Absorption of super(15)NH sub(3) volatilized from urea by Citrus trees

Background and aims: Gaseous losses of ammonia (NH sub(3)) have been observed in citrus orchards when urea is surface-applied to the soils, and this loss might significantly limit the effectiveness of the nitrogen (N) fertilizer. However, a portion of the volatilized NH sub(3) might be absorbed by the plants through the leaves. To quantify the contribution of the leaf absorption of super(15)NH sub(3), a study with sweet oranges was conducted in two field areas where trees were grown at standard (480 trees ha super(-1)) and high densities (617 trees ha super(-1)). Methods: Plastic trays were filled with soil, covered with mown grass to simulate field management conditions, fertilized with super(15)N labeled urea (12 atom % excess) and placed under each of three trees in the orchards. This experimental procedure prevented the uptake of N from the labeled urea by the roots. Two weeks after super(15)N fertilization, the trays were removed from the field, and the soil was homogenized and sampled for chemical analyses. The citrus trees under which the trays were placed were destructively harvested, and the total N concentrations and super(15)N/ super(14)N ratios were determined. Results: After urea application, the NH sub(3) losses peaked within three days and subsequently decreased to negligible amounts after 10 days. The total NH sub(3) losses accounted for 55-82 % of the applied N. Although the NH sub(3) absorption by the citrus leaves was proportional to the tree density in the field, only 3-7 % of the super(15)NH sub(3) volatilized from the soil was recovered by the citrus trees, and the NH sub(3) absorption was also influenced by the proximity of citrus trees to the site of urea application and the leaf areas of the trees. Conclusions: The citrus trees can absorb the NH sub(3) volatilized from urea, even though, the amount recovered by the trees is small and does not represent a significant proportion of total gaseous N losses, what demonstrates the importance of enhanced N use efficiency practices in field to reduce losses of NH sub(3) when urea is applied to soil surfaces.