Characteristics of permafrost degradation in Northeast China and its ecological effects:A review

ShanShan Chen,ShuYing Zang,Li Sun

Heilongjiang Province Key Laboratory of Geographical Environment Monitoring and Spatial Information Service in Cold Regions,Harbin Normal University,Harbin,Heilongjiang 150025,China

ABSTRACT Latitudinal permafrost in Northern Northeast(NNE)China is located in the southern margin of the Eurasian continent,and is very sensitive to climatic and environmental change.Numerical simulations indicate that air temperature in the perma‐frost regions of Northeast China has been on the rise since the 1950s,and will keep rising in the 21st century,leading to ex‐tensive degradation of permafrost.Permafrost degradation in NNE China has its own characteristics,such as northward shifts in the shape of a"W"for the permafrost southern boundary(SLP),discontinuous permafrost degradation into islandlike frozen soil,and gradually disappearing island permafrost.Permafrost degradation leads to deterioration of the ecologi‐cal environment in cold regions.As a result,the belt of larch forests dominated by Larix gmelinii has shifted northwards and wetland areas with symbiotic relationships with permafrost have decreased significantly.With rapid retreat and thin‐ning of permafrost and vegetation change,the CO2 and CH4 flux increases with mean air temperature from continuous to sporadic permafrost areas as a result of activity of methanogen enhancement,positively feeding back to climate warming.This paper reviews the features of permafrost degradation,the effects of permafrost degradation on wetland and forest eco‐system structure and function,and greenhouse gas emissions on latitudinal permafrost in NNE China.We also put forward critical questions about the aforementioned effects,including:(1)establish long-term permafrost observation systems to evaluate the distribution of permafrost and SLP change,in order to study the feedback of permafrost to climate change;(2)carry out research about the effects of permafrost degradation on the wetland ecosystem and the response of Xing'an larch to global change,and predict ecosystem dynamics in permafrost degradation based on long-term field observation;(3)fo‐cus intensively on the dynamics of greenhouse gas flux in permafrost degradation of Northeast China and the feedback of greenhouse gas emissions to climate change;(4)quantitative studies on the permafrost carbon feedback and vegetation carbon feedback due to permafrost change to climate multi-impact and estimate the balance of C in permafrost regions in the future.

Keywords:climate warming;permafrost degradation;greenhouse gas emissions;ecosystem impact

1 Introduction

Global mean temperature has increased by approx‐imately 0.85°C,from 1951 to 2012(IPCC,2013),and 1983-2012 was likely the warmest 30-year peri‐od in the last 1,400 years in the Northern Hemisphere.Global surface temperature will continue to increase at both global and regional scales(Zubleret al.,2016)and some studies have indicated that trends in mean annual air temperatures(MAAT)in China are consis‐tent with global averages.Minimum and maximum air temperature in China increased at rates of 0.40 and 0.25°C/decade,respectively,during 1965-2014,and regions at higher latitudes generally experienced more change at both air temperatures(Wanget al.,2018).In recent 50 years(1960-2012),mean ground temper‐ature in China increased at a rate of 0.29°C/decade(Qiaoet al.,2015).Climate change is altering global and regional air temperature regime significantly and is likely to influence subsurface thermal regimes and changes in soil and ecosystem environments(Kurylyket al.,2014).

The mid-high northern latitudes are sensitive and vulnerable to climate change.Permafrost(soil or rock that remains at or below 0°C for at least two consecu‐tive years)in Northern Northeast(NNE)China is situ‐ated at high latitudes(46°30′N-53°30′N,115°05′E-135°02′E)(Weiet al.,2011)and along the southern edge of Eurasian permafrost.Mean annual ground sur‐face temperature and mean annual air temperature in‐creased 0.61 and 0.72°C/decade in NNE China from 1972 to 2005(Luoet al.,2014).This has major impli‐cations for geomorphological,hydrological,and bio‐logical processes in permafrost regions(Olivaet al.,2017).Due to permafrost characteristics,and its sensi‐tivity to climatic and environmental changes,perma‐frost warming and thawing in the Xing'anling Moun‐tains are widespread and correspond with climate warming and human activity that have occurred dur‐ing the past several decades(Jinet al.,2007;Changet al.,2013).For example,there has been a northward movement of the southern boundary of permafrost,a deepening of active layer thickness,and a disappear‐ance of sporadic island-like permafrost.Permafrost change is closely related to the regional land ecology and hydrology,and affects the ground-atmosphere car‐bon cycle and infrastructure stability.

