Abstract: Taking one grassland and 15-, 24-, and 30 years old Mongolian pine (Pinus sylvestris var. mongolica) plantations in the southeast of Keerqin Sandy Land as test objects, this paper studied the effects of grassland afforestation on the soil chemical and biological properties, including pH value, C, N, and P concentration, inorganic N concentration, C and N mineralization rates, microbial biomass C, and enzyme activities. At the early stage of afforestation, the concentration of soil C, N, and P decreased; but with increasing stand age, they recovered gradually. Compared with those in grassland, the soil C, N, and P concentration in 24 years old plantation decreased by 29%, 34%, and 33%, respectively, but the soil C and N concentration in 30 years old plantation had less difference. Afforestation led to an increase of soil NH₄⁺-N concentration and a decrease of soil NO₃-N concentration, but had less effects on the soil N mineralization and nitrification rates. The soil C mineralization rate was decreased in the order of 24 years old plantation >30 years old plantation > grassland >15 years old plantation. Soil microbial biomass C and invertase activity decreased at the early stage of afforestation, but recovered with increasing stand age. After afforestation, soil urease activity had less change, but soil catalase activity increased gradually. Our results indicated that in Keerqin Sandy Land, grassland afforestation with Mongolian pine had significant effects on the soil chemical and biological properties, and the effects differed with stand age.

Key words: Keerqin Sandy Land, grassland afforestation, Mongolian pine plantation, soil C, N, and P, soil microbial biomass C, soil enzyme activity, forest canopy cover, ICESat-GLAS, Landsat-TM, multi-source remote sensing data, BPneural network model.