Effects of Nitrogen Application Rate and Rice–Water Mimosa Mixed Cropping on Rice Quality, Starch Structure, and Protein Composition

Abstract

Rice quality is jointly affected by nitrogen (N) management, cropping system, and environmental conditions. A two-year field experiment was conducted to evaluate the effects of rice–water mimosa mixed cropping and three N application rates (0, 75, and 150 kg N ha⁻¹) on rice milling and appearance quality, physicochemical properties, starch characteristics, protein composition, and rice-flour pasting properties. Increasing N application generally increased the brown rice rate, milled rice rate, head rice rate, gel consistency, and protein content but decreased the instrument-derived taste index, total starch content, amylose content, peak viscosity, trough viscosity, breakdown, and final viscosity. Chalkiness degree, chalky rice percentage, and solubility generally showed their lowest values at 75 kg N ha⁻¹. Compared with monocropping, mixed cropping produced the most consistent increases in milling quality and reductions in chalkiness at 75 kg N ha⁻¹ and increased the instrument-derived taste index and protein content under selected low and moderate N treatments. Mixed cropping also altered protein-fraction composition, generally increasing the relative proportions of prolamin and glutelin while decreasing the residual protein fraction. Furthermore, mixed cropping modified the short-range molecular organization of starch and the pasting properties of rice flour, although the magnitude and direction of these responses depended on N application rate and experimental year. Overall, rice-quality responses to mixed cropping depended on N management and experimental year, and among the three N rates tested, 75 kg N ha⁻¹ produced favorable values for several of the measured quality traits under the conditions of this two-year experiment.