Permafrost accounts for about 25%of land in the northern hemisphere((22±3)×106km2)and permafrost soils contain about 1,700 Gt(1 Gt=1015g)of carbon,nearly twice as much carbon as is currently in the at‐mosphere(Tarnocaiet al.,2009).Permafrost warming and rising soil temperature stimulate decomposition of soil organic matter and enhance microbial activity leading to release of stored soil organic carbon in per‐maforst,which accelerates climate warming.Within Northeast China there are also wetlands that have a symbiotic relationship with permafrost(Sunet al.,2008).As permafrost changes,the extent and distribu‐tion of wetlands can also change which is important because wetlands are a sink for atmospheric CO2and the storage of C and N(Wanget al.,2013).In addition,wetlands and peatlands located within Northeast China are known to be the largest terrestrial reservoir for C on earth;therefore,wetlands and peatlands are important contributors to global C sequestration.

Permafrost also forms an impervious water barrier and as the permafrost is gradually degenerated due to climate change,there is an elevation in soil water pen‐etration that improves soil drainage and alters soil moisture.Additionally,soil moisture and soil tempera‐ture are key ecological factors limiting vegetation dis‐tribution,plant growth,and community structure and function,and are a primary driving force for the re‐lease of carbon from permafrost.Owing to elevating air temperature,the alpine treeline in Changbai Moun‐tains has increased approximately 80 m northward in the past 160 years and if temperature continue to in‐crease,the alpine treeline will continue to move north‐ward and alpine tundra may disappear(Duet al.,2017).Permafrost degradation improves soil drainage which is favorable for tall woody vegetation growth,causing widespread shrub expansion in the arctic tun‐dra(Myers-Smithet al.,2011).Vegetation changes are likely to increase the depth of the permafrost ac‐tive layer thus releasing deep soil C and adding C to the atmosphere.

As a product of cold climate,permafrost at high latitudes is highly sensitive to climate change and can serve as an indicator of climate change(Wanget al.,2000;Jorgensonet al.,2001).In recent decades,the temperature has risen and the permafrost has seriously degraded.With the warming permafrost,forest and wetland ecosystems have changed,even deteriorated,affecting many important engineering structures and the environment of cold regions,e.g.,instability of en‐gineered infrastructure and atmospheric carbon cy‐cles.Our review aims to investigate the features of permafrost degradation,the impact of permafrost deg‐radation on ecosystem and carbon cycle change,and propose questions regarding effects of permafrost warming on carbon cycle and vegetation changes in NNE China.These questions may help to guide future research on understanding the permafrost-vegetationcarbon interactions and potential effects of decreasing permafrost on climate change.

2 Changes in permafrost distribution

The area of continuous permafrost has been re‐duced from 70%-80%to 30%-50%(Table 1),and the areal extent of permafrost in NNE China has shrunk from 3.9×105km2to 2.4×105km2and its thick‐ness range from a few meters to hundreds of meters(Zhouet al.,2000).During winter,the Da and Xiao Xing'anling Mountains are influenced by a Siberia-Mongolia High Pressure that results in a strong,sta‐ble,and extensive atmospheric inversion layer.Due to the unique geographical environment,dense vegeta‐tion,rich soil moisture content,and Quaternary depo‐sitions,the permafrost is more developed in the lowlying areas,with thicker and cold permafrost.As a consequence,the characteristic of permafrost develop‐ment differ from most elevation and latitudinal perma‐frost,so it is called the Xing'an-Baikal permafrost.Under the influence of altitude and latitude,perma‐frost temperatures vary from place to place(Strelets‐kiyet al.,2014).Additionally,as a product of cold cli‐mates,latitudinal permafrost is usually also affected and modified by surface canopies,such as snow cov‐er,topography,and soil properties,while lithology and geology are the most important influencing fac‐tors for the geothermal system.Lithology and geolo‐gy are vital for the formation and development of per‐mafrost and the ground freezing and thawing in the active layer.

