| In recent years,regional seasonal frozen ground and permafrost have changed significantly.The freeze-thaw cycle of seasonal ground soil and the distribution of permafrost have an important impact on the hydrological process,vegetation growth,soil microbial activity,agricultural and forestry production and the stability of urban infrastructure in cold regions.So far,there are few studies on the change of seasonal frozen soil melting period on a long-term scale,and there are few reports on the influence of non-climatic factors on the change of soil freezing and thawing state.Therefore,this paper takes Heilongjiang Province,an important grain production base in China,as the research area.Based on the measured data,remote sensing data and global climate models,the following parts are carried out by using mathematical statistics and numerical simulation methods:First,the variables of soil freezing and thawing process were extracted from the daily soil freezing and surface soil thawing depth of 31 stations in Heilongjiang Province.The temporal and spatial variation characteristics of soil freezing and thawing state in Heilongjiang Province in the past 60 years were analyzed in detail by using Mann-Kendall trend analysis,Sen’s slope and Hurst index.Second,based on Landsat 8 land use remote sensing data and land use transfer matrix,this paper creatively discusses the changes of soil freezing and thawing state under different land use types.In addition,based on the 11 global climate models(GCMs)of the sixth coupled model comparison program(CMIP6),combined with Stefan solution and A/Ta empirical formula,the change of frozen ground in Heilongjiang Province in the future(to 2100)is predicted.Last,the data of snow depth,soil temperature and moisture are collected to analyze the relationship between the change of soil freezing/thawing state and environmental factors,as well as the possible impact on agricultural production.The results are as follows:(1)From 1960 to 2015,the freezing and thawing state of soil in Heilongjiang province changed significantly.It is mainly reflected in: the regional average initial freezing date is delayed by about10 days,the initial thawing date and accommodation date are advanced by about 6 and 27 days respectively,the freezing period and thawing period are reduced by 17 and 21 days respectively,the duration of soil freezing and the actual number of freezing days are reduced by about 38 and 34 days respectively,and the maximum freezing depth of soil is reduced by about 48 cm.Among them,the change range of accommodation days is the largest,and the difference between the multi-year average continuous freezing period and the actual freezing days is about 9 days.Spatially,the maximum freezing depth of soil in the central and southern high-altitude areas of Heilongjiang Province decreases the fastest.(2)The change of soil freezing and thawing state is consistent with the change of temperature.Except for the first day of freezing and the first day of thawing,the maximum freezing depth,accommodation date,freezing period,thawing period,freezing duration and the actual number of freezing days of soil all mutated in 1988,and the trend was obvious after the 1990s.At the same time,under the background that the global climate will continue to warm,the Hurst index of frozen soil variables in about 2/3 of the stations shows strong sustainability,that is,the soil freezing and thawing state in Heilongjiang Province will continue to maintain the current trend in the future.(3)Among the four land use types of forest,grassland,cropland and construction land,the soil freezing and thawing cycle in forest and grassland areas of Heilongjiang Province is longer and the maximum freezing depth is deeper.Compared with cropland and construction land,due to the regulation of vegetation cover on temperature and soil surface water and heat,the soil freezing and thawing state is less affected by climate warming.The relatively bare surface in cultivated areas is more sensitive to climate change,and the soil freezing and thawing state changes greatly.Due to the existence of urban heat island effect,the change range of soil freezing and thawing state is the largest among the four land use types.(4)In the historical period,the permafrost regions with continuity between 70%-80% and 5%-30% in Heilongjiang Province have been reduced,and the permafrost regions with continuity between 30%-70% and seasonally frozen ground regions have been expanded.In the 1960 s,the areas of permafrost with continuity of 70%-80%,30%-70% and 5%-30% and seasonally frozen ground accounted for 8.43%,7.44%,21.89% and 62.24% of the total area of Heilongjiang Province respectively.With the passage of time,the permafrost area as a whole shrank to the north.After2000,it accounted for 3.08%,11.39%,17.50% and 68.04% of the total area respectively.(5)The simulation of the future frozen ground in Heilongjiang Province Based on the global climate models shows that the reduction of the maximum freezing depth of soil is the smallest in the SSP126 scenario,and the average maximum freezing depth of the whole province is basically maintained at about 158 cm from 2060 to 2100.Subject to the scenarios of SSP245,SSP370 and SSP585,the regional average maximum freezing depth in the 2090 s will be reduced by 18.33,31.25 and 43.20 cm respectively compared with the reference period(1995-2014).By 2100,the permafrost area in the study area will decrease by 47.36% and 61.34% respectively under SSP126 and SSP245 scenarios,and completely disappear under SSP370 and SSP585 scenarios.The change of permafrost region has the characteristics of "hysteresis" and "nonlinearity" relative to temperature change.(6)Temperature is the main factor leading the change of soil freezing and thawing state.During the study period,the freezing index of Heilongjiang Province decreased significantly and the thawing index increased significantly.There is a significant correlation between freezing/thawing index and soil freezing/thawing state variables.There is a significant negative and positive correlation between the maximum snow thickness and the maximum freezing depth and the first thawing date.The change of snow thickness will offset the influence of temperature rise on the maximum freezing/thawing depth to a certain extent.There is an inseparable correlation between the freezing and thawing state of seasonally frozen ground and the soil surface temperature during thawing period,and the correlation with the surface soil moisture is not significant.The change of soil freezing and thawing state has an indirect impact on agricultural production.It is necessary to strengthen agricultural monitoring,fully combine territorial knowledge and reasonably regulate planting measures,so as to make regional agricultural production adapt to climate change and realize sustainable development.In summary,in the past 60 years,the freezing and thawing state of soil in Heilongjiang Province has changed significantly,and the changes in cultivated land is more obvious.Under the background that the global climate will continue to warm,the characteristics of seasonal frozen soil and the distribution of permafrost in Heilongjiang Province will also change greatly.Under the scenario of high emission,the permafrost area in Heilongjiang Province maybe even disappear.Soil freezing and thawing state is closely related to snow cover and surface soil temperature.The impact of soil freezing and thawing state on agriculture may be both advantages and disadvantages.The research results of this paper will provide an important reference for ensuring regional food production security,ecological environment security and realizing sustainable development. |
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