Ammoniacal Nutrition of Transplanted Rice Fertilized with Large Urea Granules

With deep placement of large urea granules in wetland rice (Oryza sativa L.) cultivation, N is protected from various loss mechanisms and quite well recovered by plants. The objective of this study, which was based on soil solution measurements, was to examine the relation between the localized NH3−rich area deriving from one granule and the surrounding plants. The influence of rice on the ammoniacal N was examined in terms of a model of diffusion describing the time course of the ammoniacal N concentration. Treatments were granule mass (1, 2, 3, and 4 g). Since greater amounts of NH3 are expected for larger granules, we asked whether this would hinder N consumption by the plant, and thus delay disappearance of the ammoniacal N from the soil. The soil solution was sampled with probes buried close to the granules. Urea was quickly hydrolyzed in the soil: 78 to 91% of the reaction was performed within 4 d and 95 to 98% within 13 d. As early as 4 d, the ammoniacal N generated was 0.313 mol dm−3 for 2‐g granules, with a diffusion coefficient for NH3 of 0.998 cm2 d−1 (equivalent to an instantaneous point source of NH3). In the field, diffusion drove the time course of NH3 until 10 mmol dm−3, a level attained earlier in the lower‐mass treatments (1 and 2 g) than in the 3‐ and 4‐g treatments. Thereafter, the patterns of NH3 disappearance changed: the lower‐mass treatments displayed a faster evolution than the larger treatments. This change indicates a different speed in the consumption of ammoniacal N, possibly in relation with the vegetative or reproductive stage of the plant, throughout the carbon investment into NH3 assimilation. The granule‐rice system is proposed as a model for research on source‐sink relationship at the whole‐plant level.