4Huang G B, Zhang R Z, Li G D, et al. Productivity and sustain- ability of a spring wheat - field pea rotation in a semi - arid environment under conventional and conservation tillage systems [ J ]. Field Crops Research, 2008, 107:43 -55.
5Wolfe S A, Nickling W G. The protective role of sparse vegetation in wind erosion [ J ]. Progress in Physical Geography, 1993, 17(1) : 50 -68.
