Greenhouse Gas Emissions From Soil Of Seasonally Frozen Region Of Ecosystems

Seasonal freeze-thaw events are the widespread natural phenomenon in the northern hemisphere,and which exhibit strong effects on soil carbon and/or nitrogen biochemical processes in cold regions.Under the background of climate change,the effects and the feedback of freeze-thaw regime change on soil greenhouse gas emissions during the frozen areas have become a hotspot in ecological research.However,the data of soil greenhouse gas emissions during non-growing season are rarely reported and the related mechanisms remain unclear.Verifing the patterns and the mechanisms of the greenhouse gas emissions during the non-growing season,especially during the soil freezing-thawing processes,are profound for assessing the greenhouse effect of greenhouse gas emissions and the carbon and nitrogen budgets in the seasonal frozen regions.It also can help us to better understand the effects of freeze-thaw events on soil substances cycling and predict the possible ecological consequences of the global climate change.Focusing on the greenhouse gas emissions of the four typical seasonally frozen ecosystems?paddy field,maize field,natural forest and artificial forest?in Songnen Plain in northeast China,we conducted a 20-months field experiments,and measured the dynamic fluxes of N₂O,CO₂ and CH₄ during a full seasonal freeze-thaw cycle by using the static chamber-gas chromatograph technique.The characteristics of greenhouse gas emissions during the critical periods?early winter and early spring?were monitored intensivly via increasing the sampling frequency.A set of soil environmental factors,available nutrients and soil microbes during soil freezing and thawing,snow covered and melting processes were also measured.In addition,the effects of biotic and abiotic factors on greenhouse gas emissions during non-growing season were analyzed by using the structural equation models.The results indicated that:?1?Seasonal freeze-thaw cycle was an important driving force of soil nutrient cycling in cold areas,and there was obvious nutrient"flow"in spring thawing period.Soil nutrient transformation was still continuing after the soil was frozen and covered with snow during the winter.For instance,soil NH₄⁺and DON showed a net consumption but NO₃^-and DOC were accumulated during the winer.What's more,the soil MBC and MBN had increased rather than decreased,and the growth rate of MBC was higher than that of MBN.Which manifested as microbial biomass C:N increase with the continuation of snow cover.The contents of NH₄⁺,NO₃^-,DON and DOC increased 5-11,1-6,1-4 and 1-3 fold,respectively,compared with that before thawing.In addition,the soil microbial communities recovered rapidly during early spring,which tended to increase nitrogen fixation.However,the stimulating effect of seasonal freeze-thaw on soil nutrient release was obvious only during the early stage of spring thawing.?2?Soil greenhouse gas emissions during non-growing season had an important contribution to the annual,which accounted for 15%to 67%of the annual total emissions.The accumulative N₂O emissions during the non-growing season amounted to 0.73,0.66,0.14 and 0.17 kg·hm^-2 for the paddy field,maize field,natural forest and artificial forest,respectively,and contributed to 67%,22%,42%and 17%of the annual total.The accumulative CO₂ emissions during the non-growing season amounted to 1.28,1.57,3.36 and 3.62 t·hm^-2 for the four types of sample plots,respectively,and contributed to28%,15%,21%and 22%of the annual total.Except for the paddy field,the uptake of CH₄ in maize field,natural forest and artificial forest were 0.63,2.31 and 2.33 kg·hm^-2,respectively,accounting for48%,35%and 37%of the annual total sink.For the net greenhouse effect,the annual GWP of the paddy field,maize field,natural forest and artificial forest were 13744.23,11868.32,16026.26 and 16753.88 kg CO₂·hm^-2,respectively,in which 11%,15%,21%and 22%were from the non-growing season.Meanwhile,the contributions of the GWP of CO₂,CH₄ and N₂O to the annual GWP were differences.For instance,the greenhouse effect of the forest soil emission were primarily from CO₂ emissions?>99%?,while 65%of the greenhouse effect of the paddy field was due to CH₄.And it was 7%for the N₂O in the maize field.The soil net greenhouse effect contributed 7%.In addition,the N₂O,CO₂ and CH₄ fluxes of the farmland fields were affected by diurnal freeze-thaw cycles during spring,and the daytime average fluxes were 4 to 5 times,3 to 4 times and 6 times than night,respectively.This means that 75%to 86%of the greenhouse gas emissions during the spring thawing were focused on the daytime when the topsoil was thawing.?3?The highest N₂O emission rates(67.56²57.43?g·m^-2·h^-1)were observed during spring soil thawing period for all the fields.The N₂O pulsed emission of the four ecosystems during thawing period occupied approximately 80%of the non-growing season total.During 2015 and 2016,the spring peaks of N₂O flux of paddy field were as high as 257.43 and 210.48?g·m^-2·h^-1,respectively,and