Coupling Of Carbon-Nitrogen Cycles And Hydrological Process Of Typical Forest Ecosystems In Beijing Mountainous Area

As the important part of the terrestrial ecosystem,forest ecosystem plays an irreplaceable role in regulating the carbon and nitrogen balance of the terrestrial ecosystem,slowing down the increase of greenhouse gas concentrations such as CO₂and N₂O in the atmosphere and maintaining global climate stability.There is a close coupling relationship between ecosystem carbon-nitrogen cycle and hydrological process.The hydrological process has a significant impact on the carbon and nitrogen balance of the forest ecosystem through the control of photosynthesis,respiration and soil microbial activities.Therefore,it is necessary to pay attention to hydrological processes,and analyze the coupling relationship between carbon-nitrogen cycles in ecosystems and hydrological processes.This study selected typical forest ecosystems（Platycladus orientalis and Quercus variabilis）as research objects,analyzed the basic characteristics of C and N cycles at different interfaces in forest ecosystems,based on field monitoring,control experiments and model simulation.Next,we analyzed the coupling relationship between carbon-nitrogen cycles and hydrological processes,and simulated the effects of climate change on carbon source/sink function and the coupling between carbon-nitrogen and hydrological processes.Finally,we analyzed the possible impacts of climate change on forest ecosystems from the perspectives of water cycle,water-carbon coupling,and nitrogen supply.The results could provide scientific basis and decision-making support for ecological environment construction and sustainable resource development in North China.The main findings are as follows:（1）The carbon sequestration has coupling relationships with evapotranspiration（ET）at short-term（scale of hours）and long-term（scale of years）scales.On the short-term scale,WUE change was mainly driven by photosynthesis,temperature was the dominant influencing factor.On the long-term scale,WUE change was mainly driven by transpiration,relative humidity was the dominant influencing factor.（2）The process of soil carbon and nitrogen transformation was coupled with precipitation,soil evaporation and soil humidity.The"Birch effect"of precipitation events on soil respiration was related to rainfall and precipitation intensity.There was a significant positive correlation between soil evaporation with soil carbon and nitrogen transformation.The combination of soil temperature and humidity can explain most of the variability of soil carbon and nitrogen transformation process observed in the field.（3）According to the simulation results of the model,the P.orientalis ecosystem in the study area was a net carbon sink,with net ecosystem productivity（NEP）of 903.98 kg C·hm^-2·a^-1,accounting for6.02%of gross primary productivity（GPP）.In the study area,the forest nitrogen deposition flux was35.19 kg N·hm^-2·a^-1,and the vegetation nitrogen retention was 30.45 kg N·hm^-2·a^-1.Atmospheric nitrogen deposition was an important source of nitrogen in the ecosystem.（4）There was a significant coupling relationship between carbon,nitrogen and water cycling in forest ecosystems.Temperature and soil water supply were the main driving factors for carbon,nitrogen and water cycling.Vegetation nitrogen uptake（N-uptake）,ET,and temperature explained most of the variability in NEP,temperature and N-uptake explained most of the variability in autotrophic respiration,temperature and precipitation explained most of the variability in soil heterotrophic respiration.（5）Under the background of climate change,the carbon,nitrogen and water cycling in forest ecosystems have changed significantly.There was a significant negative correlation between WUE and global warming.Warming would significantly reduce WUE,the significant positive effect of warming on ecosystem respiration increased the uncertainty of the coupling between carbon and hydrological process.There was a significant coupling effect between vegetation water uptake and N-uptake.Insufficient soil water supply significantly affected N-uptake,while the rising atmospheric CO₂ improved forest productivity and nitrogen cycle,and led to drought stress,which accelerated nitrogen limitation in forest ecosystems to some extent.