Nitrogen dynamics in paddy soil applied with various 15N-labelled green manures

The effects of 15N-labelled green manure (GM) application on rice yield and N uptake were investigated and compared with those of inorganic fertilizer (IF) and no fertilizer (NF) application. Nine GMs (white clover, Chinese milk vetch, hairy vetch, crimson clover, oats, rye, Italian ryegrass, white mustard, lacy phacelia) were either incorporated into or surface-applied on paddy soil. Among the nine GM treatments, the rice yield and N uptake tended to be higher in the white clover and hairy vetch treatments. The rice N uptake efficiency from the applied N source was highest in the incorporated hairy vetch (56%), followed by IF (52%), and incorporated white clover (44%), whereas for the other treatments there were no significant differences. The mineral fertilizer equivalent values of hairy vetch and white clover in case of the incorporation were 108% and 84%, respectively. Therefore, hairy vetch and white clover seem to be the most efficient organic N source as alternatives to inorganic N fertilizers. The N uptake efficiency of rice and residual N in soil derived from GM were higher in the incorporation treatments than that in the surface-applied treatments, indicating that N losses from the soil-plant system were lower with the incorporation.