Effects Of Freezing-thawing Cycles On Soil Organic Carbon Dynamics In The Peatland Ecosystems From Continuous Permafrost Zone, Great Hinggan Mountains

As an abiological effect on soil environments, the freezing-thawing process directlyor indirectly influences the physical, chemical and biological properties of soils,which is an important factor controlling carbon and nitrogen dynamics in mid-highlatitude regions. Recently, there has been a growing interest in the changes thatfreezing-thawing cycle (FTC) may alter soil carbon stability, since this associatedbio-geochemical process could be very sensitive to global warming in mid-highlatitude regions. In this study, we collected the soil samples from active layer of anundisturbed permafrost peatland in the Great Hinggan Mountains, Northeast China,and then subjected them to freezing-thawing simulation experiments. We analyzed thesoil organic carbon (SOC) content and labile fractions in different frozen groundregions, discussed the characteristics of SOC dynamics under FTC and thecorresponding influencing factors controlling carbon emissions in the active soil layer,and determined the contribution of labile fractions to CO2emissions and thetemperature sensitivity of SOC decomposition under FTC. The objective of this studyis to probe into the impacts of FTC on soil organic carbon dynamics in the continuouspermafrost zone, Great Hinggan Mountains in the context of global warming. Themain conclusions are:(a) Wetland SOC and its labile fractions contents in topsoil (020cm) of frozenground regions declined with the decreasing latitude in Northeast China. The highestSOC stock was in the continuous permafrost region (mean26.02kg C m2), followedby the discontinuous permafrost region (mean22.98kg C m2), and the lowest value in the seasonally frozen ground region (mean10.08kg C m2). The labile fractions oflight fraction organic carbon (LFOC) and particle organic carbon (POC) wereconcentrated in continuous permafrost region, accounting for83%and32%,respectively, of the SOC in the020cm soil layer. However, the percentages ofdissolved organic carbon (DOC) and microbial biomass carbon (MBC) to SOC in theseasonally frozen ground region were respectively4~6times and2~3times higherthan those in continuous permafrost region. Temperature gradient was the importantfactor that affected the different contents of SOC in wetlands along a latitudinalgradient in Northeast China. However, within a given latitudes, vegetationsignificantly affected SOC and its fractions in wetlands. Soil organic carbon was53.71~245.33g kg-2in the peatland, which was higher than the values of35.84~203.65g kg-2in the marsh.(b) Freezing-thawing cycle (FTC) significantly affected SOC dynamics in theactive layer of permafrost peatland, Great Hinggan Mountains. The resultsdemonstrated that FTC significantly increased the release of soil DOC, and thehighest value was932.97mg kg-1, which was1.8times higher than that of CK. Theincrease of DOC content impacted by FTC were higher at80%maximumwater-holding capacity (MWHC), surface layer of015cm and small amplitude ofFTC (-5°C to+5°C), being1.3times,1.8times and2.3times, respectively, higherthan those at60%MWHC, deep layer of3045cm and large amplitude of FTC(-10°C to+10°C). However, FTC significantly decreased MBC content and soilenzymes activities of cellulase, amylase and invertase. The lowest values in the015cm were2868.09mg kg-1for MBC,8.01mg (g72h)-1for cellulase,37.48mg(g24h)-1for invertase and8.64mg (g24h)-1for amylase, respectively decreased by36%,19%,31%and18%compared with CK. Although the activities of microbes andenzymes gradually increased when the numbers of FTCs increased, their values werestill lower than the CK. Meanwhile, FTC significantly affected the carbon emissions.During the freezing periods, carbon emission rates gradually decreased with theextension of cultivation time, but there were still carbon emissions, being about0.07~0.70mg kg-1h-1for CO2and0.06~0.39ug kg-1h-1for CH4. Following the start of the thawing, carbon emissions rebounded quickly and reached their peaks(1.08~3.73mg kg-1h-1for CO2and4.59~12.75mg kg-1h-1for CH4), which indicatedthat the freezing process has promoted carbon emissions at the thawing stage.Additionally, the significant correlations among active organic carbon fractions,enzymes activities and CO2emissions under FTC conditions suggested that theliberated nutrients impacted by FTC could be more easily utilized by survivingmicrobes, and thus lead to the carbon flush induced by freezing-thawing.(c) The bursts of CH4emissions under laboratory-simulated FTCs clearly appearedin the peat soil of seasonally frozen ground region. The high peaks of CH4flux were13.40~55.02mg C m-2h-1for020cm soil layer, being about4~19times higher thanthe CK. However, the CH4peaks lasted for a very short time, only appearing on thefirst day of thawing, and the emission peak intensity during thawing stage decreasedwith the increasing FTC numbers. During the anaerobic incubation periods, thecumulative CH4emissions in the020cm soil layer was6127.42mg m-2, about twicehigher than CK, which was contrary to the trend of that in2040cm soil layer.Clearly, the responses of CH4emissions from different soil layers to FTC weredifferent. Meanwhile, FTCs significantly increased the DOC and DIC releases fromsoil and water, and enhanced its instability, being about1.2~1.5times higher than theCK. The released labile substrates under FTC play an important role in the microbialproduction of CH4and the resultant high emission rates upon thawing.