Synergistic Effects of Rice Husk Biochar and Urea Fertilizer on Rice Growth Performance, Soil Properties, and Nitrogen Use Efficiency

This study evaluated the effect of the combined application of rice husk biochar (RHB) and urea fertilizer on soil properties, plant growth, yield, grain quality, and nitrogen use efficiency metrics in the Low Country Dry Zone of Sri Lanka. A pot experiment, arranged in a completely randomized two-factor factorial design with 4 replicates, was used for the study. The experiment consisted of five biochar carbon rates B0: 0%, B1: 1%, B2: 2%, B3: 3% and two urea levels (U1: 157.5 kg/ha, U2: 225 kg/ha), with treatments repeated across the Yala (from June to September 2023) and Maha (November 2023 to March 2024) seasons. Partially burnt paddy husk (PBPH; 625 kg/ha which corresponds to the 0.01% carbon rate in soil) is a recommended type of biochar-based soil amendment for rice cultivation; therefore, a treatment with the recommended level of PBPH and two urea levels was also included in the study. Results showed that RHB significantly improved soil chemical properties, rice plant growth, yield and nitrogen use efficiency metrics. The B3U2 treatment exhibited an 11% higher grain yield per plant and a 16% increase in nitrogen use efficiency compared to the existing conventional practice (B0U2). Moreover, grain iron content (IC), protein content (PC), and amylose content (AC) in the B3U2 treatment were 7%, 1%, and 15% higher, respectively, compared to the B0U2 treatment. Some growth and yield parameters, as well as grain physical properties, did not significantly differ between the low nitrogen (B3U1) and high nitrogen (B3U2) treatments. Compared to PBPH, RHB demonstrated superior effects on all measured parameters, although its preparation is comparatively more difficult. The findings of the present study revealed that the combined application of RHB and urea fertilizers is beneficial in rice cultivation. However, further research under field conditions is needed to understand the long-term impacts of RHB application.