ALBERT

Description: Background and aims Gaseous losses of ammonia (NH 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 3 might be absorbed by the plants through the leaves. To quantify the contribution of the leaf absorption of 15 NH 3 , a study with sweet oranges was conducted in two field areas where trees were grown at standard (480 trees ha −1 ) and high densities (617 trees ha −1 ). Methods Plastic trays were filled with soil, covered with mown grass to simulate field management conditions, fertilized with 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 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 15 N/ 14 N ratios were determined. Results After urea application, the NH 3 losses peaked within three days and subsequently decreased to negligible amounts after 10 days. The total NH 3 losses accounted for 55–82 % of the applied N. Although the NH 3 absorption by the citrus leaves was proportional to the tree density in the field, only 3–7 % of the 15 NH 3 volatilized from the soil was recovered by the citrus trees, and the NH 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 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 3 when urea is applied to soil surfaces.