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 (NO(3)(−)) 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 NO(3)(−) accumulation in spinach 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 NO(3)(−) 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 NO(3)(−) accumulation. The co-fertilization of N-Mo-Mn gave the highest values in all studied attributes and the lowest NO(3)(−) 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 + 50 mg L(−1)). Our findings proposed that the co-fertilization of N-Mo-Mn could enhance spinach yield and its quality, while reducing NO(3)(−) accumulation in leaves, resulting agronomical, environmental and economic benefits.