Changes In Nitrogen Mineralization Of Soils At Different Altitudes On The Western Slope Of The Helan Mountain Range, Northwest China

The study was conducted in major types of grasslands along the middle Halan mountain range in Alex Desert, Inner Mongolia during the period from July to Augest in 2004. Basing on theories of plant nutrition, plant ecology, restoration ecology, experimental ecology and soil science, we studied spatial variation in nitrogen mineralization with the increase in altitude in relation to the variation in vegetation and soil environment. We collected data by taking soil samples in experimental plots that were established in different types of grassland along a gradient of altitudes between 1370 m and 2940 m and analyzing the contents of soil NH₄⁺-N and NO₃^--N in the collected samples.The results of this study showed the following important points:1. The mean values of soil NH₄⁺-N and NO₃^--N tended to decrease with decreasing elevation from 1370 m to 2940 m.2. Net nitrogen mineralization rates decreased rapidly firstly, and then increased with decreased elevation, showing a "V" shape from 1370 to 2940 m. Net nitrogen mineralization rates were found to be the highest (0.272mg·kg^-1·d^-1) at the 2940 m site and the lowest (0.001 mg·kg^-1·d^-1) at the 2100m site.3. Regression analyses indicated that soil mineral nitrogen content had a strong positive relationship with soil water content, soil organic matter, total nitrogen, soil bulk density, plant density, aboveground biomass, vegetation cover, N acculation of the aerial parts of grass. At the same time, soil mineral nitrogen content was negatively correlated with soil bulk density and soil PH but no significant relationships were foind between the net nitrogen mineralization rates and the factors including soil water content, soil organic matter, soil total nitrogen, soil bulk density and soil PH.4. The vertical zonation of vegetation was strongly developed along the middle Helan mountain range in Alex, Inner Mongolia.With decreasing elevation, the vegetation composition and dominant species were found to change considerably in the study region. Kobresia humilis and Polygonum viviparum were the dominant species in plot I at a 2940 m height. Stipa glareosa and Artemisia frigida were the dominant species in plot II at a 2100m height. Stipa glareosa, Cleistogenes squarrosa and Salsola passerine were the dominant species in plot IIIat a 1700m height. Eragrostis poaeoides, Setaria viridis and Zygophyllum xanthoxylum were the dominant species in plots IV(exclution) at a 1370m height. Artemisia scoparia and Caragana microphyllawere the dominant species in plot V (grazing) at a 1370m height.5. For grasses, the sequence of the N acculation of the aerial parts is I >IV> II >III> V, while for shrubs, this sequence is III>IV> V.