Available nitrogen in forest soil of Pinus sylvestris var． mongolica plantations in Zhanggutai sandy lands

Dynamics of soil NH⁺₄ -N, NO^-₃ -N, mineral N and microbial biomass N were measured in two management systems (fenced and unfenced) of Pinus sylvestris var. mongolica plantations in Zhanggutai sandy lands, north China. The results show that the contents of soil NH⁺₄ -N, NO^-₃ -N, mineral N and microbial biomass N are low. Soil NH⁺₄ -N and mineral N are higher in the fenced stands than those in the unfenced stands (P＜0.001), but soil NO^-₃ -N reverses (P＜0.001), and microbial biomass N is not different (P＜0.001) in both types of stands. Each soil NH⁺₄ -N, NO^-₃ -N, mineral N and microbial biomass N is distinctly different for months from April to October according to variance analysis(P＜0.001). Soil NH⁺₄ -N, NO^-₃ -N and mineral N show a normal seasonal change with highest concentrations in August and lowest in April and October. But soil microbial biomass N does not show a seasonal fluctuation, and is strongly correlated with the soil moisture (P＜0.001). The interactions of stands and months on soil NH⁺₄ -N and mineral N are significant (P＜0.001), these interactions on soil microbial biomass N are evident (P＜0.05), and not significant on soil NO^-₃ -N (P＜0.05). All of these mentioned above demonstrates that it is advantageous to improve soil N availability, stand production and protect environment, when the P. sylvestris var. mongolica stands on sandy soil are intensively managed (from unfenced to fenced). Then authors suggest that the prohibition of grazing and litter raking should be popularized in order to avoid stand decline and to maintain stand stability.