Table 1 Basic feature of permafrost in Northeast China

2.1 Changes in the southern limit of permafrost(SLP)

In recent decades,China has published a series of permafrost maps.Shi and Mi(1988),for example,drew a permafrost distribution map called Map of Snow,Ice and Frozen Ground in China(1:4,000,000 scale).Guoet al.(1981)summarized permafrost divi‐sions and regional characteristics and drew a perma‐frost distribution map of Northeast China with a scale of 1:3,000,000.Permafrost maps not only summarized previous geocryological research,but also,have been applied to evaluating changes in the area of perma‐frost and researching the responses and feedbacks of permafrost to climate change.Variation in SLP can be used as an indicator of permafrost change.Numerous studies have evaluated SLP migration,but the accu‐rate position of the SLP remain evasive due to the dif‐ficulty of monitoring permafrost and paucity of sur‐vey and measured data.However,research on SLP is important in evaluating the relationship between cli‐mate change and permafrost.

Xin and Ren(1956)defined the SLP in Northeast China based on a large amount of survey data about hydrology,geology,and production and construction experiences. The Study Group on Permafrost in Northeast China conducted field investigations in many areas,including Jiayin,Dedu,A'ershan,and New West Brag Banner(Xinba'erhuyouqi)in 1983,which greatly improved the aforementioned perma‐frost map in 1956.Guoet al.(1981)combined theo‐retical distributions and mathematical models to draw a geographic SLP in Northeast China,which was actu‐ally a geographical belt with isotherms of MAAT at 0±1°C,with 0°C as the centerline.Compared with the 1970s,the SLP in Northeast China has shifted by 100-150 km.According to the relationship between the occurrence of ice wedges and air/ground tempera‐tures,Xie(1985)concluded that the SLP in NNE Chi‐na during the Late Pleistocene(about 40°N-42°N)co‐incided with the present MAAT isotherm of 6-10°C,while the present SLP is at 48°N.The SLP in China during the Holocene moved southward by 2°N in com‐parison with that of 1990s.Luet al.(1993)divided the recognized"W"-shaped geographical SLP in the Da and Xiao Xing'anling Mountains;i.e.,near the A'er‐shan Mountains in the west,extending northeast‐wardly along the eastern slopes of the Da Xing'an‐ling Mountains,passing the south slopes of the Xiao Xing'anling Mountains to the south of the Nenjiang River Basin;then turning to the southeast passing Yic‐hun,and returning to the northeast.As for the SLP in the vicinity of Jiayin,Sunet al.(2007)proposed that the orogenic effects of the Da and Xiao Xing'an Mountains should be considered.The SLP in the Da Xing'an Mountains was south of Huanggangling Mount and 120 km south of Aer Mount.The SLP in the Xiao Xing'an Mountains was in the south of mid‐dle mount near the Hulan River.Zhouet al.(2000)re‐ported that the geophysical SLP in Northeast China basically follows the MAAT isotherm of-5°C;how‐ever,the geographical SLP is generally used for per‐mafrost zoning.

