Effects Of Permafrost Degradation On Carbon Source/Sink Of Wetlands In Northeast China

Under the background of global warming,permafrost in the northern hemisphere is undergoing extensive degradation.The high latitude permafrost zone in Northeast China is located at the southern edge of the Eurasian permafrost zone,and its southern boundary（near Yichun）has moved northward by about 130 km since the 1970s,the total area of the permafrost zone has been reduced by 37%-38%.With further warming,continuous permafrost degradation is bound to have a huge impact on the wetlands and its carbon cycle above the permafrost.At present,most of the relevant studies focus on the comparison of different wetalnd types or different treatments in the same climate zone.In the Northeast permafrost zone,there are no studies on the response model of the direction and scale of carbon source/sink of the same wetalnd type in different zones to permafrost degradation.In this study,three zones（permafrost zone-Mohe,discontinuous permafrost zone-Nanwenghe,and seasonal permafrost zone-Yichun）with different degrees of degradation distributed along the latitudinal gradient were selected to represent the permafrost degradation process.Using static dark chamber-gas chromatography and relative growth equations,the annual net carbon sequestrations and soil heterotrophic respiration carbon emissions of seven wetalnd types（marsh-C,thicket wetland-G,five forested wetland types（alder wetland-M,birch wetland-B,larch wetland-LT,larch fen-LX,and larch bog-LN））in different zones were compared and analyzed from May 2017 to April 2018.According to the ecosystem net carbon budget balance,the effects of permafrost degradation on the carbon source/sink of wetlands in Northeast and its mechanism were revealed.The results indicated that:（1）Soil greenhouse gas emissions（CO₂ and CH₄）from seven wetland types regularly changed along the permafrost degradation gradient.In terms of CO₂:The mean annual CO₂fluxes from seven wetland types were 7.7%-15.1%significantly higher in the discontinuous permafrost zone(132.53-185.94 mg·m^-2·h^-1)than in the permafrost zone(123.01-161.52 mg·m^-2·h^-1).However,in the seasonal permafrost zone(97.33-231.81 mg·m^-2·h^-1),two different response patterns were showed,i.e.,the mean annual CO₂ fluxes from C and G regularly increased along the permafrost degradation gradient;while five forested wetland types first increased and then decreased.Permafrost degradation also changed the pattern of CO₂ emissions in the growing season and non-growing season.In the discontinuous permafrost zone,the contribution rate of CO₂ emissions from seven wetland types in the growing season increased to varying degrees;while,differentiation occurred in the seasonal permafrost zone（C,G,B and LN increased;M,LT,and LX decreased）.In terms of CH₄:The mean annual CH₄ fluxes from most wetland types in the discontinuous permafrost zone(-0.006-0.041 mg·m^-2·h^-1)were not significantly different from those in the permafrost zone(-0.007-0.406 mg·m^-2·h^-1)（excluding C and LN）;however,the mean annual CH₄ fluxes from the seven wetland types in the seasonal permafrost zone(0.075-23.439 mg·m^-2·h^-1)explosively increased by several orders of magnitude.Permafrost degradation also changed the pattern of CH₄ emissions,when permafrost was degraded to seasonal permafrost,there was only one response type in which the contribution rate of CH₄ emissions in the growing season was greater than that in the non-growing season.（2）Soil annual carbon emissions from the seven wetland types were significantly higher（7.8%-15.2%）in the discontinuous permafrost zone(3.17-4.45 t·ha^-1·yr^-1)than in the permafrost zone(2.94-3.89 t·ha^-1·yr^-1).However,in the seasonal permafrost zone(2.60-5.68t·ha^-1·yr^-1),two different response patterns were showed,i.e.,the annual carbon emissions from C,G and M showed an increasing trend along the permafrost degradation gradient,while the other four forested wetland first increased and then decreased.Permafrost degradation also changed the pattern of annual carbon emissions.The contribution rate of CO₂-C of most wetland types had no significant change（excluding C and LN）in the discontinuous permafrost zone compared with that in the permafrost zone,but seven wetland types significantly decreased in the seasonal permafrost zone.However,the contribution rate of CH₄-C along the permafrost degradation gradient is opposite to that of CO₂-C and is complementary.