Soil Nitrogen Pools And Their Seasonal Dynamics Characteristics Of Different Types Grasslands In Three River Source Area

Grassland degradation is widespread and severe in Three River Source Area of Qinghai-Tibet Plateau.In order to explore management approach for sustainable development of vegetation and soil quality,we study the effect of land degradation on soil nitrogen content(total nitrogen,amino acid-nitrogen,NH₄⁺-N,NO₃^－-N),examine the relative influence of man-made rehabilitation grassland on soil nitrogen nutrition The research was conducted on alpine meadows in Dari county of Qinghai Province and The results show:1.The content of total N in the 0-10cm native alpine Kobresia meadow soil is 3807.93 mg/kg(the average_of five months).The grassland degradation led to a swift decline of soil total N,it only accounts for 44.35%of that in native meadow soil. After by man-power rehabilitation,the content of soil total N bit higher than severely degraded grassland accounts for 44.8%of that in native meadow.There is no significant difference for soil total N between severely degraded land and man-made rehabilitation grassland.2.The average content of NH₄⁺-N in the 0-10cm native alpine Kobresia meadow soil is 31.95 mg/kg.But the content of NH₄⁺-N in severely degraded grassland reachs up to 59.51 mg/kg,it is 1.86 times higher than that in native meadow. After by man-power rehabilitation,the content of NH₄⁺-N is 43.92 mg/kg,it is 1.38 times higher than that in native meadow.With the growth of plants,the content of NH₄⁺-N in three types of grassland soils all represented inverted "V" form,the content was highest in June and then decreased with time.3.The content of NO₃-N in the 0-10cm native alpine Kobresia meadow soil is 4.29 mg/kg,the content of soil NO₃-N in severely degraded grassland and the man-power rehabilitation grassland is 4.41 g/kg and 4.55 g/kg.There is no significant difference for soil NO₃-N content among three types of grasslands.Seen from the seasonal dynamics,the content of soil NO₃-N in three types of grassland all represented inverted "V" form.The results show that the effect of grassland degradation and man-power rehabilitation on soil NO₃-N content is not obvious.4.This study analyses the species and the content of soil amino acids in threee types of grassland used RP-HPLC technique.All three types of meadow contain all kinds of amino acids except tryptophan.The order of content of soil amino acids in different types of grassland is that native meadow(6308.78 mg/kg)＞man-power rehabilitation grassland(2977.08 mg/kg)＞severely degraded grassland(2911.26 mg/kg).The effect of grassland degradation on the soil amino acids content is great. The content of soil amino acids in severely degraded grassland accounts for 47.19% of that in native meadow.What more,the content of soil amino acids through nine years rehabilitation has not obvious increase.5.Native meadow soil contain five kinds of predominant amino acids in whole growing season.The content of glycine(1096.41μg/g) is the highest,it about accounts for 17.36%of all kinds of amino acids,then alanine,praline,serine,leucine. These five kinds of amino acids accounts for 53.52%of total content of amino acids. The soil predominant amino acids in severely degraded grassland and man-power rehabilitation grassland are glycine,alanine glutamic acid and serine.