xing'an Larch Forest in Daxing'an Mountains.According to the current local nitrogen deposition rate,the experimental plots were set as four treatments:control(CK,0 kg N ha-2yr-1),low nitrogen(LN,25 kg N ha-2yr-1),medium nitrogen(MN,50 kg N ha-2yr-1)and high nitrogen(HN,75 kg N ha-2yr-1).This paper mainly studies the response of forest tree biomass and soil respiration to nitrogen addition.The test results are as follows:(1)The growth of trees in northern forest is still limited by the level of soil nitrogen.Under the action of nitrogen addition,the total biomass of trees and the biomass of stems,leaves,branches and roots are increased.In Xing'an larch forest,increasing nitrogen can promote tree growth and increase forest tree carbon storage.(2)Soil respiration(RS)was different in different months.It was the highest in July and relatively low in May and September.Ln had a significant promoting effect on RS,RS had no significant change under Mn treatment,and HN inhibited RS in September.The contribution of heterotrophic respiration(RH)to RS was relatively high,and the proportion of root autotrophic respiration(RA)in RS was relatively low.The trend of respiratory value of RS and RA was similar under each treatment.(3)Simulated nitrogen deposition will affect the soil respiration temperature sensitivity coefficient(Q₁₀).On the whole,Q₁₀ of RA is greatly affected by nitrogen addition,and the variation range is 2.057-3.164.Ln significantly increased the Q₁₀ of RA,and HN significantly promoted the Q₁₀of RS and RA.Nitrogen application could affect soil respiration by affecting soil physical and chemical properties.The results showed that soil autotrophic respiration had a significant negative correlation with soil total carbon content,and soil total phosphorus had a significant positive correlation with soil respiration and its components.Soil total nitrogen content increased with the increase of nitrogen application concentration.