Changes Of Carbon And Nitrogen In Main Ecological Processes Of Betula Platyphylla Forests In Daxing’an Mountains And Its Model Simulation

The Northern Daxing’an Mountains is the only high latitude cold temperate forest region,which is also an important ecological function area and ecological sensitive area in China.Betula platyphylla is the broadleaf tree species with the widest distribution area,and is also the pioneer tree species for natural forest regeneration in this area.Betula platyphylla has an irreplaceable ecological status in the Daxing’an Mountains.In this study,the carbon and nitrogen storage and distribution characteristics,coupling characteristics of carbon and nitrogen in the photosynthetic carbon fixation process,coupling characteristics of carbon and nitrogen in the litterfall decomposition process,coupling characteristics of carbon and nitrogen in soil carbon release process,and coupling characteristics of carbon and nitrogen in soil nitrogen mineralization process were studied in 25-,40-,and 61-year-old Betula platyphylla forest ecosystems of the Daxing’an Mountains.Through Forest-DNDC model simulation,the characteristics of carbon and nitrogen cycle in Betula platyphylla forest ecosystem are predicted to provide basic data and scientific reference for the management of Betula platyphylla forests of this region under the background of future climate change.The major achievements are as follows:（1）The carbon stocks of 25-,40-and 61-year-old Betula platyphylla forest ecosystems in Daxing’an Mountains were 73.04、176.98,and 268.74 t?hm^-2,respectively,which increased significantly with the increase of stand age.The soil layer contributed the most to the carbon stock of the forest ecosystem,accounting for 86.54%～89.60%,followed by the order of tree layer,with a contribution rate of 8.61%～11.73%.The understory ground cover layer contributed the least to the carbon stock,which was only 1.49%～3.12%.The nitrogen stocks of 25-,40-and 61-year-old Betula platyphylla forest ecosystems in Daxing’an Mountains were7.19、9.83 and 12.41 t?hm^-2,respectively,which was mainly contributed by the soil layer,with a contribution rate of 97.82%～98.74%.The nitrogen stock of the tree layer was 0.05～0.18t·hm^-2,and the contribution rate to the nitrogen storage of the forest ecosystem is 0.70%～1.45%.The contribution rate of understory ground cover was only 0.56%～0.73%.（2）The net photosynthetic rate of Betula platyphylla leaves increased with the increase of forest age.The mean net photosynthetic rate during the study period followed by the order of 61a(3.24±0.29μmol CO₂·m^-2·s^-1)>40 a(2.74±0.25μmol CO₂·m^-2·s^-1)>25 a(1.65±0.13μmol CO₂·m^-2·s^-1);The mean transpiration rate during the study period followed by the order of 40 a(1.71±0.17 mmol H₂O·m^-2·s^-1)>25 a(1.0±0.12 mmol H₂O·m^-2·s^-1)>61 a(0.72±0.12 mmol H₂O·m^-2·s^-1).The mean water use efficiency（WUE）during the study period followed by the order of:61 a(4.48±0.79μmol·mmol^-1)>40 a(1.60±0.46μmol·mmol^-1)>25a(1.50±0.20μmol·mmol^-1).The mean photosynthetic nitrogen use efficiency（PNUE） during the study period followed by the order of:61 a(0.16±0.02μmol·g^-1·s^-1)=40 a(0.16±0.02μmol·g^-1·s^-1)>25 a(0.09±0.01μmol·g^-1·s^-1).There was a significant positive exponential correlation between net photosynthetic rate and leaf nitrogen content of Betula platyphylla forest.The coupling equations were as follows Pn=0.001e^0.375N,Pn=0.002e^0.446N,and Pn=0.004e^0.340Nfor 25-,40-,and 61-year-old stands,respectively.（3）During the study period,the litterfall input of Betula platyphylla forests in the Daxing’an Mountains was 100.07～220.81 g?m^-2,which was mainly concentrated from August to September.There was a significant negative linear correlation between the decomposition rate of litter and the C:N ratio in Betula platyphylla forests of Daxing’an Mountains with r values of-0.901 and-0.944 for leaves and branch,respectively.The coupling equations are y=1342.449-37.724x and y=1131.320-31.954x for leaves and branches,respectively.The decomposition coefficients of leaves and branches were 1.213 and 0.845,respectively.The decomposition rate of leaves was higher than that of branches.The half decomposition time of leaves and branches was 0.571 years and 0.820 years respectively,and the decomposition time of 95%was 2.470years and 3.545 years respectively.The release of carbon and nitrogen both showed enrichment–release during litter decomposition.