Response of urinary purine derivatives excretion to yeast RNA infusion levels in the yak abomasum Impacts of biological soil crusts on soil nitrogen in different rainfall regions in hilly areas of the Loess Plateau, China Effects of Astragalus sinicus manure and fertilizer combined application on biological properties of soil in Anhui double cropping rice areas along the Yangtze River Effect of incorporating corn residues into soil on nitrogen transformation in soil-plant ecosystem