Effects of grazing exclusion on soil net nitrogen mineralization and nitrogen availability in a temperate steppe in northern China

This study was conducted to examine the potential effects of ecological restoration by grazing exclusion on soil net nitrogen (N) mineralization over the growing season in the temperate steppe in this area. Both laboratory and in situ incubations with intact soil cores were used. Strong temporal variability in in situ net nitrification, net mineralization and available soil N were observed. Long-term (25 years) grazing exclusion stimulated in situ net nitrification whereas short-term (5 years) grazing exclusion had no effect. Neither long- nor short-term grazing exclusion affected net mineralization. Available soil N increased with time since grazing exclusion. Laboratory incubation showed that soil temperature affected net mineralization significantly and soil moisture effect depended on soil moisture range. The relationships of net mineralization with soil temperature and soil moisture observed in the laboratory incubation could not explain the temporal dynamics of in situ net mineralization. This study suggests that the grazing exclusion plays a positive role in regulating soil N availability, and thus stimulates the plant growth in the temperate steppe ecosystem in northern China.