Recovery Of Soluble Organic Nitrogen And Its Adsorption Characteristics Of Seabuckthorn In The Loess Hilly Area

Soil soluble organic nitrogen?SON?can be directly absorbed and utilized by plants,which is of great significance to forest soil development and biological productivity.Because SON has strong fluidity,it is prone to occur the leaching loss and runoff migration,resulting in the loss of soil available nitrogen.Thus,soil SON has become a hot issue in the research focused on the forest soil nitrogen cycle.Three vegetation restoration plots in the loess hilly area of northern Shaanxi,including 5 year?a?seabuckthorn forest,15 a seabuckthorn forest,and 20 a seabuckthorn forest,were selected to determine the soil SON in the seabuckthorn forest,the grassland was used as controls.In the present study,the correlation between soil SON and vegetation types,restoration years,seasonal changes,slope position,soil depth,and other physical and chemical properties.Those findings will predict the evolution of soluble organic nitrogen in planted forests with vegetation restoration,and provide the theorical basis for understanding the correlation between SON and soil physical and chemical properties and enzyme activity.Based on that,it will reveal the change principle and driving forces of the artificial forest SON during the vegetation restoration.Meanwhile,the adsorption characteristics of soil SON were studied through indoor adsorption experiments to explore the migration and adsorption of soil SON in different plantations during the vegetation restoration.The findings will provide a basis for evaluating the environmental effects of SON.The main conclusions of the current study were as follows:?1?The SON content in the surface layer?0-30 cm?of seabuckthorn forest in the loess hilly region was significantly decreased and then increased with the increase of vegetation restoration years?p<0.05?.The average content of SON in each plot were followed the order:20 a seabuckthorn forest(88.48 mg kg^-1)>15 a seabuckthorn forest(56.06 mg kg^-1)>5 a seabuckthorn forest(43.55 mg kg^-1)>grassland(48.31 mg kg^-1).Compared to grassland,the content of SON in 20 a seabuckthorn forest increased by93.94%,and increased by 14.92%in the 15 a seabuckthorn forest,while decreased by9.85%in the 5 a seabuckthorn forest.In addition,the slope positions significantly affected the content of the soil SON,and the relationship of magnitude were followed the order:middle slope>downhill>uphill.The soil SON content under various vegetation restoration types accounted for an average of 77.91%of Total organic nitrogen?TSN?content,which is the main component of TSN.Moreover,the content of SON in the same soil layer in each plot varied significantly with the seasons,exhibited a trend of decreasing first and then increasing.The highest content was found in spring and the lowest content was found in summer.The content of soil SON began to rise at the autumn and winter,and the content in winter was close to spring.In the 0-30 cm soil layer,the soil SON content decreases significantly with the increase of the soil layer depth.?2?The soil fertility of the surface layer?0-30 cm?of seabuckthorn forest in the loess hilly region significantly improved with the restoration of vegetation.Compared with the grassland,the ratios of soil nutrients in each plot were:nitrate nitrogen?43.15%-193.13%?,ammonium nitrogen?24.35%-127.51%?,organic matter?29.35%-103.7%?,alkali-decomposed nitrogen?36.17%-301.98%?,available phosphorus?41.65%-99%?,conductivity?38.1%-8.83%?,water content?6.8%-18.15%?urease activity?60.37%-185.9%?,sucrase activity?38.16%-103.55%?;while soil available potassium,pH,catalase activity decreased by 5.18-23.6%,2.3%-4.2%,28.57%-33.3%,respectively.In addition,under different vegetation restoration types,various physical and chemical properties and enzyme activities varied significantly with the seasons.Soil organic matter exhibited a trend of rises firstly and then declines and rises again with seasonal changes;while soil water content and urease exhibited a trend of decreases firstly and then rises and declines again with seasonal changes.The parameters of alkaline nitrogen,available potassium,available phosphorus content,pH,conductivity and invertase activity following the seasonal changes exhibited a"V"-shaped change trend;while catalase exhibited an inverted"V"change trend with the seasonal changes.Meanwhile,the slope positions significantly affected the above parameters.The correlation between slope positions and moisture content,electrical conductivity,organic matter,alkali nitrogen,available phosphorus,urease,and sucrase were followed the order:down slope>up slope>in slope;while the correlation with pH value were followed the order:Middle slope>uphill>downhill;the correlation with available potassium were followed the order:uphill>downhill>in slope.Moreover,the soil nutrients of each vegetation type varied significantly with the increase of soil depth.The content?activity?of SON,nitrate nitrogen,organic matter,alkali-decomposed nitrogen,available potassium,conductivity,urease,and sucrase in 0-10 cm soil layer were significantly greater than that in 10-30 cm soil layer;while the content?activity?of available phosphorus,pH,water content,and catalase in 0-10 cm soil layer were significantly lower than that in the 10-30 cm soil layer.?3?The correlation analysis revealed that the content of soil SON in seabuckthorn forest in the loess hilly region was affected by a multitude of factors.The content of soil SON was positively correlated with the parameters of TSN,water content,organic matter,alkali-decomposed nitrogen,available potassium,and urease;while it was adversely correlated with pH,available phosphorus,and sucrase.?4?The adsorption capacity of soil SON in the layer?10-20 cm?of seabuckthorn forest in the loess hilly region was significantly increased with the increase of vegetation restoration years.In 20 a,the adsorption capacity of seabuckthorn forest is the strongest,while the grassland exhibited the weakest.Additionally,the adsorption amount of SON increased with the increase of the amount of exogenous SON added,while its adsorption rate exhibited a trend of increasing firstly and then decreasing.Moreover,vegetation restoration could increase the adsorption capacity of soil to SON,and effectively prevent the loss of soil nitrogen.However,the adsorption of soil to SON in different vegetation types is still at a low level.Thus,more work is required to unravel the adsorption law and influencing factors of soil SON.