Annual Costs of Runoff Nutrient Losses From A Fertilized Furrow-Irrigated FIELD.

Fertilizer’s rising costs and dwindling reserves are driving the need to utilize this resource more effectively.  Knowledge gaps concerning furrow-irrigated fields persist; e.g. the extent of nutrient losses incurred in runoff from such fields is unknown.  In 2003 and 2004, we measured sediment, NO₃-N, NH₄-N, K, dissolved reactive P (DRP), and total P (TP) concentrations in runoff from furrow irrigated field plots (6-7 irrigations y^-1) cropped to corn (Zea mays L.) in the semi-arid climate of southern Idaho.  Annual treatments included:  13 (yr 1) and 34 Mg ha^-1 (yr 2) stockpiled dairy manure (M); 78 (yr 1) and 195 kg N ha^-1 (yr 2) inorganic N fertilizer (F); or  control--no amendment (NA).  Available N in manure applied each year was similar to amounts applied in fertilizer.  Constituent concentrations (mg L^-1) in runoff ranged widely among all treatments:  sediment, 10 to 50,000; NO₃-N, 0 to 4.07; NH₄-N, 0 to 2.28; K, 3.6 to 46.4; DRP, 0.02 to 14.3; and TP, 0.03 to 41.5.  Averaged across years and irrigations, fertilizer and manure treatments increased NO₃-N runoff concentrations (M=0.30, F=0.26, NA=0.21 mg L^-1) and mass losses (M=0.50, F=0.42, NA=0.33 kg ha^-1) relative to the control.  In addition, the manure treatment increased irrigation runoff concentrations of DRP (M=0.19, F=0.09, NA=0.08 mg L^-1) and K (M=1.13, F=0.79, C=0.62 mg L^-1) compared to fertilizer and control plots.  Average K and DRP runoff mass losses were 2.0x and 2.4x greater in manure-treated than control plots.  Neither F nor M affected season-long cumulative infiltration.  Runoff DRP and inorganic-N losses appeared to be influenced more by the timing of the amendment application and environmental conditions, than by the quantity of nutrients applied.  The replacement cost of nutrients lost in runoff from fertilized or manured furrow-irrigated soils was substantial, averaging $88 ha^-1 y^-1, (2011) and should be considered in benefit-cost analyses for furrow-to-sprinkler irrigation conversions.