Is the source-sink relationship in transplanted rice receiving deep-placed urea supergranules dependent upon the geometry of transplanting?

Deep placement of urea supergranules in wetland rice (Oryza sativa L.) or correct urea band application enables to protect nitrogen (N) from various loss mechanisms, but recovering of fertilizer N by plants depends upon geometric and agronomic factors. The objective of this study was to characterize the diffusion of ammoniacal N from the two urea sources, point or line application, in a typical paddy soil. A model of ammonia diffusion was developed for the two geometries. The relation between the N application rate and the transplanting geometry was studied in two fields using probes attached to urea supergranule of different mass (2 and 4 g). The transplanting pattern was adapted for obtaining 58 or 116 kg N ha(-1) in the 4 g application. The ammoniacal nitrogen concentration was compared to the diffusion model prediction. The values of the diffusion coefficient were found to be 1.160 and 1.107 cm(2) d(-1). Ammonia disappearance below 10 mmol L-1 did not follow the same kinetics in the two treatments corresponding to 4 g application. Relative to the 2 g treatment, root ammonia uptake in the 4 g treatment was delayed and slowed in the classical geometry of 20 cm x 20 cm (116 kg N ha(-1)) when it was mainly delayed in the geometry provided with 58 kg N ha(-1). This manipulation of the source-sink relationship enables to foresee possibilities for the development of new fertilizers adapted to wetland rice cultivation.