Combined Effects of Nitrogen and Zinc Applications on Nutrient Use Efficiency, Grain Quality and Biofortification in Rice

Abstract

A pot experiment was conducted using a split-plot design with four replications to determine the optimal Nitrogen (N) and Zinc (Zn) levels for maximizing nutrient uptake, recovery efficiency, agronomic use efficiency, milling quality, and grain Zn concentration in rice. The main plots comprised four N rates based on Department of Agriculture (DOA) guidelines (102 kg N ha⁻¹, 27 kg P₂O₅ ha⁻¹, and 117 kg K₂O ha⁻¹): 0% (N0), 75% (76.5 kg N ha⁻¹, N75), 100% (102 kg N ha⁻¹, N100), and 125% (127.5 kg N ha⁻¹, N125). The subplots included three Zn treatments: 0 kg Zn ha⁻¹ (Zn0), 20 kg Zn ha⁻¹ applied basally to the soil (Zn20), and 40 kg Zn ha⁻¹ (20 kg Zn ha⁻¹ basal + 20 kg Zn ha⁻¹ split into two foliar sprays at the heading and grain-filling stages, Zn40). The results showed that N uptake peaked under N125 (1.23 g pot⁻¹), whereas Zn uptake was highest under Zn40 (0.08 g pot⁻¹). The combined treatment (N125Zn40) demonstrated strong synergistic effects, yielding the highest nutrient uptake, Zn recovery efficiency, and agronomic N use efficiency. However, N100Zn40 gave the highest NRE, whereas N125Zn20 provided the highest AZnUE. Regarding milling quality, N125Zn20 produced the highest milling recovery (68.28%), and N125Zn40 produced the highest head rice recovery (63.75%), along with the lowest broken rice percentage (6.04%). Zn concentrations under N125Zn40 were 48.75 mg kg⁻¹ in brown rice and 46.05 mg kg⁻¹ in white rice, whereas N100Zn40 obtained the highest Zn content in bran (177.02 mg kg-1), and N75Zn40 provided 189.39 mg kg-1 in husk. Overall, combining 125% of the recommended N rate with both basal and foliar Zn applications (N125Zn40) is the most effective strategy to enhance nutrient efficiency, grain Zn biofortification, and post-harvest milling quality in rice.