Irrigation water amount and salinity dictate nitrogen requirement

Abstract

Crop relative yield (RY) may be reduced by matric water potential or salinity stresses but also by nitrogen (N) deficiency. Elucidation of interactions among these factors through modeling using the ENVIRO-GRO program to compute RY and N leaching from the root zone for corn (Zea mays) was our objective. Ten-year simulations included growing seasons and fallow periods. Simulation variables were applied water divided by potential evapotranspiration (AW/PET) equal to 0.9, 1.1, 1.3, and 1.42; irrigation water salinity (ECiw, dS/m) values of 0.5, 1, 2, 3, and 4; and inorganic N applications ranging from 270 to 350kg/ha. RY for simulations that included winter precipitation were approximately 5% greater than identical simulations without precipitation for 2≤ECiw≤4. Except for deficit irrigation, all AW/PET values resulted in RY equal to 100% for ECiw≤2 when N was not limiting. For ECiw>2 progressively greater AW/PET was required for full yield. For less saline waters, water stress was limiting for the AW/PET=0.9 treatment and RY was unaffected by the N application rate. For a given N application rate, increasing water application caused decreasing RY and increasing leached N. When N applications were greater than required for maximum RY, a decrease in N application resulted in an equal decrease in leached N. When decreasing N applications caused decreasing RY, the reduction in leached N was less than the reduction in N application. For the very saline waters (ECiw=4), AW/PET=1.1 was inadequate to leach the salts and water stress was the limiting factor rather than N application. Water flow below the root zone is controlled by both water application and N deficiency which reduces plant water uptake. Regulatory attempts to restrict N leaching to groundwater that prescribe management of N applications exclusively are likely to fail because N leaching depends on water flow as well as N application.