ABSORPTION OF TOPDRESSED NITROGEN BY RICE THROUGH MASS-FLOW UNDER DIFFERENT RELATIVE HUMIDITIES

To evaluate the contribution of mass flow to the absorption of topdres sed N by paddy rice, a pot experiment was conducted under different ra tes of water uptake at the panicle formation stage. The rate of water uptake was modified by using two different values of relative humidity (RH, 75% and 95%). Labelled N was used to determine the amount of abs orbed fertilizer-derived N in the rice plants. The amount of ammonium N (NH4+-N) in the floodwater was depleted within 48 h after topdressin g (HAT) in all the treatments. The concentration of NH4+-N in the soil solution decreased with depth in the top 0.03 m of soil. N in the soi l solution from 0 to 0.03 m soil depth was almost depleted by 48 HAT. The percentage of fertilizer-derived N recovered by plants and the pat tern of N accumulation were the same in all the RH treatments. However , in the course of the entire experiment, the amount of N absorbed thr ough mass flow was larger in the 75% RH treatment during the first 48 h. Moreover, under 75%, RH conditions, the percentage of topdressed N in the plants that was associated with N absorbed through mass flow wa s 78% at 48 HAT and 29% at 168 HAT. These values mere significantly hi gher than the previously reported values for basal N (2.5-6.3%). There fore, it is concluded that the role of mass flow in N absorption is mo re significant for topdressed N than for basal N.