Response Of Inorganic Nitrogen To Water And Heat In Permafrost Peatlands Of The Daxing'an Mountains During The Autumn Freeze-thaw Period

High altitudes and latitudes are subject to temperature changes that cause permafrost soils to freeze and thaw.This process affects soil physicochemical properties and soil microbial activity,thus influencing soil nitrogen mineralisation processes and the accumulation of inorganic nitrogen and the emission of the greenhouse gas nitrous oxide(N₂O).The Daxing'an is the main perennial permafrost peatland distribution area in northern China and experiences frequent freezing and thawing in autumn due to the diurnal temperature difference,which plays an important role in the accumulation of inorganic nitrogen.Permafrost peatlands in the Daxing'an Mountains are one of the areas sensitive to the response to global warming.In order to investigate the spatial and temporal variation of inorganic nitrogen in permafrost peatlands under the background of global warming and permafrost degradation,this study was carried out from September to November in 2019 and 2020 to investigate the spatial and temporal variation of inorganic nitrogen in three types of permafrost peatlands in the Daxing'an Mountains,and to analyse the spatial and temporal variation of inorganic nitrogen in shallow and deep soils before,during and after the autumn freeze-thaw in 2019 A multivariate linear regression model was developed to compare the inorganic nitrogen content with soil temperature and water content in the shallow and deep layers of the peatlands during the autumn freeze-thaw period in 2019 and 2020,and to analyse the differences in inorganic nitrogen in the shallow and deep layers,as well as the differences in soil temperature and water content in the shallow and deep layers,The results of the study showed that:(1)During the autumn freeze-thaw of 2019,the shallow soil temperatures varied more than the deeper soils,with the shallow soil temperatures dropping significantly compared to the pre-fall freeze-thaw period,while the deeper soil temperatures showed fluctuating changes,with the highest surface temperatures in the Calamagrostis angustifolia peatland and the lowest in the Eriophorum vaginatum peatland.The highest average soil temperatures were also found in the Calamagrostis angustifolia peatland and the lowest in the Eriophorum vaginatum peatland.Only the shallow soil temperature in the Calamagrostis angustifolia peatland varied significantly between 2019 and 2020(p<0.05),while the changes in soil temperature in the Larix gmelina-sphagnum peatland and the Eriophorum vaginatum peatland were not significant(p>0.05).(2)Soil moisture content was significantly higher in the early autumn freeze-thaw period than in the late autumn freeze-thaw period in 2019,and the highest mean overall moisture content was found in the Calamagrostis angustifolia peatland.However,in 2020,the soil moisture content of the Eriophorum vaginatum peatland showed a fluctuating upward trend throughout the autumn freeze-thaw period,with the highest soil moisture content in the Larix gmelina-sphagnum peatland.The difference in soil moisture content between the Calamagrostis angustifolia peatland and the Larix gmelina-sphagnum peatland during the autumn freeze-thaw period between the two years was significant(p<0.05),but the difference in soil moisture content between the Eriophorum vaginatum peatland was not significant(p>0.05).(3)In 2019,the 0?50 cm soil ammonium nitrogen(NH₄⁺-N)content in the peatlands of in Calamagrostis angustifolia peatland,Larix gmelina-sphagnum peatland and Eriophorum vaginatum peatland ranged from 1.00±0.00 mg·kg-1 to20.60±0.20 mg·kg^-1;nitrate nitrogen(NO₃^--N)ranged from 0.02±0.01 mg·kg^-1 to14.64±1.11 mg·kg^-1.The range of variation in the content of ammonium nitrogen(NH₄⁺-N)in 0?30 cm soil in 2020 was 3.35±0.15 to 15.10±1.30 mg·kg^-1and nitrate nitrogen(NO₃^--N)content was 0.91±0.10 to 5.38±0.03 mg·kg^-1.During the autumn freeze-thaw period in both years All three perennial permafrost peatlands showed significant differences in inorganic nitrogen content(p<0.05).(4)Soil inorganic nitrogen was found to be dominated by NH₄⁺-N in three perennial permafrost peatlands,and inorganic nitrogen levels were significantly higher in the late autumn freeze-thaw period than in the early period.Although hydrothermal interactions did not significantly affect inorganic nitrogen during this period,the extent to which inorganic nitrogen responded to environmental factors varied between freeze-thaw stages:shallow inorganic nitrogen dynamics correlated with changes in shallow temperature and water content before,during and after autumn freeze-thaw in 2019,respectively,but shallow inorganic nitrogen content in Eriophorum vaginatum peatlands throughout autumn freeze-thaw in 2019 and 2020only responded to 10-20 cm water content(R~2=0.344,p<0.01;R~2=0.354,p<0.05).(5)The study shows that the initial accumulation of inorganic nitrogen occurs in perennial permafrost peatlands during the autumn freeze-thaw period,and that the shallow environmental factors respond to inorganic nitrogen to the greatest extent.This study can supplement the data on the effects of autumn freezing and thawing on soil inorganic nitrogen in permafrost peatlands in the Daxing'an Mountains and provide basic data to support the study of greenhouse gas release from permafrost peatlands in response to global warming.