Biochar Based Slow-Release Urea Fertilizer: Production and Assessing the Effects on Growth of Lowland Rice and Nitrogen Dynamics in an Alfisol

Use efficiency of N fertilizers in lowland rice cultivation is very low. Biochar can be used to modify the N releasing rates of fertilizers and increase their efficiency. Maximum amount of urea that can be intercalated into biochar (BC) and the effects of biochar based slow-release urea fertilizer (SRF) on soil solution N contents and growth of rice (Oryza sativa) were investigated. Two feedstock materials (sawdust and tea waste) were slow pyrolysed at 500°C to produce BC and N were intercalated into BC using urea solutions with seven concentrations (U1=0% <U2<U3<U4<U5<U6<U7=100% w/v). The maximum intercalated N contents in sawdust BC and tea waste BC were 28% and 22%, respectively. Five treatments: two SRFs at 17.25 mg N kg^–1 soil (75% recommended N) and three urea levels (0, 75, and 100% N) were applied to rice plants (Bg 251) in a pot experiment. Application of SRF significantly improved the greenness and growth of rice plants when compared with 0% N control and were comparable with 100% urea-N treatment. The apparent recovery of N (ARN) was significantly (p<0.05) improved in SRFs (60-71%) compared to urea only treatments (38-52%). Nitrate-N contents in soil solution were not significantly different (p<0.05) among treatments while NH4⁺-N content in 100% urea treatment was significantly (p<0.05) higher throughout. Based on urea intercalation efficiency and the ARN, we concluded that SRF made using sawdust biochar is having a higher potential than SRF made using tea waste biochar to improve the efficiency of urea in lowland rice cultivation.