6Wang Xiaoying, Tong Yanan, Gao Yimin, et al. Spatial and temporal variations of crop fertilization and soil fertility in the Loess Plateau in China from the 1970s to the 2000s [J]. PLoSOne,2014,19(11):1-13.
7Tong Y A,Emteryd Ove,Lu D Q,etal. Effect of organic ma- nure and chemical fertilizer on nitrogen uptake and nitrate leaching in a Eumorthic anthrosols profile [J]. Nutrient Cyc- ling Agroecosystems, 1997,48 : 225-229.