Abstract: Studies of nutrient cycling in farmland ecosystems with multiple cropping systems show that in fields Ⅰ(double cropping rice after wheat), Ⅱ(rice-soybean after barley) and Ⅲ(double cropping rice after rape), there are surplus in Ca and Mg, deficient in P, K, Fe and Mn, and greater extravagant in N. N, Pand Kin farmland ecosystems are imported by fertilizers and exported by agricultural products. Ca and Mg are controlled by water cycle. The remnant amount of Nis maximum (23—71kg·ha^-1), and the residual rate of Fe is as high as 64—87%.

Key words: Nutrient balance, Nutrient cycling, Residual rate, Farmland ecosystem, gravity model, least cost model, ecological network, biodiversity.