Biochar Enhances Rice Grain Quality and Starch Functionality While Maintaining Yield Under Reduced Nitrogen Input

Improving rice grain quality while reducing nitrogen (N) fertilizer input is a key challenge for sustainable rice production. Although biochar can enhance soil fertility and N use efficiency, its effects on grain quality and starch functionality under reduced N input remain unclear. Two-year pot experiments involving japonica rice cultivars were conducted with different N rates (0.7N–1.1N) under no-biochar (NB) and biochar (BI) conditions. Under non-biochar conditions, grain yield increased with N input, with no significant difference between 1.0N and 1.1N. Biochar application increased grain yield by 4.0–13.3% compared with the corresponding no-biochar treatments, and the yield of BI-0.9N was comparable to that of NB-1.0N. Nitrogen reduction decreased grain chalkiness and improved eating quality, whereas excessive N reduction lowered head rice rate. Biochar improved milling quality and eating quality, while its effects on appearance quality were relatively limited. Compared with 1.0N, 1.1N increased protein content but reduced amylose content and starch viscosity parameters, whereas 0.9N showed the opposite trend. Biochar further increased starch viscosity parameters, with peak and final viscosities increasing by 4.4–30.2% and 2.9–38.9%, respectively, under reduced N conditions, and altered starch thermal properties. Correlation analysis indicated that milling quality correlated with protein content and starch solubility, while eating quality correlated with peak viscosity and breakdown value. In this study, biochar combined with 10% N reduction (BI-0.9N) maintained grain yield while improving grain quality and starch functional properties compared with conventional N management (NB-1.0N), indicating its potential as a nitrogen-saving management option for japonica rice production.