Plastic film mulching affects the critical nitrogen dilution curve of drip-irrigated maize

Critical nitrogen dilution curves (CNDC) can efficiently diagnose nitrogen (N) condition of crops. Unfortunately, no CNDC exits for mulched drip-irrigated crops. Here we determine the CNDCs of drip-irrigated maize (Zea mays L.) with and without plastic mulch and investigate effects of plastic mulch on soil NO3- -N concentration in root zone (Nsoil), maize morphology, and CNDC changes. Aboveground dry biomass (DM), crop N concentration, soil N concentration, and related morphological characteristics of maize were measured in Northwest China under five N application rates with and without plastic mulch to calculate critical N concentration (Nc) and develop CNDCs. Plastic film mulching decreased Nsoil by 16.5 % and increased DM by 25.9 %. Mulching increased DM more than root growth and resulted in a lower root to shoot ratio compared with that of no mulching. Plastic film mulching affected CNDC and grain yields by increasing DM, thus lowering root to shoot ratio. CNDCs were Nc = 3.58DM- 0.36 [R2 = 0.98] for drip-irrigated maize and Nc = 4.17DM-0.35 [R2 = 0.94] for mulched drip-irrigated maize. Mulching lowered nitrogen nutrition index at silking stage and increased it at physiological maturity stage. These results indicate that CNDC can be used to correctly determine N application rates for mulched dripirrigated maize production.