（3）The main controlling factors of carbon（CO₂ and CH₄）emissions from different wetland types had different responses to permafrost degradation.During the growing season,CO₂ fluxes from C and G were promoted by soil temperature in three zones,while those from five forested wetland types were mainly promoted by soil temperature in permafrost zone and discontinuous permafrost zone,but turned to be inhibited by water table in seasonal permafrost zone;the main controlling factor of CH₄ fluxes from seven wetland types was thaw depth（promote）in permafrost zone,turned to the combined control of water level and thaw depth（promote）in discontinuous permafrost zone,and turned to thaw depth（promote）in the seasonal permafrost zone.During the non-growing season,permafrost degradation had no significant effect on the main controlling factor of CO₂ fluxes from the seven wetland types（all promoted by soil temperature in the three zones）;CH₄ fluxes from the seven wetland types were promoted by both soil temperature and soil water content in the permafrost zone and the discontinuous permafrost zone,but only promoted by soil temperature in the seasonal permafrost zone.（4）The net primary productivity and annual net carbon sequestration of vegetation of seven wetland types ranged from 6.76 to 11.22,5.69 to 11.03 and 4.92 to 9.50 t·ha^-1·yr^-1 and from 2.97to 5.37,2.38 to 5.05 and 2.30 to 4.46 t·ha^-1·yr^-1in the permafrost zone,discontinuous permafrost zone and seasonal permafrost zone,respectively.There are six different response patterns along the permafrost degradation gradient,namely:（1）first stable and then rising type（LN）（different from the increasing type of NPP）;（2）decreasing type（M）;（3）invariant（C）;（4）first stable and then descending type（LT and LX）;（5）first descending and then stable（B）and（6）first descending and then recovery（G）.Among them,the net primary productivity and annual net carbon sequestration of LN were 18.9%and 22.5%significantly higher in the seasonal permafrost zone than in the permafrost zone,therefore,LN was the best wetalnd type to cope with climate warming and permafrost degradation.（5）The carbon source/sink of different wetland types had different responses to permafrost degradation.The carbon source strength of C(-0.86,-1.72 and-2.66 t·ha^-1·yr^-1)gradually increased along the permafrost degradation gradient.G shifted to carbon sources(-0.82 and-0.98 t·ha^-1·yr^-1)in discontinuous permafrost zone and seasonal permafrost zone.Five forested wetland types were carbon sinks(0.46-2.43 t·ha^-1·yr^-1)in permafrost zone;and maintained carbon sinks(0.30-1.88 t·ha^-1·yr^-1)in discontinuous permafrost zone,but the sink strength generally significantly decreased（22.6%-70.3%）;in the seasonal permafrost zone,M shifted to be a strong carbon source(-2.15 t·ha^-1·yr^-1),and the other four wetland types were carbon sinks(0.31-1.76 t·ha^-1·yr^-1)(among which only the carbon sink strength of LN(1.76 t·ha^-1·yr^-1)is 2.8times higher than that in permafrost zone,and the sinks of the other three wetland types(0.31-1.35 t·ha^-1·yr^-1)decreased（LT and LX）or remained unchanged（B）).（6）From the perspective of net radiative forcing,the global warming potential（GWP）of different wetland types had different responses to permafrost degradation.Positive GWP of C(3.94,6.33 and 13.70 t·CO₂-eq·ha^-1·yr^-1)（warming effect）stabilized and then increased along the permafrost degradation gradient.The G shifted to positive GWP(3.02 and 20.88 t·CO₂-eq·ha^-1·yr^-1)in the discontinuous zone and seasonal permafrost zone.The GWP of the five forested wetland types were negative(-8.91--1.62 t·CO₂-eq·ha^-1·yr^-1)in the permafrost zone,having cooling effects;remained negative GWP(-6.90--1.10 t·CO₂-eq·ha^-1·yr^-1)in the discontinuous permafrost zone,but the cooling effect generally significantly reduced by about 1/5～7/10;in the seasonal permafrost zone,only the negative GWP of LN was 288.3%significantly higher than that in the permafrost zone and played a strong climate cooling effect,the other four forested wetlands shifted to positive GWP(0.84-53.56 t·CO₂-eq·ha^-1·yr^-1)and had warming effects.This study used the scientific method of spatial variation instead of temporal variation to compare and analyze the difference of ecosystem net carbon budget balance in different zones for the same wetland type,and comprehensively evaluated the effects of permafrost degradation on carbon source/sink of wetlands in Northeast China.This study not only helps us to accurately predict the response pattern of carbon source/sink of wetlands in Northeastern to further permafrost degradation,but also provides data basis and theoretical support for the government to make accurate decisions on permafrost degradation in the future.