Nutrient resorption efficiency and proficiency in economic wood trees irrigated by treated wastewater in desert planted forests

Little is known about the impacts of recycled wastewater irrigation on adult tree species, especially the nutrient resorption efficiency (RE) and proficiency (RP) during leaf senescence and whether trace metals are resorbed or not during this process. Here, we present a study on the concentrations of nutrients and trace metals in green and senesced leaves, collected from Eucalyptus camaldulensis and Casuarina spp., in four planted forests irrigated by wastewater (or groundwater as control). Stoichiometric ratios (C:N, C:P, and N:P), RE and RP from senesced leaves of the trees were investigated. The concentrations of trace metals showed normal levels and were below phytotoxic concentrations in green and senesced leaves except for Mn. The intra-specific variations in nutrient concentrations in senesced leaves showed larger differences among the sites than those found in green leaves. The studied tree species excluded excess elements by decreasing its proficiency. The RE for N and P was high under poor soil nutrient conditions, but very low under high availability of nutrients. Resorption of trace metals from senesced leaves was low and presented negative mean values for most metals at all sites. RP values were low or equal to zero for the estimated elements. C:N and C:P ratios were high at low nutrient availability i.e. control site, while N:P ratios were non-significant among sites in green or senesced leaves. Our results illustrate that forest trees have highly variable nutrient resorption capacities that vary under high nutrients availability and indicate that under continuous wastewater irrigation, low or negative resorption efficiency for trace metals and other nutrients is an efficient protective strategy.