Role of biochar coated and blended nitrogen fertilizers on maize yield and nitrogen use efficiency

Increasing nitrogen use efficiency (NUE) remains a major challenge in maize production due to substantial losses from conventional fertilizer applications. Efforts have been made to develop enhanced efficiency nitrogen (N) fertilizers by modifying the N release rate. Changing fertilizer microsite pellet chemistry by including suitable reactive surfaces could increase N retention and NUE. This study evaluated the field performance of biochar-based N fertilizers. Specifically, urea was coated with four biochars, i.e., iron (Fe)-aluminum (Al)- magnesium (Mg) pre-doped biochar, biologically active biochar, pristine biochar, and oxidized biochar at a ratio of 10:1, while potassium nitrate (KNO3) was blended with pristine biochar and Fe-Al-Mg pre-doped biochar. Urea was also blended with coal and pristine biochar. The surface charge of the biochars differed markedly, ranging from negative (−34 cmolc kg-1) in the chemically and biologically oxidized biochars, to nearly neutral in the pristine biochar (−2.6 cmolc kg-1), and becoming positive in the mineral-impregnated biochar (~ 1cmolc kg-1) at pH 7.0. The blending was performed by mixing 2 g biochar, 0.288 g urea/KNO3, and 1 g clay, respectively. These fertilizers were compared with neem-coated urea, guti-urea (urea super granule), and conventional prilled urea. Twelve treatments were tested in a randomized complete block design under field conditions. Among the formulations, urea coated with biologically activated biochar produced the highest grain yield (9.84 t ha⁻¹), representing a 174% increase over the unfertilized control (3.58 t ha⁻¹) and a 29% increase over conventional urea (7.63 t ha⁻¹). This treatment also achieved the highest NUE (74.4%), compared with 47.6% for conventional urea. This enhanced performance might be attributed to a relatively slower N release and improved plant N uptake than other treatments. These findings highlight the agronomic potential of biologically functionalized biochar-based fertilizers as a strategy that may reduce N losses and support sustainable maize production.