Characteristics of nutrient balance of different cropping systems in paddy field of red earth.

The characteristics of nutrient balance of 4 kinds of cropping systems were studied in paddy field sys-tems of red earth in northern Hunan. The results show that the sequence of nutrient output of N, P and K is the cropping system of rice-rice-rapethe cropping system of rice-rice-rape (green manure)the cropping system of rice-rice-green manurethe cropping system of rice-rice. The N surplus of the cropping system of rice-rice-green manure is 18.4kg/hm2·a,which is the biggest among 4 kinds of cropping systems. N nutrient of the cropping sys-tem of rice-rice-rape(green manure) is basically balanced. The cropping system of rice-rice shows the N deficit. The N deficit in the cropping system of rice-rice-rape is up to 38. 9kg/hm2·a,which is the biggest. P and K of 4 kinds of cropping systems all show deficient, but the cropping system of rice-rice-rape is the biggest, being 11.2 kg/hm2·a and 118.7kg/hm2·a, respectively, and the cropping system of rice-rice-green manure is the smallest. Returning of organic matter of the cropping system of rice-rice-green manure is the biggest and favourable to accu-mulation of organic matter, and return of organic matter of the cropping system rice-rice is the smallest and shows negative balance in organic matter.