Characteristics Of Soil Available Nitrogen In Burned Area Of Cold Temperate Coniferous Forest

Fire disturbance is an important factor leading to the change of soil nitrogen（N）nutrition habitat in boreal forest.The objective of this study was to evaluate the effects of wildfires on soil N content and forms in a cold temperate coniferous forest in the Daxing’an Mountains（NE China）nine years after the fire.Soil samples were collected from 0-5cm layer under Ledum palustre-Larix gmelinii（LL）,Rhododendron dauricum-Larix gmelinii（RL）and Pinus sylvestris var.mongolica（PM）forests.Then,the contents of both mineral N and available N were analysed and used to calculate the different ratios as indicators of N forms.And analyze the relationship between habitat factors and N nutrition level.Aiming to explore the characteristics of soil available N in the mid-term restoration of coniferous forest burned areas in cold temperate zones.The specific research conclusions are as follows:1)The responses of the soil NH₄⁺-N、NO₃^--N and mineral N in the three forest types to wildfire was not significant（except for the early freezing period of PM）,while the early freezing period of PM in the burned plots was significantly lower than those of unburned plots.2)The content of soil soluble amino acid N in LL had no significant difference between burned plots and unburned plots.During the growing season,there was no significant difference in RL and PM.However,the two forests in the non-growing season was different,the former decreased significantly after freezing and thawing,and increased significantly before freezing,while the latter was the opposite.3)The total amount of free amino acid N in the burned plots of three forest types were mostly higher than that in the unburned plots,and its seasonal dynamics varied with forest types.The composition of free amino acid N in the soil of the three forest types is diverse,Asp,Glu,Gly,Ala,Tyr,Lys and His were the relative dominant free amino acids.4)The category dominance of free amino acid N in burned plots varied with forest types.The neutral amino acids of LL forest in June always had a clear advantage,and lost their advantages in August and October after fire,and the basic amino acids and acidic amino acids showed a relative advantage.Acidic amino acids predominate in unburned plots in RL forest,and neutral amino acids predominate after fire except in August.The contents of acidic and neutral amino acids in PM unburned plots and burned plots were higher than those of basic and sulfur-containing amino acids.5)The forms of soil mineral N in the three forest types always showed NH₄⁺-N as the main form.Wildfire did not significantly affect the dominance of mineral N form in LL and RL forest.The dominance of soil mineral N forms in PM significantly decreased in the early freezing period.6)The forms of soil available N varied with forest types,especially at some key time points,which were still significantly different from those of unburned plots.Among them,there was no significant difference in the dominance of soluble amino acids N between the burned and unburned plots of LL;The dominance of soil soluble amino acid N in the RL burned plot decreased after the snowmelt season and increased before freezing;in the PM burned plot,it increased to varying degrees after the snowmelt season and the growing season,but decreased significantly before freezing.7)Compared with unburned plots,habitat factors related to soil N nutrient levels in burned plots decreased.After 9 years of wildfire,the soil temperature in the LL forest was significantly positively correlated with the content of soluble amino acid N and free amino acid N,and the soil habitat factors in the RL forest had no significant correlation with the N level.In PM forest,soil temperature and reduction potential was significantly positively correlated with soil free amino acid N content,and soil moisture was significantly positively correlated with soil NO₃^--N content.