Nutrients and Associated Ion Concentrations in Irrigation Return Flow from Flooded Rice Fields

Abstract

AbstractA 3‐year field study of nutrient and common ion concentrations in irrigation return flow (IRF) from flooded rice fields utilized replicated plots which received either recommended or excessive fertilizer rates with continuous flow or intermittent flood irrigation. All P and K and 40% of the (NH4)2SO4 nitrogen (N) were applied preplant and incorporated. The remaining N was applied just prior to permanently flooding (40%) and at 2‐mm panicle stage (20%). The irrigation and plot water were analyzed for NH4+‐N, NO3−‐N, NO2−‐N, PO43−‐P, K+, Ca2+, Mg2+, Na+, Cl−, and SO42−. Highest nitrate‐N concentrations occurred early in the season before the permanent flood period, but did not exceed drinking water standards. Nitrite‐N concentrations in the IRF were low at all times. Maximum NH4+‐N concentrations occurred following N fertilizer applications which were not incorporated into the soil and persisted in the floodwater for 5 to 7 days. These peak NH4+‐N concentrations exceeded acceptable drinking water standards concentration by a factor of 10 or greater. Concentrations of PO43−‐P and K+ in the floodwater were similar to those in the surface water used for irrigation. The concentration of the other common ions in the floodwater did not greatly exceed those in the irrigation water and all were within concentrations acceptable for drinking water. Method of fertilizer application had more influence on IRF nutrient concentrations than did fertilizer rates or irrigation management practices. Under the conditions of this study, it appears that only the ammonium concentrations may have a detrimental impact on the quality of IRF from flooded rice fields. This potential problem could be minimized by preventing IRF for a period of 1 week following surface applications of (NH4)2SO4 fertilizer.