Abstract:

Nitrogen content of soil particulate fractions and its relationship to the vegetation and the elevation in Qilian Mountains were investigated. The results showed that the ratio of soil particulate fractions, soil particulate organic matter\|nitrogen(PON) ratio, nitrogen content of soil particulate fractions, soil PON content did not significantly change with the altitude. The ratio of soil particulate fraction in 0～15 cm soil depth at 3 000 m,3 200 m and 3 500 m of shady slope, or 3 800 m of sunny slope, or that in 15～35 cm soil depth at 3 200 m,3 500 m of shady slope,or 2 800 m of semishady slope was the highest among the all elevations slope aspects and soil depths. PON ratio in 0～15 cm soil depth at 3 000 m and 3 200 m of shady slope,or 3 300 m and 3 800 m of sunny slope, or that in 15～35 cm soil depth at 3 300 m of sunny slope, or 3 000 m of shady slope was also the highest. The nitrogen content of PON in 0～15 cm at 3 600 m of shady slope was the lowest, while the nitrogen content of PON in 0～15 cm at 3 800 m of sunny slope,or 2 800 m of half shady slope, or that in 15～35 cm at 3 400 m of shady slope, or 3 300 m of sunny slope was the highest. The nitrogen content of soil particulate fraction in 0～15 cm soil depth at 3 400 m of shady slope, or 3 300 m and 3 800 m of sunny slope,or that in 15～35 cm under 3 400 m of shady slope and 3 300 m of sunny slope was the highest. There were also significant effects of vegetation on the ratio of soil particulate fraction, the ratio of soil PON, soil PON and nitrogen content of soil particulate fraction. The ratio of soil particulate fraction in 0～15 cm soil depth forest or in shrub meadows was the highest, and the difference for this ratio between in forest and shrub meadows or between in desert steppe and arid steppe were not significant, and that in 15～35 cm soil depth in forest, shrub meadows or arid steppe was the highest. While there were no significant difference for the ratio of soil particulate fraction among in forest, shrub meadows and steppe. The ratio of PON in different soil depth in forest and alpine meadows were the highest, and there were no significant difference for the ratio among under shrub meadows, steppe and arid steppe in 0～15 cm soil depth, and the ratio in steppe or arid steppe in 15～35 cm soil depth were the lowest (p<0.05). The nitrogen content of soil particulate fraction in different soil depth in shrub meadows was the highest, and that in desert steppe or arid steppe was the lowest. The nitrogen content of soil PON in different soil depth under forest or shrub meadows was the highest, while that in desert steppe or arid steppe was the lowest, and there were no significant difference for the nitrogen content among forest, alpine meadows and shrub meadows. The ratio of soil particulate fraction, nitrogen content of soil particulate fraction, soil PON content and soil PON ratio all increased with increasing total soil nitrogen content (p<0.02), and soil PON content increased with increasing nitrogen content of soil particulate fraction (p<0.001), and ratio of soil PON increased with increasing soil PON content (p<0.05).The results showed that the soil unprotected soil nitrogen content increased with increasing elevation, generally, and unprotected soil nitrogen ratio in forest and meadows was the highest among the all different types of vegetations. 

Key words: altitude, vegetation, total soil nitrogen, soil particulate fraction nitrogen, soil particulate organic matternitrogen, Qilian Mountains