2.2 Characteristics of permafrost degradation

The latitudinal permafrost region in NNE China is located in the southern margin of the Eurasia conti‐nental permafrost zone(Jinet al.,2007).The perma‐frost exhibits high temperature and thin thickness and the thermal regimes of permafrost are unstable;which is susceptible to external climatic,environmental,and anthropogenic disturbances.Owing to the vulnerable physical environment,the ecosystem in cold regions is highly sensitive to climate and human-induced changes.With the climate warming and intensive hu‐man activities,the permafrost degradation in NNE China is obvious and extensive and has the common characteristics of permafrost degradation and also has its own characteristics.Its common features are as follows:rising ground temperatures,deepening ac‐tive layer,forming and enlarging taliks,and elevat‐ing permafrost base(Heet al.,2009).For example,in Northeast China,the northern slopes of the Da Xing'anling Mountains are the most developed area of permafrost in NNE China,where permafrost has also degraded,even disappeared,and taliks have ex‐panded significantly(Guet al.,1994).Borehole No.14 at the permafrost observation site of the Ituri(Yit‐uli'he) Riverside Railway Research Institute has been influenced by surrounding urbanization and cli‐mate warming.The mean annual ground tempera‐tures(MAGT)of permafrost has been increasing,es‐pecially at the depth of 13 m,where it increased 0.2 and 0.4°C,respectively,in 1984-1997 and 1997-2010(Zhouet al.,1996;Changet al.,2013).Its own characteristics shows that the degradation of continu‐ous permafrost in the north is quantitative including:decreasing in the permafrost table,reduction in conti‐nuity of permafrost extents,elevation of ground tem‐peratures,and thinning of permafrost layer.For ex‐ample,ground temperature at 10 m in depth in the Amur area of Mo'he County,northern Heilongjiang Province,rose from-3.7°C in 1975 to-1.5°C in 1978(Wanget al.,1996;Jinet al.,2000).Most of the southern permafrost zones have qualitatively changed as the island permafrost shrinks and degen‐erates into areas of seasonal frost and the SLP shifts northwards.Compared to the 1970s,the area of spo‐radic discontinuous and isolated patchy permafrost has shrunk by 9×104to 10×104km2(Jinet al.,2007).In Northeast China,the response of permafrost to cli‐mate warming has also displayed strong spatial dif‐ferentiation.The general rules of permafrost degrada‐tion are that southern permafrost in NNE China de‐grades faster than the north;degradation of perma‐frost in mountainous areas begins earlier than in val‐leys and basins;on sunny slopes permafrost thaws more rapidly than on the shadowy slopes;and hu‐mans activities have more direct and dominant im‐pacts on permafrost degradation compared to natural factors(Wanget al.,1996).

Figure 1 Distribution of permafrost in Northeast China

Based on meteorological and snow depth data,Lüet al.(2008)applied a freezing index model to fore‐cast permafrost distribution in Northeast China.The results indicated that permafrost was dominated by discontinuous permafrost.Weiet al.(2011)estab‐lished the equivalent latitude model of permafrost,in‐dicating some areas where the current surface temper‐ature is+0.5°C and-0.5°C would have permafrost in 50 and 100 years later;the area of stable permafrost with the MAGT less than-1.0°C will be gradually re‐duced from 1.07×105km2in 2011 to 8.8×104km2in 2060,and further to 5.6×104km2in 2110;and areal extents of unstable permafrost and seasonal frost will increase.Based on the law of energy balance between the atmosphere and soil,Zhanget al.(2018)analyzed the dynamics of permafrost from 1980 to 2010 in NNE China.Based on mean annual air temperature,annual precipitation,and mean annual wind speed,they reported that the permafrost area decreased by 13.67%from 1980 to 2010;however,the areal extent of sparsely-island permafrost expanded because pre‐cipitation increased in this region,especially in the southeast area.On the other hand,the discontinuous and continuous permafrost degradation into islandlike frozen soil caused an increase in island perma‐frost.Permafrost degradation may lead to adverse and irreversible changes in the environment including wet‐land shrinkage or disappearance and land desertifica‐tion.Permafrost degradation may also cause changes in regional or local climatic and hydrology.Minimiz‐ing or avoiding artificial activities that affect perma‐frost is thus advised for protecting the permafrost environment.

3 Impact of permafrost degradation on the ecosystem

Forests,wetlands and permafrost are the three ma‐jor elements of a cold environment and are interdepen‐dent to form the permafrost ecosystem in Northeast China(Sunet al.,2000,2007).Climate change and widespread degradation of permafrost can alter waterheat process of soils,which has potentially affected species composition,extent and distribution of plant communities, resulting in community succession.Eventually, the permafrost eco-environment will change dramatically in Northeast China.

3.1 Impact on wetland ecosystems

Permafrost and wetlands have symbiotic relation‐ships(Sunet al.,2007).Permafrost promote wetland development because it is almost impervious in pre‐venting rainwater and soil water from filtering under‐ground,providing ample water supplies for shrubs and herbaceous plant growth.In addition,wetlands on permafrost are favorable to the development of perma‐frost and have a function of preventing permafrost degradation(Sunet al.,2000).The reason is that wet‐land plants produce a shade effect, reducing the amount of heat absorbed by the ground.On the other hand,peat moss soil has high insulating capacity and subsequently suppresses soil temperature,stimulating permafrost formation and growth.

