关键词: 京津冀区域, 城市化, 活性氮, 数据包络分析, 氮补偿

Abstract: A consecutive fiveyear experiment was conducted to study the responses of relative biomass of Stipa species in meadow steppe, typical steppe and desert steppe to different grazing intensities (no, light, moderate or heavy grazing) in Inner Mongolia in relation to precipitation and air temperature patterns over time. The relative biomass of Stipa baicalensis was inversely related to grazing intensity in meadow steppe, irresp ctive of precipitation. In contrast, the relative biomass of S. grandis in typical steppe increased with precipitation (r=0.326) in the control plots, yet it decreased (r=-0.319) with precipitation in the light grazing plots and was significantly greater than that of the control plots in dry years. S. breviflora’s relative biomass decreased with grazing intensity in desert steppe, but the differences among treatments were not statistically significant. S. breviflora’s relative biomass was also negatively correlated with precipitation in nongrazing (r=-0.459), light grazing (r=-0.583), and moderate grazing plots (r=-0.440), yet positively with air temperature in all treatments (r=0.282-0.299) except for heavy grazing. In conclusion, in meadow steppe where water was not a limiting factor, fluctuations in precipitation and air temperature had no great influence on the dominant species in the community, while gra-zing was the main factor of grassland degradation. Fluctuations in precipitation and air temperature became the key factors of grassland degradation in typical and desert steppe where the precipitation was less.

Key words: nitrogen offset, reactive nitrogen, data envelopment analysis, Beijing-Tianjin-Hebei agglomeration., urbanization