maize field were 207.44?199.15?g·m^-2·h^-1,natural forest were 71.86,110.89?g·m^-2·h^-1 and artificial forest were87.04?67.56?g·m^-2·h^-1.The peak value were 7 to 15-fold higher than annual mean fluxes.However,the spring emission pulse only continued 10 to 18 days.Therefore,spring thawing was critical period to monitor N₂O emissions for seasonally frozen ecosystems.Our results confirmed that the combined impact of high soil moisture,flush available nitrogen and rapid recovery of microbial biomass triggered the vigorous N₂O emissions during the spring thawing.In addition,the soil moisture?but not the soil temperature?was the key abiotic factor controlling the entire non-growing season N₂O emissions,it can explained 34%to 63%of the farmlands and woodlands N₂O fluxes variation.?4?Although the major CO₂ emissions of soil of the seasonally frozen ecosystems happened during the growing season,the cumulative CO₂ quantity during the long-term non-growing season were still accounted for 15%to 30%of the annual total.Hence,winter CO₂ efflux from soils played an unneglectable role in the C cycling during the cold regions.The soil CO₂ emissions in the freezing and thawing periods accounted for more than 80%of the non-growing season total amount.Therefore,taking more efforts to monitor soil CO₂ fluxes during these two periods are helpful to accurately estimate the annual C budget of the seasonal frozen ecosystems.Soil temperature was the main environmental factor limiting winter soil respiration,it could explain 66-72%of the fluxes variation.Furthermore,the effect of DOC on soil CO₂ fluxes during non-growing season was not significant,but the positive effect of total available nitrogen was 0.24-0.27,this indicated that soil respiration was restricted by N source availability rather than C.In addition,the temperature sensitivity of soil respiration during non-growing season were significantly higher than that in growing season.The Q₁₀value of snow covered and thawing periods ranged from 2.6 to 20.3,and the Q₁₀ of the maize field during the thawing period was even as high as 40.Therefore,winter climate warming are likely to accelerate soil respiration multiply,and consequently input more soil carbon into atmosphere.?5?On the one hand,the soil CH₄ fluxes were different among the four land use types.The paddy field under irrigation was an important source of atmospheric CH₄,but the low temperature reduced CH₄ emission in northeast China(only 305 kg·hm^-2 during growing season).However,the role of maize field,natural forest and artificial forest were acting as atmospheric CH₄ sink for most of the year.The annual net uptake of natural forest and artificial forest were 6.7 and 6.3 kg·hm^-2,respectively,which were higher than that of maize field(1.3 kg·hm^-2).On the other hand,the seasonally freeze-thaw cycle could changed the function of soil‘source-sink'.The trend of CH₄ fluxes of the maize field,natural forest and artificial forest were show as‘sine function'throughout the year,and the uptake rates of growing season were higher than that of the non-growing season.But the amount of absorption during non-growing season still contributed to as high as 35%to 48%of the annual total.The paddy soils were also change to be the sink of atmospheric CH₄ during the non-growing season.In particular,the two kinds of farmlands shifted from the sink of CH₄ to the source during the spring thawing period.Soil moisture was the only factor that had a significant effect on CH₄ flux in farmland soil??=0.82?,however,soil moisture,MBN and MBC showed positive effects??=0.59,0.79,0.43?while the soil temperature and TDN had negative effects??=-0.44,-0.23?.Due to the uptake of CH₄ during non-growing season,the increase of soil moisture and microbial biomass would reduce the uptake of CH₄,and the increase of soil temperature and available nitrogen would promote the uptake of CH₄..This indicated that the CH₄ uptake was favored under high temperature and dry conditions in winter.In a word,the greenhouse gas emissions from soil during non-growing season can not be ignored.The change of winter temperature,snow cover and soil freeze-thaw pattern will influence soil nutrient transformation,microbial activities and greenhouse gas emissions indirectly by affecting soil temperature and moisture.The innovation points of this study were as follows:1)We compared the four freeze-thaw periods of greenhouse gas fluxes,and determined that the spring thawing period was the key point to monitor the greenhouse gas emission during the non-growing season in seasonal frozen ecosystems.2)In-situ sampling is more practical than laboratory simulation test.We synthetically analyzed the effects of temperature,moisture,snow cover and soil nutrient on greenhouse gas fluxes during the non-growing season.3)The differences of greenhouse gas emissions among different cropping systems?dryland and paddy field?,vegetation types?natural and artificial forests?,and fertilization systems?farmlands?and natural systems?woodlands?were compared in the seasonally frozen region.