It is well known that permafrost degradation in NNE China is widely common due to climate change and human activities.For example,permafrost patch‐es disappeared; the south boundary of permafrost moved northward and the active layer deepened(Jinet al.,2007).Since permafrost has a symbiotic rela‐tionship with wetlands(Sunet al.,2007),permafrost degradation caused the decrease in wetland area and vegetation productivity of wetlands,and led to chang‐es in species distribution and species diversity of wet‐land communities in NNE China,especially,the per‐mafrost wetlands of the Great Hing'an Mountains(Sunet al.,2010,2011).The original wetland of up‐per level permafrost shows a similar trend as that of permafrost(Sun,2000;Zhouet al.,2003).

Moreover,species diversity of wetlands existed in latitude zonal in permafrost regions(Sunet al.,2011).That is,temperature is critical for species diversity and shrub species decreased linearly with decreasing latitude,but increased for herbaceous plants.When mean annual temperature increase by 1°C,shrub di‐versity and number decrease 0.33 and 2,respectively,and herbaceous species diversity and number increase 0.29 and 9,respectively.In 2011,Liuet al.(2011)used the global circulation model to project that the temperature in the Great Hing'an Mountains,located in the southern edge of the Eurasia permafrost(Zhouet al.,2000),would rise by 2-4°C over 100 years.Accordingly,wetland areas in this region tends to de‐crease,and will decrease by 60%by 2100(Liuet al.,2011).In the same climate condition,herbage produc‐tivity will increase 2-4 times in predominantly contin‐uous permafrost and 57%-114%in island permafrost(Sunet al.,2010).The herbaceous species diversity is the same as herbage productivity(40%or 2 times),but shrub species diversity may decrease,or even dis‐appear in some parts of lower latitude regions(Sunet al.,2010,2011).Wetland degradation in the Great Hing'an Mountains may not only accelerate perma‐frost degradation but also affect greenhouse gas emis‐sions in that native wetlands are considered as a sink for atmospheric CO2,providing an important contribu‐tion to global C sequestration.Thus,it is necessary to research the influence of vegetation types on perma‐frost degradation and permafrost-carbon feedbacks in the future.

3.2 Impact on forest ecosystems

Permafrost warming and degradation is changing the environment in cold regions,resulting in altering the forest ecosystems,such as vegetation species,bio‐mass,and vegetation coverage and productivity(Da‐viset al.,2000).Permafrost degradation is obvious in the Da Xing'anling Mountains.The warming perma‐frost may have caused the tree-line altitude to rise for the main forest species,such asLarix gmeliniiandL.chinensis,and the coniferous forest gradually evolved into a mixed forest of deciduous coniferous and broadleaved species(Gu and Zhou,1994).The original larch(Larix gmelinii)forest in island permafrost zone of Dayangshu region degenerated into a secondary birch(Betula)forest due to permafrost disappearance(Zhouet al.,2003).Compared with Siberia in the north,permafrost degradation in the boreal forest eco‐system brings about the expansion of shrub-lands and the reduction of tundra and coniferous forest.In the south,areal extents of forested grasslands and grass‐lands have increased,and coniferous forest was trans‐formed to forested grassland or grassland systems(Tchebakovaet al.,2009).

However,with climate warming,permafrost deg‐radation would prolong the growth period of vegeta‐tion owing to the active layer thawing in advance,and delayed freezing.In 1982-2009,the vegetation NPP in permafrost regions of Northeast China was on the rise,but its growth rate was not as fast as that of in Eastern Siberia(Satoet al.,2016).In addition,perma‐frost degradation resulted in the reduction of soil moisture and caused rapid evaporation of available soil water.Thus,water availability in shallow soils is a vital factor for vegetation growth in permafrost re‐gions,especially in the growing season,resulting in grassland degradation,grass biomass and NPP reduc‐tion(Miaoet al.,2012).By analyzing NDVI index of vegetation in Northeast China,studies have shown that vegetation coverage has a downward trend(Guoet al.,2008),dramatically fluctuating grassland ND‐VI,and strong spatial heterogeneity in vegetation ND‐VI(Miaoet al.,2011).

