Effect on the content and distribution of total N After 28 yr of continuous cultivation of rice, the total N content of topsoil in the four plot types were all increased compared to the initial soil (1.58 g kg-1; Fig. 1-B). Total N content of the plowed soil layer in the three green manure plots(i.e., R-R-RP, R-R-MV and R-R-RG) were 4.8, 8.9 and
4.1% higher than that of the R-R-WF plot, respectively,and the differences were significant. Thus winter green manure increased SOC, and stimulated soil biological
N fixation (Ladha et al. 2000), so soil N content was increased. In addition, rice biomass was higher in plots with winter green manure than other plots; the root
residue in soil was also conducive to improve SOC and N content. For the R-R-MV plot, the trend of total N content of the plowed soil layer in the different plots was
consistent with the content of organic C. The content of organic C was lower in the R-R-MV than in R-R-RP and R-R-RG plots; however, total N content was higher in R-R-MV than in R-R-RP and R-R-RG plots.