Recycled wastewater (RWW) has become a common water source for irrigating golf courses and urban landscapes. Ponderosa pine (Pinus ponderosa) is commonly used in urban landscape settings in the Rocky Mountain West. To evaluate the effects of RWW irrigation on quality and needle ion accumulations of ponderosa pine, eight landscape facilities near metropolitan Denver, CO, were selected for the experiment. Among these sites, four had been irrigated exclusively with domestic RWW [electrical conductivity (EC) = 0.84 dS/m] for 5, 6, 15, and 20 years, respectively. The other four with similar turf species, age ranges, and soil textures had been irrigated with surface water (EC = 0.23 dS/m). Ponderosa pines grown on sites irrigated with RWW exhibited 10 times higher needle burn symptoms than those grown on sites irrigated with surface water (33% vs. 3%). Tissue analysis indicated that ponderosa pine needles collected from sites receiving RWW exhibited 11 times greater Na+ concentration, 2 times greater Cl, and 50% greater B concentrations than samples collected from the control sites. Stepwise regression analysis revealed that the level of needle burn was largely influenced by leaf tissue Na+ concentration. Tissue Ca level and K/Na ratio were negatively associated with needle burn symptoms, suggesting that calcium amendment and K addition may help mitigate the needle burn syndrome in ponderosa pine caused by high Na+ in the tissue.

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Author notes

This report was financed in part by a grant from The Horticultural Research Institute, 1000 Vermont Ave., NW, Suite 300, Washington, DC 20005; the Colorado Agricultural Experimental Station (Project 658); and the U.S. Department of the Interior, Geological Survey, through the Colorado Water Resources Research Institute and Grant No. 01HQGR0077.

2Associate Professor. Corresponding author. <Yaling.Qian@colostate.edu>.

3Former Post-Doctoral Research Associate.

4Professor.

5Associate Professor.