Additionally,in high latitudes and high mountains,permafrost degradation not only affects vegetation eco‐logical type,community structure,and species compo‐sition,but will eventually cause changes in the service and function of the vegetation ecosystem.This de‐serves further study.Moreover,changes in vegetation structure and composition are the result of interactions between climate,hydrology,topography and other in‐terfering factors.Thus,it is difficult to predict future vegetational feedback on permafrost degradation.

4 Impacts of permafrost degradation on greenhouse gas emissions

4.1 Changes in stocks of soil carbon and nitrogen

Terrestrial ecosystem C and N play an important role in driving climate change because of emissions of C and N oxides into the atmosphere(Fuet al.,2010).Soil organic carbon(SOC)is the largest ter‐restrial C pool,about two times larger than C storage in the aboveground biomass or the atmosphere(Postet al.,1990).This vast amount of soil organic carbon stock may be vulnerable to climate change and sus‐ceptible to rapid decomposition when the natural geographical environment changes with global warming(Savage and Davidson,2001).However,the permafrost region in NNE China is warming in‐creasingly,were Northeast China contains 26.43 Pg C(Wanget al.,2002)of SOC,of which wetland SOC declined with decreasing latitude (Wanget al.,2012).Additionally,soil depth and vegetation type affected soil C and N concentrations and storage(Wanget al.,2013)because permafrost degradation increased the depth of permafrost active layer and vegetation change may expose the permafrost,lead‐ing to the release of C of deep soil.Wetland or forest to the wetland-forest ecotone of thicket peatland would intensify the extent of C and N losses,and therefore affect soil C and N storage.Namely,soil carbon and nitrogen are released into the atmosphere in the form of carbon dioxide and methane,which has a positive feedback on climate,thus accelerating permafrost degradation.

Soil temperature and moisture can play an active role in determining soil carbon and nitrogen mineral‐ization in all ecosystems.Similarly,many studies have illustrated that they also influence permafrost peatland of soil carbon mineralization in the Great Hing'an Mountains(Wanget al.,2010;Wanget al.,2014;Songet al.,2018).Wanget al.(2010)came to the conclusion that carbon mineralization rates of continuous permafrost peatland increased with in‐creasing temperature in the Great Hing'an Moun‐tains,and the rates increased with increasing mois‐ture up to 60%water holding capacity(WHC),and then decreased at 100%WHC,probably due to the absence of available oxygen. Rising temperatures significantly increased soil C mineralization in a per‐mafrost peatland,the Q10values of the carbon miner‐alization rates in the shallow soil(0-15 cm)and deep soil(15-30 cm)were 3.95 and 2.91,respectively(Songet al.,2018).The same situation exists in wet‐lands of this region.

These findings indicate that permafrost soil is sen‐sitive to climate and environment change.Permafrost soil would be potentially mineralized under future cli‐mate change,influencing permafrost soil C balance in NNE China.

4.2 Changes in greenhouse gas emissions

A large amount of organic carbon was deposited in wetlands and peatlands,which plays an essential role in global carbon balance.Wetlands have a symbi‐otic relationship with the underlying permafrost,con‐taining a large amount of carbon stock(Jinet al.,1999;Schuuret al.,2008).The thinning or disappear‐ance of permafrost can accelerate biogeochemical pro‐cesses.Wetlands in cold regions may be transformed into a carbon source,which will have a positive feed‐back to accelerate climate warming.

4.2.1 Impacts of process change for freeze-thaw cycle on greenhouse gas emissions

Permafrost degradation can change the process of freeze-thaw-cycle in the active layer,especially freez‐ing and thawing time.Moreover,it would change the original physical hydrothermal properties of perma‐frost soils.Permafrost degradation have brought about changes in permafrost carbon storage and release,af‐fecting the soil carbon cycle.Major greenhouse gases(e.g.,CO2,CH4and N2O)are produced when the ac‐tive layer and permafrost tables thaw,and permafrost degradation changes the seasonal distribution patterns and magnitude of greenhouse gas emissions(Sunet al.,2011;Songet al.,2012).CH4emissions in wet‐lands produce multiple peaks and peak period ad‐vancement owing to permafrost degradation.In the non-growing season,wetland CH4emissions also have high peaks.

