Abstract: Leguminous green manure crops such as sweet clover and Astragalus huangheensis,can provide definiteamount of biologically fixed-Nto make N-budget balance, promote soil physical and chemical proper-ties,reduce soil and water erosion,and thus,increase per unit area yield and total output when grown inrotation or intercropping with cereal crops and fruit trees,can also promote the development of animalhusbandry,push forward the cycling of the whole agricultural ecosystem and enhance its overall function.Azolla (Azolla filiculoides Lamk),a species of aquatic fern possessing a strong ability of biological N-fix-ation,is a good green manure crop in paddy fields and good green forage for animals.

Key words: maize, air temperature, sowing date, Northeast China.