Soil Greenhouse Gas Fluxes Of Different Forest Types And Their Responses To Climate Change In Permafrost Regions Of Daxing'an Mountains,Northeast China

Permafrost is an important part of the cryosphere,which is extremely sensitive to climate change.Climate warming has caused the degradation of permafrost and affected the carbon and nitrogen cycle of the soil system.The Daxing'an Mountains is an important forestry base,and it is also the core distribution area of high-latitude permafrost in my country.This area is experiencing a greater increase in temperature,so it is an ideal research place to reveal the response of forest ecosystems to climate change and human activities in cold regions.Due to the combined effects of human activities and climate warming,permafrost is accelerating degradation,which intensifies the decomposition of ancient carbon and releases more greenhouse gases,and produces important feedbacks to climate change.In this study,a combination of in-situ observation in the field and indoor simulated cultivation was used.The typical forest soils of Larix gmelini(LF),Pinus sylvestris(PF),and Betula platyphylla(BF)in the permafrost regions of the Daxing'an Mountains were selected as the research object,to explore the dynamic characteristics of greenhouse gases in different forest types and their main controlling factors,quantitatively study the contribution of greenhouse gas emissions from different soil layers of permafeost to climate change,and reveal the response mechanism of greenhouse gas emissions from permafrost ecosystems to climate change.The main findings are as follows:(1)There showed significant seasonal variations in soil greenhouse gas fluxes(CO₂,CH₄ and N₂O)of LF,PF and BF in the permafrost regions of the Daxing'an Mountains(p<0.05),and differences in soil CO₂ and CH₄ fluxes between different forest types were significant.The soils from the three typical forests were all sources for CO₂ and N₂O,and sink for CH₄ during the entire observation period.During the two observation periods(2016.5-2018.4),the soil CO₂ flux showed a unimodal change trend,and the annual average emission flux was PF(291.76 mg·m^-2·h^-1)>LF(273.84mg·m^-2·h^-1)>BF(243.29 mg·m^-2·h^-1).The soil CH₄ flux also showed a single-peak change trend,and the annual average absorption flux performance was BF(30.46ug·m^-2·h^-1)>PF(24.59 ug·m^-2·h^-1)>LF(19.33 ug·m^-2·h^-1).The soil N₂O flux was more complicated.The first observation period was bimodal,and the second observation period was unimodal.The annual average flux was in the PF(13.33 ug·m^-2·h^-1)>LF(11.80 ug·m^-2·h^-1)>BF(11.45 ug·m^-2·h^-1).Temperature was the main factor affecting soil CO₂ and N₂O fluxes of different forest types in permafrost regions,while soil CH₄fluxes were affected by both soil temperature and water content.(2)Ground vegetation and snow cover have significant effects on soil CO₂,CH₄,and N₂O fluxes in permafrost regions.During the winter snow cover period and vegetation growing season,soil CO₂,CH₄,and N₂O fluxes account for important proportions,respectively.In-situ observations in the field found that the fluxes of soil CO₂,CH₄,and N₂O ranged from 5.21±2.15?980.79±42.70 mg·m^-2·h^-1,-61.12±9.56?34.31±9.22 ug·m^-2·h^-1,and-3.99±4.25?49.46±2.02 ug·m^-2·h^-1,respectively.Ground vegetation removal reduced soil CO₂ and N₂O emissions of various forest types,but increased soil CH₄ absorption.Snow cover removal reduced soil CO₂ and N₂O emissions and increased soil CH₄ emission.Soil CO₂ flux was mainly affected by soil temperature and had a significant positive correlation with soil temperature(p<0.01).Soil CH₄ flux was mainly affected by soil water content and temperature,which was significantly positively correlated with soil water content and significantly negatively correlated with temperature(p<0.05).Soil N₂O flux was relatively affected by soil temperature.Under different ground vegetation and snow cover treatments,soil CO₂,CH₄,and N₂O fluxes of various forest types were also affected by soil p H,nitrate nitrogen,ammonium nitrogen,total nitrogen and organic carbon.(3)Incubation experiments found that under different hydrothermal conditions,the soil CO₂,CH₄,and N₂O fluxes of different sections in the permafrost region of Daxing'an Mountains ranged from 0.49?2.43 mg·kg^-1·h^-1,-0.14?0.01ug·kg^-1·h^-1,and-0.21?0.10 ug·kg^-1·h^-1.Temperature had a significant impact on soil greenhouse gases in different sections,while the impact of water content was relatively small.Under variable temperature cultivation,the soil CO₂flux of each section changed significantly with the change of temperature.The CO₂ flux of each layer of soil showed the distribution pattern of A>B>C.Interactive analysis found that soil layer and temperature were the main factors affecting CO₂ flux.10°C was an important temperature threshold that affected soil CH₄ flux,and the soil CH₄ absorption flux of each profile showed the distribution pattern of B>C>A.Soil CH₄ was greatly affected by soil temperature and soil layer.Soil temperature and soil layer had a great influence on soil N₂O.The constant temperature incubation found that the N₂O flux in different sections of the soil was A>B>C.Incubation at variable temperature found that the soil N₂O of A and B was absorbed,and the C was released.Warming significantly increased soil CO₂ and N₂O emissions,and promoted soil CH₄ absorption.(4)There were some differences in soil microbial diversity and richness index in different sections of permafrost regions in Daxing'an Mountains,which were mainly reflected in abundance,and the types of microbial communities were basically the same.The dominant populations were Chloroflexi,Proteobacteria,Actinobacteria,Acidobacteria.Soil TOC and p H were the main physical and chemical factors affecting bacterial communities in each soil layer.Soil TOC and p H were the main physicochemical factors affecting bacterial communities in each soil layer.Diversity analysis found that the Chao1 index and Observed species index of the A-layer soil bacterial community were higher than those of the B-layer and C-layer soil.The differences in the bacterial communities of the A-layer and B-layer soils were significantly greater than that in the C-layer soil.(5)The cumulative fluxes of CO₂,CH₄,and N₂O in the three typical forest soils in the permafrost region of the Daxing'an Mountains were 13.45±0.34?16.54±0.39t·hm^-2?-1.77±0.08?-0.51±0.07 kg·hm^-2?0.61±0.02?0.81±0.02 kg·hm^-2,respectively.The contribution rate of soil CO₂ and N₂O flux in the growing season reached64.10%?91.89%.The greenhouse gases global warming potential of the three typical forest soils were 15.934 t·hm^-2 in LF,16.445 t·hm^-2 in PF,and 14.064 t·hm^-2 in BF.The global warming potential of soil greenhouse gases in different sections was 954.42mg·kg^-1 in A layer,693.90 mg·kg^-1 in B layer,and 521.17 mg·kg^-1 in C layer.Comprehensive analysis shows that under the background of continued climate warming in the future,permafrost will further degrade,thereby releasing more greenhouse gases,enhancing the atmospheric greenhouse effect,and generating positive feedback on climate warming.This study provides an important scientific basis for clarifying the carbon and nitrogen cycle process in permafrost regions in mid-high latitudes and improving the Earth system model.At the same time,it is of great significance for maintaining ecological security in cold regions and coping with regional climate change.