Long-term effects of rice husk biochar on the milling traits of rice (Oryza sativa L.) grown in the low country dry zone of Sri Lanka: evidence from a four-season field experiment with urea application

A four-season consecutive field experiment was conducted from the 2023 Yala season to the 2024/25 Maha season under low country dry zone conditions in Sri Lanka. The objective was to evaluate the combined effect of rice husk biochar (RHB) and urea fertilization on the rice milling traits hardness, chalkiness, bran weight (BW), total milled rice (TMR), hull percentage, degree of milling (DM), brown rice (BR), and head grain (HG) percentages. The rice variety At 362 was used as the test cultivar and the experiment followed a split-plot design with three replicates. The main plot treatments consisted of four RHB levels: 0, 1, 2, and 3 t ha^-1 (designated as BC0, BC1, BC2 and BC3) with partially burnt paddy husk (PBPH) applied at the Department of Agriculture recommended rate of 625 kg ha^-1. Sub-plot treatments included four levels of urea fertiliser: 0, 30, 70, 100 % of the recommended dose (U0, U30, U70, U100, where U100 = 225 kg ha^-1). Agricultural-grade urea commonly used by local farmers was applied following standard local practice. Results indicated that a one-time application of RHB combined with seasonal urea significantly enhanced total grain yield peaking at 6.50 t ha^-1 in season 3 and improved rice milling traits. Higher biochar and urea application rates led to consistent improvements in grain yield and all positive milling attributes, while significantly reducing hull percentage and grain chalkiness across all seasons, demonstrating a residual biochar effect. In comparison, PBPH resulted in significantly lower grain yield and quality improvements compared to RHB treatments. Correlation analysis revealed a strong positive relationship between grain hardness, DM % and TMR %, and a strong negative relationship between hull percentage and BW %. Forecasting models suggest that rice milling traits can be further optimized with RHB application rates exceeding 3 t ha^-1. Overall, the integration of RHB with urea provides greater and more sustainable benefits for rice milling traits while increasing productivity under low-country dry zone conditions in Sri Lanka.