Characteristics And Simulation Of Surface Greenhouse Gas Flux In Age Series Of Chinese Fir Plantation In Huitong

The ecological problems caused by increasing concentrations of greenhouse gases in the atmosphere are a global concern and the study of CO₂,CH₄ and N₂O fluxes is an important basis for mitigating the greenhouse effect.As an important part of terrestrial ecosystems,forests play an important role in influencing global climate change.As the largest tree species planted in China,the study of surface greenhouse gas fluxes in Chinese fir plantations has received widespread attention.In this paper,we used static dark box-gas chromatography to determine the greenhouse gas fluxes at the surface of Chinese fir plantation forest age series（6a,11a,14a,24a,32a,100a）in Huitong during MarchDecember 2021,and analyzed the dynamic characteristics of greenhouse gas fluxes at the surface of Chinese fir plantation forest age series and their warming potential in combination with environmental factor tests.The study was carried out to validate the Forest-DNDC model using CO₂ observations and to predict the trends of surface CO₂ fluxes in Chinese fir plantation forests under future climate change.The main results of the study are as follows:（1）There were highly significant differences in soil physicochemical properties in all age sequences of Chinese fir plantations.The soil mucilage content（15.98%）was significantly higher in the 6a Chinese fir plantations than in other stands;the soil soluble organic carbon（DOC）（450.91mg/kg）was significantly higher in the 24a Chinese fir plantations than in other stands;the soil bulk weight（1.42 g/cm3）and soil water-filled porosity（72.84%）were significantly higher in the 32a Chinese fir plantations than in other stands;and the soil pH（4.58%）was significantly higher in the 100a Chinese fir plantations than in other stands.Soil pH（4.58）and soil Eh value（326.56mv）were significantly higher in 100a Chinese fir plantations than in other stands.（2）The surface CO₂ flux of Chinese fir plantation was mainly controlled by temperature,and the CH₄ and N₂O fluxes were mainly controlled by temperature and humidity.Surface CO₂ flux peaked in June-August and lowest in December.The surface CH₄ flux showed multiple absorption peaks from August to October,and a low value from May to July.The surface N₂O flux showed a peak value in May-July and a low value in March and October-December.The annual average surface CO₂ emission flux decreased first and then increased with the increase of forest age.The annual average CH₄ absorption flux increased first and then decreased with the increase of forest age.The annual average N₂O emission flux increased first and then decreased and then increased with the increase of forest age.There were significant differences in surface greenhouse gas fluxes between different forest ages and months（P<0.05）.（3）The trends of annual emissions of CO₂,CH₄ and N₂O gases from Chinese fir plantations forests were not consistent with increasing stand age.The annual surface CO₂ emissions of Chinese fir plantations forests showed a general trend of decreasing and then increasing,then decreasing,then increasing again with increasing forest age,with the maximum in 6a Chinese fir plantations forests（28.66 t/hm²）and the minimum in 32a fir plantation forests（24.99 t/hm²）.Annual surface CH₄ emissions from Chinese fir plantations showed a general trend of increasing and then decreasing with increasing stand age,with the largest amount in 11a Chinese fir plantations（-4.24 kg/hm²）and the smallest in 32a Chinese fir plantations（-1.98 kg/hm²）.Annual N₂O emissions from Chinese fir plantations show a trend of increasing,then decreasing and then increasing with increasing stand age,with the largest amount in 100a Chinese fir plantations（2.157 kg/hm²）and the smallest in 6a Chinese fir plantations（0.44 kg/hm²）.（4）The ambient temperature is the main factor affecting the surface CO₂ flux of the forest age sequence of the Chinese Fir plantation,the soil 5cm moisture and soil water fill porosity are the main factors affecting the CH₄ flux of the surface of the forest age sequence of the Chinese Fir plantation,and the ambient temperature and soil temperature and humidity are the main factors affecting the surface N₂O flux of the age sequence of the Chinese Fir plantation.6a,11a,14a,24a,32a,and 100a Chinese fir plantation age series were significantly higher than those in other stands.Soil temperature sensitivity in plantation forests was 2.2,1.44,1.78,1.59,1.43 and 1.89,respectively.soil physicochemical properties had different effects on greenhouse gases in Chinese fir plantations.14a Chinese fir plantation surface CO₂ and CH₄ fluxes were influenced by DOC;24a fir plantation surface CO₂ fluxes were influenced by pH and SOC.Surface CO₂ fluxes in 100a Chinese fir plantations were influenced by soil Eh and SOC.Surface CH₄ fluxes in 6a,11a and 24a Chinese fir plantations were influenced by SOC;surface CH₄ fluxes in 11a and 32a Chinese fir plantations were influenced by soil Eh.Surface N₂O fluxes in 6a Chinese fir plantations were influenced by pH,SOC;surface N₂O fluxes in 24a Chinese fir plantations were influenced by pH,SOC.fluxes were influenced by pH,Eh;32a N₂O fluxes at the surface of Chinese fir plantations were influenced by Eh,DOC.（5）The corrected Forest-DNDC model allows the simulation of surface CO₂ fluxes in Chinese fir plantation forest age series.When comparing the measured and model simulated values,the coefficient of determination R² for 6a,11a,24a and 32a Chinese fir plantations was above 0.84,the model efficiency coefficient ME was above 0.75 and the consistency index d value was above 0.9.The simulated and measured values fitted well.The simulated projections of 6a Chinese fir plantation surface CO₂ emissions under climate scenarios for the next 10 years show that annual Chinese fir plantation surface CO₂ emissions gradually increase with increasing time,and compared with annual 6a fir plantation surface CO₂ emissions in 2021,the annual 6a Chinese fir plantation surface CO₂ emission fluxes in 2022-2027,2029 decrease by 3.3%-28.4%,and 6a Chinese fir plantation annual surface CO₂ emission fluxes increased by 6.4%-12.5%in 2028 and 20302031.In summary,the surface greenhouse gas flux of the age sequence of Chinese fir plantation has obvious seasonal characteristics,temperature is the main factor affecting the surface CO₂ and N₂O flux of Chinese fir plantation,soil moisture is the main factor affecting the surface CH₄ flux of Chinese fir plantation,different soil physical and chemical properties have different effects on the surface greenhouse gas flux of Chinese fir plantation,Forest-DNDC model can effectively simulate the surface CO₂ flux of Chinese fir plantation.