Abstract: A full factorial experiment was conducted to study the effects of understory removal and nitrogen addition (8 g·m^-2) on the soil NO₃^--N and NH₄⁺-N concentrations, potential net nitrogen mineralization rate (PNM) and nitrification rate (PNN), microbial biomass C (MBC) and N (MBN), MBC/MBN, urease and acid phosphomonoesterase activities, and Olsen-P concentration in a Pinus sylvestris var. mongolica plantation in Keerqin Sandy Land during a growth season. Understory removal decreased the soil NH₄⁺-N concentration, PNM, MBC, and MBN/MBN significantly, increased the soil Olsen-P concentration, but had little effects on the soil NO₃^--N concentration, PNN, and urease and acid phosphomonoesterase activities. Nitrogen addition increased the soil NO₃^--N concentration, PNM and PNN significantly, but had little effects on the other test properties. The interaction between understory removal and nitrogen addition had significant effects on the soil NH₄⁺-N concentration, but little effects on the soil NO₃^--N concentration. However, the soil NO₃^--N concentration in the plots of understory removal with nitrogen addition was increased by 27%,  compared with the plots of nitrogen addition alone, which might lead to the leaching  of NO₃^-  It was suggested that understory vegetation could play an important role in affecting the soil chemical and biological properties in Mongolian pine plantations, and hence, the importance of understory vegetation should not be neglected when the forest management and restoration were implemented.

Key words: Keerqin Sandy Land, soil microbial biomass C and N, potential net N mineralization rate, potential net nitrification rate, soil enzyme activity, Olsen-P.