Effects of snow cover and soil moisture on CO2 and N2O fluxes from typical semi-arid grassland soil subjected to freeze-thaw cycles Photosynthetic Characteristics of a Semi-arid Sandy Grassland Community in Inner Mongolia Study on Plant Niche under Fencing Measures in the Semi-arid Grassland of North China Shrub-mediated “fertile island” effects in arid and semi-arid grassland HJ NDVI cross-calibration for grassland in Northern China