Using spatiotemporal substitution method

we constructed a sequence which was formed by natural grassland and Caragana microphylla stands of 7(young)

12(intermediately aged) and 27(old) years old. In addition

we measured and analyzed the variations of the soil nutrients including organic matter

total nitro-gen

alkali-hydro nitrogen

and available phosphorus. The results show that： (1) The soil organic matter contents within 0-200 cm displayed an ascending order of the cover as meadows〈 young stand〈intermedi-ately aged stand.old stand; the increasing trend of organic matter was more significant in the deeper layers. (2) The grassland soil had lower total nitrogen contents than the soil of young Caragana microphylla stand. The total nitrogen contents in the soils decreased with the increase of the forests age

as the biggest differ-ence was found between the young and intermediately aged stands. Alkali-hydro nitrogen showed the exactly same trend as total nitrogen. (3) The available phosphorus content in the whole soil profile is： aged forests ~young forests〈middle-aged forests〈meadows. (4) The spatial variability of all the variables in the three differently aged Caragana microphylla forests was not significant in the rows and between the rows. The degree of correlation was found high between organic matter and total nitrogen

and low between alkali-hydro nitrogen and available phosphorus.