Freezing-thawing Process Of Carbon And Nitrogen Mineralization In Forest Soil On The Daxing'anling Mountains

Freezing and thawing are ubiquitous phenomena in the soil surface at high altitudes and latitudes.Freezing and thawing affect the mechanism of soil nutrient cycling and is one of the important factors to change forest ecosystems.Freeze-thaw cycle is the key link of biochemical cycle on the earth.It not only changes the physical and chemical properties of soil,but also changes the microbial species and community structure in soil.Different types of forest vegetation grow in the permafrost region of Daxing'an Mountains.Under the background of global warming and degradation of permafrost,the changes of freeze-thaw cycle will inevitably affect the carbon and nitrogen content of forest soil and the process of plant nutrient uptake in the region.Soil carbon and nitrogen transformation is an important factor to maintain productivity and stability of forest ecosystem.Under the influence of climate warming,the frequency,temperature and duration of freeze-thaw of forest soil in permafrost region will change,which will inevitably affect the carbon and nitrogen cycle process of forest ecosystem.Therefore,the typical forest soils of Pinus sylvestris var.mongolica forest and Betula platyphylla forest in Daxing'an Mountains permafrost region were selected as the research objects.The effects of freeze-thaw process on active carbon components and nitrogen mineralization in forest soils and the effects of different vertical depths and freeze-thaw temperatures on carbon and nitrogen cycles in different vegetation types of soils were discussed through indoor freeze-thaw simulation experiments.Potential trends during the period provide basic data for the study of carbon sink function and nitrogen accumulation potential of forest soils.The main conclusions are as follows:(1)Freeze-thaw treatments significantly decreased the rates of soil organic carbon mineralization in the two forests.The rates of soil organic carbon mineralization in Betula platyphylla forest treated by constant temperature and freeze-thaw treatments were significantly lower than those in Pinus sylvestris forest(p<0.05).Freeze-thaw treatment had a significant effect on soil organic carbon mineralization accumulation.The carbon mineralization accumulation of Betula platyphylla forest under constant temperature treatment and freeze-thaw treatment was significantly lower than that ofPinus sylvestris forest(p<0.01).Freeze-thaw treatment had a significant effect on soil nitrogen mineralization.Freeze-thaw treatment was beneficial to increase soil inorganic nitrogen content.The inorganic nitrogen content of Betula platyphylla forest soil treated with constant temperature and freeze-thaw treatment was significantly lower than that of Pinus sylvestris forest soil(p<0.05).The relationship between carbon and nitrogen mineralization in freeze-thaw and constant temperature treatments was positive correlation.Compared with the same carbon emission,the inorganic nitrogen content in freeze-thaw treated soil increased.(2)The effects of freeze-thaw times and freeze-thaw temperature on SOC and LFOC contents were not significant(p>0.05).The SOC content in 0~10 cm soil of Pinus sylvestris var.mongolica forest was higher than that of Betula platyphylla forest during large freeze-thaw cycle and small freeze-thaw cycle,while that in 10~20cm and20~30 cm soil of Betula platyphylla forest was higher than that of Pinus sylvestris var.mongolica forest.LFOC in the two soils was stable during freezing and thawing,and the LFOC content in the soil of Betula platyphylla forest was always higher than that of Pinus sylvestris var.mongolica forest.Soil dissolved organic carbon(DOC)showed a trend of first increasing and then decreasing.Freezing-thawing significantly increased soil DOC content(p<0.05).During the freezing-thawing process,the DOC content in0~10 cm soil of Pinus sylvestris var.mongolica forest was higher than that of Betula platyphylla forest,and the DOC content in 10~20 cm and 20~30 cm soil was lower than that of Betula platyphylla forest.The contents of SOC,DOC and LFOC decreased with the increase of soil depth under the two forest types(p<0.05).(3)Soil microbial biomass carbon(MBC)showed a trend of first increasing and then decreasing,and MBC content in all layers of Pinus sylvestris var.mongolica forest was always higher than that of Betula platyphylla forest.With the increase of freeze-thaw times,soil microbial biomass nitrogen(MBN)decreased gradually.During the freeze-thaw cycle,MBN content in 0~10cm,10~20cm and 20~30cm soil of Pinus sylvestris var.mongolica was always higher than that of Betula platyphylla.Freeze-thaw significantly reduced the contents of MBC and MBN in soil(p<0.05),but the effect of freeze-thaw temperature on MBC and MBN was not significant(p>0.05).With the increase of freeze-thaw times,soil ammonium nitrogen decreased first and then increased,while nitrate nitrogen increased first and then decreased.Freeze-thaweffect significantly increased the content of soil ammonium nitrogen and nitrate nitrogen(p<0.05).Larger freeze-thaw temperature difference would promote nitrogen mineralization(p<0.05).During freeze-thaw process,the content of soil ammonium nitrogen and nitrate nitrogen in Betula platyphylla forest was always higher than that in Pinus sylvestris var.The contents of MBC,MBN,ammonium nitrogen and nitrate nitrogen decreased with the increase of soil depth under the two forest types(p<0.05).