Biochar-Coated Urea Boosts Maize Yields and Slashes Nitrogen Waste in Field Trial

Nitrogen fertilizer is both the engine of modern agriculture and one of its most wasteful inputs. Crops typically acquire only about 35 percent of the nitrogen applied as urea, with the rest escaping into air and water through ammonia volatilization, leaching, and denitrification. In South Asia, losses of 40 to 70 percent of applied nitrogen are common, driving eutrophication, soil acidification, and a heavy carbon footprint. A new open-access field study published in Discover Plants by S. M. M. Alam, Shamim Mia, and colleagues at Patuakhali Science and Technology University in Bangladesh, together with collaborators in Australia and the Netherlands, reports that a simple change in fertilizer chemistry—wrapping urea granules in specially modified biochar—can dramatically close that gap.

The team’s central idea was to change the chemistry of the fertilizer microsite, the tiny zone of soil immediately surrounding each pellet where urea hydrolyzes within hours, pushing local pH above 8.5 and triggering ammonia losses. By coating or blending urea with biochars carrying a wide range of surface charges, the researchers aimed to slow nitrogen release and improve retention right where it matters. Biochar, a carbon-rich material produced by heating biomass in low-oxygen conditions, has long been studied as a soil amendment, but its properties vary enormously with feedstock, pyrolysis conditions, and post-production treatment. Rather than treating biochar as a single material, the team engineered four distinct versions and tested them head to head.

The biochars were produced from sawdust in a semi-airtight brick kiln, with temperatures climbing from 300 degrees Celsius at two hours to a peak of 807 degrees at 28 hours before declining. One batch was left pristine, with a nearly neutral surface charge of about minus 2.6 centimoles of charge per kilogram at pH 7.0. A second was chemically oxidized with hydrogen peroxide, which flooded the surface with carboxylic and phenolic functional groups and pushed the negative charge to minus 34.3 centimoles per kilogram. A third was biologically activated by inoculating it with microorganisms extracted from forest, agricultural, and pasture soils and incubating it for 15 days at alternating temperatures of 40 and 25 degrees. The fourth was pre-doped with iron, aluminum, and magnesium chlorides before pyrolysis, which impregnated the carbon matrix with metals and flipped the surface charge to slightly positive, around 1 centimole per kilogram, with a correspondingly tiny cation exchange capacity of about 2.4.

These four biochars were applied in two different architectures. In the coated formulations, urea granules were tumbled at 30 revolutions per minute with biochar, starch, and soybean oil at a urea-to-biochar ratio of 10:1, creating an external diffusion barrier around each pellet. In the blended formulations, biochar, fertilizer, and clay were physically mixed and pressed into new composite granules, so that nitrogen release depended on internal pore connectivity rather than an outer shell. Potassium nitrate was also blended with pristine and metal-doped biochars, and urea was blended with coal as an additional comparison. Because blending dilutes the nutrient content—the coated fertilizers contained roughly 40 percent nitrogen while the blends held only about 2.5 percent—application rates were adjusted so that every treatment delivered the same nutrient supply.

The field trial took place at the Agronomy Field of Patuakhali Science and Technology University in Dumki, Bangladesh, from January to May 2023, on a slightly alkaline silty clay loam with very low organic carbon of 0.98 percent and total nitrogen of just 0.05 percent. Twelve treatments were arranged in a randomized complete block design with three replications, spanning the biochar formulations, neem-coated urea, guti-urea (urea super granules), conventional prilled urea, and an unfertilized control. The hybrid maize variety Bahubali-881 was sown at 25 centimeters within rows spaced 60 centimeters apart, and nitrogen was split three ways: at land preparation, 40 days after sowing, and 75 days after sowing, just before flowering.

