Impacts of Irrigation Water Quality On Water Extractable Dissolved Organic Carbon, Nitrogen, and Phosphorus In Urban Soils of Texas.

Urban soils are exposed to increased concentrations of sodium in urban ecosystems through road salt run off, irrigation with wastewater treatment effluent, and municipal water with high sodium concentrations.  Dissolved organic carbon (DOC) concentrations in urban streams in south-central Texas have been found to be correlated with the stream sodium adsorption ratio (SAR) (Aitkenhead-Peterson et al. 2009).   The objective of this study was to determine the relationship between irrigation water quality and the solubility of soil carbon and nutrients across cities with a wide range of irrigation water SAR (SAR_iw) values.  Thirty five composite soil and an irrigation water samples were collected in late summer toward the end of the irrigation season from irrigated soils under turf in 26 cities across Texas.    The pH, electrical conductivity (EC), DOC, phosphate, nitrate, ammonium, alkalinity, sodium, calcium, and magnesium were all measured in a 10:1 soil to water extract and in irrigation water.  Saturated paste extract EC, pH, and SAR, the total carbon and nitrogen and percent sand were measured in the soil.  Results of multiple regression analysis indicated the SAR and alkalinity of irrigation water and total soil nitrogen explained 82% of the variability in water extractable DOC and 77% of dissolved organic nitrogen.  SAR_iw was also a significant factor in solubility of phosphate (R² = 0.54) and ammonium-N (R²=0.59).  Analysis of samples based on SAR indicated that solubility of carbon and nutrients is controlled by SAR starting at SAR= 5.  Based on these result we recommend reducing SAR_iw to less than five to minimize the risks of large losses of carbon and nutrients to urban surface waters.

Aitkenhead-Peterson JA, Steele MK, Nahar N, Santhy K (2009) Dissolved organic carbon and nitrogen in urban and rural watersheds of south-central Texas: land use and land management influences. Biogeochemistry DOI: 10.1007/s10533-009-9348-2, 96: 119-129.