Nitrogen-molybdenum-manganese co-fertilization reduces nitrate accumulation and enhances spinach (Spinacia oleracea L.) yield and its quality

Spinach (Spinacia oleracea L.) is considered a nitrogen (N) intensive plant with high nitrate (NO3-) accumulation in its leaves. The current study via a two-year field trial introduced an approach by combining N fertilization from different sources (e.g., ammonium nitrate; 33.5% N, and urea; 48% N) at different rates (180, and 360 kg N ha-1) with the foliar spraying of molybdenum (Mo) as sodium molybdate, and/or manganese (Mn) as manganese sulphate at rates of 50 and 100 mgL-1 of each or with a mixture of Mo and Mn at rates of 50 and 50 mg L-1, respectively on growth, chemical constituents, and NO3- accumulation in leaves. Our findings revealed that the highest rate of N fertilization (360 kg N ha-1) significantly increased most of the measured parameters e.g., plant length, fresh and dry weight plant-1, number of leaves plant-1, leaf area plant-1, leaf pigments (chlorophyll a, b and carotenoids), nutrients (N, P, K, Fe, Mn, Zn), total soluble carbohydrates, protein content, net assimilation rate, and NO3- accumulation, but decreased leaf area ratio and relative growth rate. Moreover, plants received urea-N fertilizer gave the highest values of all previous attributes when compared with ammonium nitrate –N fertilizers, and the lowest values of NO3- accumulation. The co-fertilization of N-Mo-Mn gave the highest values in all studied attributes and the lowest NO3- accumulation. The best treatment was recorded under the treatment of 360 kg N-urea ha-1 in parallel with the combined foliar application of Mo and Mn (50+50mg L-1). Our findings proposed that the co-fertilization of N-Mo-Mn could enhance spinach yield and its quality, while reducing NO3- accumulation in leaves, resulting agronomical, environmental and economic benefits.