Comparison of Soil Nitrate and Phosphorus Concentrations Prior to and Five to Eleven Years after Recycled Water Irrigation

Abstract

Increasing demand on water supplies in western US and more stringent wastewater discharge standards have made recycled water a common water source for irrigating urban green spaces. Studies are needed to evaluate nitrate-N leaching potential and phosphorus (P) movement along soil profile when recycled wastewater is used for irrigation. We collected and analyzed soil samples at the commencement and 5 and 11 years after recycled water irrigation on 2 golf courses, 5 metropolitan parks, and 1 school ground. Samples were taken at 0-20, 20-40, 40-60, 60-80, and 80-100 cm depths on golf courses and at 0-20 and 20-40 cm depths at other locations. Soil samples were tested for soil pH, soil nitrate, and AB-DTPA extracted P level. No increase in soil nitrate-N was observed over 5 and 11 years with recycled water irrigation, suggesting leaching of nitrogen to the groundwater was not a great concern. Soil P concentration at the surface soil depth was the highest 11 years after recycled water irrigation. Moreover, soil P increasing below the surface layer was observed on sites with sandy soil, suggesting long-term recycled water irrigation could impose some risk for P leaching on sandy soil.