The standout result came from urea coated with biologically activated biochar. This treatment produced the highest grain yield of the entire experiment, 9.84 tonnes per hectare, a 174 percent increase over the unfertilized control’s 3.58 tonnes and a 29 percent increase over conventional urea’s 7.63 tonnes. The same plots produced the tallest plants at 202.73 centimeters, the longest cobs at 19.12 centimeters, the most grains per cob at roughly 590, the heaviest 1000-grain weight at 332.7 grams, and the highest SPAD chlorophyll reading at 54.7. Root biomass reached 3.81 tonnes per hectare, and straw yield peaked at 7.78 tonnes per hectare. Most strikingly, nitrogen use efficiency hit 74.4 percent, compared with just 47.6 percent for conventional urea—meaning nearly three-quarters of the applied nitrogen ended up in the crop rather than the environment.

The efficiency metrics told a consistent story across the board. The researchers calculated not only overall nitrogen use efficiency but also agronomic efficiency, the yield gain per unit of nitrogen applied; physiological efficiency, how well plant nitrogen converts into grain; and apparent recovery efficiency, the fraction of applied nitrogen recovered in plant tissue. The biologically activated biochar-coated urea led every index. Chemically oxidized biochar coatings and iron-aluminum-magnesium pre-doped coatings also matched or beat conventional urea and the commercial enhanced-efficiency products, neem-coated urea and guti-urea. When results were pooled by formulation type, coated fertilizers averaged 9.43 tonnes per hectare of grain and 65.32 percent nitrogen use efficiency, outperforming blended formulations, and oxidized biochar types averaged 9.78 tonnes per hectare and 69.3 percent efficiency.

Intriguingly, the soil itself offered few clues to the yield differences. Measurements of pH, ammonium nitrogen, and nitrate nitrogen during the growing season showed no statistically significant differences among treatments, and post-harvest analysis found most nutrients unchanged, with the notable exception of potassium, which was highest where potassium nitrate had been blended with biochar. Soil organic matter did vary, rising in several biochar-blended treatments. The authors attribute the modest soil effects to the small quantities of biochar applied, suggesting that the fertilizers’ benefits occurred locally around each pellet rather than across the bulk soil. Grain nutrient composition likewise remained stable, with no significant differences in micronutrients such as iron, zinc, and manganese, indicating that the yield gains came without sacrificing nutritional quality.

Multivariate statistics sharpened this interpretation. Pearson correlation analysis showed grain yield strongly and positively linked to plant traits—straw yield, grains per cob, root biomass, cob length, plant height, and 1000-grain weight—with correlation coefficients between 0.78 and 0.96, while residual soil nitrate after harvest correlated only weakly at 0.38. Redundancy analysis separated yield-related plant traits from soil nutrient variables along the first ordination axis, confirming that variation in harvest-time soil chemistry did not explain treatment performance. The authors are careful to note that mechanisms such as slower nitrogen release and improved synchronization between nitrogen supply and crop demand remain plausible explanations drawn from prior literature rather than processes directly measured in this study; urease activity and nitrogen transformation pathways were not evaluated.

Even with those caveats, the practical implications are substantial. Biologically activated biochar-coated urea is built from sawdust waste, soil microbes, starch, and soybean oil—no synthetic polymer coatings that linger in soil—making it an attractive candidate for smallholder maize farmers in Bangladesh and beyond, where yields still fall short of the regional irrigated potential of roughly 12.3 to 13.8 tonnes per hectare. The authors call for direct quantification of nitrogen release dynamics and loss pathways, along with validation across multiple seasons, locations, and economic conditions before large-scale adoption. If those follow-up studies hold up, a coating of microbe-conditioned charcoal could become one of the cheapest levers available for feeding more people while wasting less of the nitrogen that modern farming depends on.

Subject of Research: Biochar-coated and blended nitrogen fertilizers for improving maize yield and nitrogen use efficiency

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

Article References: Alam, S. M., Mia, S., Saha, G., Masud, M. M., Al-Amin, M., Mim, M. J., Islam, M. M., & Jindo, K. (2026). Role of biochar coated and blended nitrogen fertilizers on maize yield and nitrogen use efficiency. Discover Plants, 3(1), Article 372. https://doi.org/10.1007/s44372-026-00835-2

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00835-2

Keywords: biochar, urea, nitrogen use efficiency, maize, slow-release fertilizer, soil chemistry, Bangladesh, agronomy, sustainable agriculture, pyrolysis, enhanced-efficiency fertilizer, field trial