(d) During the short mineralization incubation periods, the SOC mineralizationunder FTC were2048.37mg C kg-1for015cm and483.99mg C kg-1for1530cm,decreased by28%and76%, respectively, compared with CK. Noticeably, FTCsignificantly restrained the SOC mineralization, especially for the deep layer of1530cm. Although the FTC significantly increased the DOC releases from soils, its DOCuse ratio was only15%during the mineralization periods, slight lower than the CK(21%), which was similar to the trends of the MBC, soil enzymes activities ofcellulase, amylase and invertase. Meanwhile, we also found that the inhibition effectof FTC on SOC mineralization were consistent with the carbon emissions duringfreezing-thawing process, indicating that the carbon emissions from active layer of permafrost peatland impacted by FTCs may be lower than the carbon emissions underglobal warming-induced higher temperatures above0°C.(e) Based on the carbon mineralization incubation experiments, the POCmineralization rate from015cm soil layer was2~64times higher than that of LFOC,but their gap closed under freezing (only2~26times). Compared with the CK,freezing increased the carbon mineralization rates at the early stage (about twice)from different labile carbon fractions and particle soil sizes. However, theircumulative CO2emissions were not significantly different between freezing and CK,except for LFOC (60.00mg kg-1under freezing,44.92mg kg-1under CK) and DOC(51888.45umol L-1under freezing,30089.80umol L-1under CK). For the differentlabile carbon fractions, the contribution of POC to the total CO2emission was27~36%, being1.5~2.7times and2.9~7.9times higher than the fractions of LFOCand DOC; For the different particle soil sizes, the contribution of the part of1mm250um to the total CO2emissions was69~71%, being27~28%and32~37%higher than the parts of>1mm and250um53um. Meanwhile, freezing increasedthe contents of DOC, NH4+and NO3release from different labile fractions and soilsizes, about1.2times,1.4times and1.3times as high as CK. However, the existedforms of inorganic nitrogen were different. Overall, NH4+was the major form at thepart of POC and different sizes of soils, but NO3was the major form at the LFOC,indicating that the metabolic pathway of nitrogen in the light fraction may be differentfrom other components.(f) In the southward transplanted FTC experiment, the greenhouse gas emissionrates of CO2, CH4and N2O from continuous permafrost peatland (CP) and seasonallyfrozen ground marsh (SF) exhibited obvious emission peaks at thawing stage. Theemission flush reached to159.83mg m-2h-1(CP) and86.83mg m-2h-1(SF) for CO2,0.26mg m-2h-1(CP) and4.07mg m-2h-1(SF) for CH4, and72.14ug m-2h-1(CP) and22.15ug m-2h-1(SF) for N2O that from sink at freezing stage changed to source at thethawing stage. During the field freezing-thawing season, the cumulative greenhousegas emissions were598.35g m-2(CP) and255.35g m-2(SF) for CO2,1273.24mg m-2 for N2O. Meanwhile, there were significant correlations between CO2, CH4and10cmsoil temperature. Soil labile fractions of DOC, MBC, LFOC and POC also played animportant role in greenhouse gas emissions during freezing-thawing periods. At thesame FTC conditions, the potential releases of CO2and N2O during freezing-thawingperiods were much higher in the peatland of the permafrost zone than that in themarsh of seasonally frozen regions, indicating that permafrost degradation may resultin greater amounts of CO2, CH4and N2O releases from wetlands in cold region intoatmosphere, and thus intensify the greenhouse effect.(g) During temperature gradient incubation experiment, the gas emission rates ofCO2, CH4and N2O increased with temperature increase. Below zero temperature(-10°C to-0.5°C), the temperature sensitivity of gases (expressed as Q10) were5.99~23.34for CO2,10.70~103.54for CH4,7.17~432.68for N2O, respectively, about2~10times (CO2),3~16times (CH4) and2~53times (N2O) higher than those of theabove zero temperature (+0.5°C to+25°C). Meanwhile, the Q10values of CH4andN2O below zero temperature were about1.8~4.4times and1.2~18.2times higher thanthe Q10values of CO2, and the Q10values of015cm soil layer were1.7~2.7times(CO2),3.4~4.1times (CH4) and2.3~9.7times (N2O) higher than the values of3045cm soil layer. Although FTC decreased the greenhouse gas emission rates withtemperature change, their Q10values under FTC were higher than the CK, especiallyfor the below zero temperature, about1.6~2.3times (CO2),1.3~2.8times (CH4) and6.1~26.0times (N2O) as high as CK. The high Q10values at below zero temperaturesuggested that after spring freezing-thawing periods the soils in the high latitude ofpermafrost peatland of Great Hinggan Mountain would be more sensitive to the globalwarming.