（4）The soil respiration rate（Rs）of Betula platyphylla forest followed the order of:40 a(4.64±0.50μmol·m^-2·s^-1)>61 a(4.34±0.51μmol·m^-2·s^-1)>25 a(2.45±0.19μmol·m^-2·s^-1).The temperature sensitivity(Q₁₀)values of Betula platyphylla forests decreased with the increase of stand age.The Q₁₀ values of 25-,40-,and 61-year-old Betula platyphylla forests were8.28,3.29,and 2.32,respectively.Soil temperature and water content could be combined to explain 64.0%～84.2%of the change on Rs.There was a positive linear correlation between Rs and soil organic carbon content in Betula platyphylla forest.The results of the coupling equation were as follows:Rs=1.459+0.039C;Rs=1.567+0.069C and Rs=0.013+0.077C for 25-,40-,and 61-year-old stands,respectively.There was a positive exponential correlation between Rs and soil nitrogen content.The results of the coupling equation are as follows:Rs=0.308e^1.357N,Rs=1.279e^0.540N,and Rs=-1.235e^0.475Nfor 25-,40-,and 61-year-old stands,respectively.Under the treatment of litter reduction,Rs reduced by 15.23%～24.26%due to the reduction of fresh substrates.Under the treatment of doubling litter,Rs increased by 23.58%～27.38%due to the increase of the amount of easily decomposed organic matter,resulting in the priming effect.Under the background of climate warming,increasing litter input and nitrogen deposition in the future,the carbon release flux of Betula platyphylla forests in the Daxing’an Mountains will significantly increase.（5）The soil inorganic nitrogen（Min-N）content of Betula platyphylla forests ranged fro m 2.57 to 12.91 mg?kg^-1.Ammonium nitrogen（NH₄⁺-N）accounted for a larger proporti on of Min-N in this region.The content of Min-N had significant vertical spatial variat ion,with higher content of Min-N in the 0-10 cm soil layer than that of the 10-20 cm soil layer.With the increase of stand age,the mean content of soil inorganic nitrogen followed the order of:61 a(8.08±0.34 mg?kg^-1)>40 a(6.21±0.35 mg?kg^-1)>25 a(5.13±0.33 mg?kg^-1).The order of mean soil mineralization rate（Rm）was as follows:61 a(0.67±0.07 mg?kg^-1?d^-1)>40 a(0.46±0.05 mg?kg^-1?d^-1)>25 a(0.10±0.03 mg?k g^-1?d^-1).Ammonization（Ra）dominated mineralization（Rm）,which accounted for 66.22%～77.87%when averaged across both 0-10 cm and 10-20 cm soil layers.There was a significant positive correlation between Rm and soil microbial biomass carbon content,and microbial biomass nitrogen content.The results of the coupling equation are as foll ows:Rm=-0.817+0.003MBC+0.004MBN,Rm=-2.895+0.016MBC+0.009MBN,and Rm=-9.678+0.018MBC+0.039MBN for 25-,40-,and 61-year-old stands,respectively.（6）The RMSE%value and MAE%value of the forest-DNDC model after parameter calibration are less than 30%and 25%,which can meet the accuracy requirements of the model.The simulated results showed that the annual net primary productivity（GPP）of 25-,40-,and 61-year-old Betula platyphylla ecosystems were 4.82,5.06,and 7.67 t C?hm^-2?y^-1,respectively,and41.00%～49.93%of which would release into the atmosphere through respiration.The annual Rm is 41.47,50.11,and 54.18 kg N?hm^-2?y^-1,respectively.The annual nitrogen flux released into the atmosphere through NO,N₂O,and N₂ were 0.68-0.83 t N?hm^-2?y^-1.The fitting equations between soil C and N release fluxes of Betula platyphylla forest were N=0.006C^4.774+1.391;N=0.001C^5.029+1.294 and N=1.816C^0.537-0.389for 25-,40-,and 61-year-old stands,respectively.Under the background of climate change with the increase of atmospheric carbon dioxide concentration,temperature,and precipitation in the future,the GPP of Betula platyphylla ecosystem in the Daxing’an Mountains would increase.In addition,soil carbon and nitrogen fluxes will also increase under the influence of climate change.