With climate warming and permafrost degradation,the surface active layer of permafrost thaws earlier than before.As a consequence,the peak period of methane emission advanced.On the other hand,pro‐longing the growing season may increase the propor‐tion of CH4emissions in the growing season through‐out the year.The longer thawing period has boosted the peatlands potential emissions of CO2and N2O in permafrost regions in Northeast China(Wanget al.,2014).During the spring freeze-thaw transition period,the maximum rate of CH4emission from the wetland is 48.6 g/(cm2·h),which is three times that of wetland CH4in the growing season(Songet al.,2012).As the active layer thickness increases,not only will perma‐frost degradation alter greenhouse gas emission pat‐terns and magnitudes,but also ancient carbon stored in permafrost would be emitted as CO2or CH4.

4.2.2 Impacts of abiotic factor on greenhouse gas emissions

Climate warming in Northeast China has an enor‐mous impact on permafrost,soil and forest ecosystem environments in this region.With permafrost warm‐ing,soil temperature,soil moisture,and vegetation have changed.Yet,these factors are the primary driv‐ing force of organic carbon decomposition and emis‐sions,the change of which influence the pool of soil carbon and nitrogen.On the one hand,elevating tem‐perature will simulate the decomposition of soil or‐ganic matter and enhance enzyme activity,which ac‐celerates the release of ancient carbon(stored in per‐mafrost)into the atmosphere.Also,soil moisture(wa‐ter table)would alter the original aerobic and anaero‐bic environment,damaging the original carbon bal‐ance,because the release rate and type of soil organic carbon from permafrost soil differs in different oxidiz‐ing environments.

Permafrost carbon feedback has received a lot of attention.Numerous studies have been carried out by static chamber and eddy covariance methods to mea‐sure greenhouse gas emissions from permafrost soils in Northeast China(Table 2).As shown in Table 2,CO2,CH4,and N2O flux from different permafrost types have some discrepancy in the same ecosystem.Namely,CO2flux was the largest,followed by CH4,and N2O flux the lowest.According to published lit‐erature,Table 2 shows regional variations in CO2,CH4,and N2O flux in the wetlands or peatlands of Northeast China.CO2and CH4flux increased with mean air temperature from continuous permafrost to sporadic permafrost as a result of methanogenic ac‐tivity enhancement.These results implies that perma‐frost degradation due to climate warming would greatly stimulate CO2and CH4emission from the peatlands in Northeast China,and the peatlands will become a potential carbon source for the atmosphere(Wanget al.,2010).Expect temperature,water table level,active layer depth and soil active organic car‐bon to play vital roles in greenhouse gas emissions(Wanget al.,2013;Songet al.,2014;Liuet al.,2015;Cuiet al.,2018).Songet al.(2014)indicated that CH4and CO2emissions significantly increased with permafrost thaw under aerobic and anaerobic conditions.Moreover,carbon release under aerobic conditions was greater than under anaerobic condi‐tions.However,Cuiet al.(2018)demonstrated that soil moisture has a weak correlation with N2O flux;soil temperature and active layer depth have a con‐siderable influence on N2O flux.The aforementioned facts demonstrate that permafrost soils have a great‐er potential for decomposition than the active layer soil.If N2O flux continues to rise,it will potentially drive a noncarbon feedback to ongoing climate change.

As mentioned above,permafrost degradation is widespread with climate warming,leading to the re‐lease of carbon from permafrost soil to the atmo‐sphere,in turn the permafrost carbon feedback also has a negative or positive feedback on the climate.Thus,it is necessary to further study the impact of per‐mafrost degradation on the climate. Additionally,Northeast China has a relatively high fire frequency,and under a warming climate,fire disturbance would bring about large quantities of organic C released into the atmosphere.Fires increase CO2emissions to the atmosphere not only during the combustion process,but also in an extended post-burning period by the biogeochemical process(Zhaoet al.,2012).Yet,little is known about the evolution of carbon stored in per‐mafrost after burning,which is potentially important in the global carbon cycle.

