Soil Inorganic Nitrogen and Microbial Biomass Carbon and Nitrogen Under Pine Plantations in Zhanggutai Sandy Soil*1

The dynamics of soil inorganic nitrogen (NH+4 -N and NO−3 -N) and microbial biomass carbon (Cmic) and nitrogen (Nmic) under 30-year-old fenced Pinus sylvestris L. var. mongolica Litvin (SF), unfenced P. sylvestris L. var. mongolica Litvin (SUF), and unfenced Pinus densiflora Siebold et Zucc. (DUF) plantations in the Zhanggutai sandy soil of China were studied during Apr. to Oct. 2004 by the in situ closed-top core incubation method. All mentioned C and N indices in each stand type fluctuated over time. The ranges of inorganic N, Cmic, and Nmic contents in the three stand types were 0.7–2.6, 40.0–128.9, and 5.4–15.2 μg g−1, respectively. The average contents of soil NH+4 -N and Cmic under the three 30-year-old pine plantations were not different. However, soil NO−33 -N and total inorganic N contents decreased in the order of SUF ≥ SF ≥ DUF, the Nmic content was in the order of SF = SUF > DUF, and the Cmic: Nmic ratio was in the order of SUF = DUF > SF. Seasonal variations were observed in soil inorganic N, microbial biomass, and plant growth. These seasonal variations had certain correlations with microbe and plant N use in the soil, and their competition for NH+4 -N was mostly regulated by soil N availability. The influence of tree species on inorganic N and Nmic were mainly because of differences in litter quality. Lack of grazing decreased the Cmic:Nmic ratio owing to decreased carbon output and increased the ability of soil to supply N. The soil N supply under the P. sylvestris var. mongolica plantation was lower than under the P. densiflora plantation.