COMPARISON OF THREE DIFFERENT WOODY PLANT SPECIES FOR THEIR ABILITY TO UTILIZE NUTRITIONAL ELEMENTS SUPPLIED IN THE FORM OF BIOSOLIDS AND MINERAL FERTILIZERS

Crop production systems of the future will need to focus on recycling rather than flow- through of mineral nutrients. Much of the nutrients that leave agricultural systems with crop products eventually end up in household waste and sewage. Returning nutrientrich products from wastewater treatment to agricultural soils must be done in an environmentally sound and culturally acceptable manner. More than 300,000 hectares of forest have been planted in the United Arab Emirates (UAE), consisting mainly of native species, such as Prosopis cineraria (Ghāf) and Vachellia tortilis (Samr). It has been proposed that sewage sludge, the dry residue from wastewater treatment, is returned to crop production via land application to these forests, as they are not directly used in food production for human consumption nor are they in close proximity to human settlements. However, little is known about how native desert trees would respond to such an additional fertilizer supply. In the present study, the ability of Ghāf and Samr trees to utilize nutrients provided either in the form of sewage sludge as biofertilizers or mineral fertilizer salts was compared. Wild jasmine shrubs (Clerodendrum inerme) were included as a third species in this experiment because they are an exotic and faster growing woody plant. The young jasmine shrubs were grown in the greenhouse in pots filled with sandy dune soil to which nutrients were added at three different levels. The nutrients were supplied either in the form of sewage sludge or mineral fertilizers. The soil prepared for fertilization with sewage sludge was mixed with 3.2, 6.4 and 12.8 g of dry sewage sludge per kg of dry soil, respectively. This was equivalent to 60, 120 and 240 mg N per kg dry soil. Plants were harvested nine months after the start of the experiment, and their dry weight and shoot nutrient uptake were evaluated. Compared to the desert tree species, wild jasmine shrubs had significantly higher dry weights at the end of the experiment. Nevertheless, none of the species showed a positive growth response to the increase in the supply of nutritional elements. Wild jasmine shrubs showed increased uptake of macronutrients with increasing fertilizer supply and were equally capable of utilizing nutrients from sewage sludge and mineral fertilizers. No increase in elemental uptake in response to increasing fertilizer supply was observed in the indigenous trees. The growth of Ghāf trees responded negatively to a high supply of sewage sludge but not to a high supply of mineral fertilizer. The results of our study suggest that the ability to absorb and utilize nutrients supplied in the form of sewage sludge may be limited in desert trees such as Ghāf and Samr. The potential to utilize sewage sludge could be increased by planting exotic species which have higher growth and element uptake potential compared to indigenous trees.