Various nitrogen sources were tested in this study in order to fertilize pine leaves and oak leaves instead of using the traditional fertilization which is based on the nitrogen absorption through the root. After assessing the efficiency of leaf nitrogen uptake of the two studied species, the researcher team from Universidad de Alcalá and Politécnica de Madrid concluded that this fertilization system can be an efficient tool to complement radical fertilization regimes in order to improve plant nurseries and to plantations in nutrient-poor soils or arid conditions.
Foliar feeding is used in agriculture to rapidly and precisely control the nutrition of plants. This technique has not been tested in the forestry area, but its application for nursery production can provide solutions to improve plant quality produced for afforestation.
Researchers have used four types of nitrogen fertilizers in this study (urea, nitrate, ammonium and glycine) in two Mediterranean species typically used in restoration: the holm oak (Quercus ilex L.) and the Aleppo pine (Pinus halepensis Mill.). By using a stable isotope of nitrogen (N), researchers assessed the absorption efficiency of various nitrogen sources for both species. They observed that the best absorbed source was urea, followed by ammonia, glycine, and finally, the nitrate. These differences among the four sources can be explained due to their physicochemical properties as polarity, hygroscopicity, and solubility of compounds.
Likewise, the holm oak absorbs via foliar better than the pine, and this is associated to different anatomical properties at leaves scale, such as stomatal density. Researchers have found within every species a close relationship between cuticle permeability and foliar absorption although this relationship can vary depending on the fertilizer used. Besides, the usage of organic sources such as glycine is something new in these types of research.
Throughout this study, researchers confirmed that the two species studied can absorb intact glycine via foliar. The foliar feeding caused an increase of the plant nitrogen content in the oak and the pine studied and all the products used.
The results confirm that foliar feeding is a suitable tool to complement the traditional methods of nursery fertilization based on the radical absorption (pointing out the urea as the most efficient fertilizer). Besides, foliar feeding could be a tool potentially useful in forest plantations and in situations in which absorption via radical is not possible. Likewise, the differences observed at the absorption rates between both species will allow researchers to predict the effects of atmospheric deposition on forest systems depending on the dominant species.