Effect of Single Application of Rice Husk Biochar and Seasonal Application of Urea on Grain Nutritional and Morphometric Traits of Rice: Four Seasonal Field Experiment

A four-season field experiment was conducted for rice variety At 362 from the 2023 Yala season to the 2024/25 Maha season in the low-country dry zone of Sri Lanka, to examine the effects of single application of rice husk biochar and seasonal urea applications on grain nutritional and morphometric traits of rice. Grain amylose, iron (Fe) and ash percentages, antioxidant capacity, grain width, grain length, length-to-width ratio, grain thickness, equivalent diameter, grain volume, grain surface area and grain sphericity were measured. Four biochar rates (0, 1, 2, and 3 t/ha) and the recommended rate of partially burnt paddy husk (625 kg/ha), along with four urea levels (0, 67.5, 157.5 and 225 kg/ha), were arranged in a split-plot design with three replicates. The analysis of variance showed that rice husk biochar had a significant effect on all measured parameters across the four seasons. The application of urea also had a strong impact on some parameters such as antioxidant capacity, Fe content, grain volume, grain length and grain length-to-width ratio, although it did not always significantly affect on other measured parameters. Seasonal variation significantly influenced all measured parameters. Regarding interactions, the biochar × urea and urea × season effects were not significant, while the biochar × season interaction was significant for ash and Fe, indicating that the effect of biochar varied across seasons. The three-way interaction (biochar × urea × season) was not significant for any parameter. Dunnett’s test results indicated that most traits were significantly improved at higher levels of rice husk biochar compared to the partially burnt paddy husk treatment, except for antioxidant capacity. Overall, 3 t/ha of rice husk biochar combined with recommended urea level performed better than recommended rate of partially burnt paddy husk in enhancing and maintaining grain nutritional and morphometric traits over four seasons under low-country dry zone conditions in Sri Lanka.