5 Conclusions and prospect

The cryosphere is one of the five spheres in the earth system.Permafrost is an important part of the cryosphere and is very sensitive to climate change.With climate warming,permafrost in NNE China has undergone significant changes.For example,the per‐mafrost table has lowered;the active layer has deep‐ened;the talik has expanded;and the permafrost ex‐tent has decreased.The response of permafrost to cli‐mate warming has led to a series of changes in cold regions environment,such as the reduction of wetland area,species succession,and variation in vegetation coverage and productivity.The change in soil physi‐cal properties and variations in freeze-thaw cycles af‐fect the release of carbon from permafrost,influenc‐ing carbon balance and carbon cycles at regional and global scales.

At present,there are numerous studies on the fea‐tures and phenomena of degrading permafrost,the cold regions ecosystems and construction,and the maintenance of engineering infrastructures in NNE China.However,some issues and research still are in‐sufficient,such as mechanisms for permafrost degra‐dation in boreal wetlands and peatlands, feedback mechanisms for permafrost carbon and microbes.The impacts of permafrost change on the environment are becoming increasingly evident in NNE China.The fu‐ture changes of permafrost will inevitably have a wide and profound impact on the ecological and envi‐ronmental security and water resources in NNE Chi‐na.However,these mechanisms and processes are in‐adequately studied and poorly understood.Thus,the following aspects should be urgently addressed:

(1)Permafrost dynamic process and its response mechanism to climate change:It is necessary to study the mechanism and features during the process of spa‐tiotemporal variations on permafrost degradation in NNE China under changing climate.The essence of permafrost change is the soil water phase change,that is,simple solid-liquid water transformation process,but permafrost degradation will bring about changes in climate,ecology and environment with varied time and space scales.In the future,it is urgent and necessary to establish and improve the long-standing monitoring network of permafrost regions,especially for essential variables,such as soil moisture,ground temperature,and active layer depth.Only on the basis of these basic data,can we establish coupled models of permafrost distribution in order to predict the dynamic process of permafrost.Therefore,we can better propose and for‐mulate scientific and prudent mitigating measures to changes of permafrost in Northeast China.Using the coupled global and regional climate system models un‐der typical concentration pathways(RCP)estimate cli‐mate response to permafrost change in Northeast China in the 21st century,quantitatively evaluate the impact of climate change on permafrost,and carry out indepth research on the physical processes and feedback mechanisms between cryosphere and climate.Scientif‐ic prediction of future climate and permafrost changes enhance the level of scientific understanding of climate system change and reduce the uncertainty of climate system simulations and future projections.

(2)Multi-factor interactions and mutual coupling mechanisms for permafrost changes:The study on bio‐geochemical processes and their mutual feedbacks will be conductive to promote in revealing the mechanism for permafrost change.It is vital to understand the ef‐fects of permafrost degradation on wetlands,forest eco‐system,and carbon and nitrogen cycles in terrestrial ecosystems,to investigate the mechanism of wetlands degradation and clarify change of ecosystem service function,and to assess carbon source and sink transi‐tions in the future.It is significant to the analysis and understanding of the effects of changes in permafrost and active layer on global carbon cycles by quantita‐tively estimating the stock of permafrost carbon in Northeast China and its changes, surface-to-atmo‐sphere exchange flux of CO2,CH4and N2O in perma‐frost regions,and clearly determining the important fac‐tors for carbon conversion(Dinget al.,2014).In addi‐tion,permafrost degradation could prolong the growth season and increase photosynthetic rate,which could sequester C from the atmosphere and restraining cli‐mate warming through a negative mechanism,namely,it can offset,to some extent,the release of carbon from permafrost.In the long term,whether carbon absorbed by ecosystems can offset carbon emissions from perma‐frost degradation is worthy of in-depth study.This is crucial for future research in the impact of permafrost change on climate and accurately estimating the bal‐ance of C in permafrost regions in the future.

Acknowledgments:

This study was supported by the National Natural Sci‐ence Foundation of China(No.41571199).We are grateful to Professors Richard S.Halbrook and Hui‐Jun Jin for their guidance on this manuscript.