The research team, led by Asheesh Kumar and K. M. Manjaiah, synthesized two versions of the composite by grafting acrylic acid and acrylamide polymers onto bentonite nanoclay, replacing 20 percent of the acrylic acid with starch from either maize flour or mango kernel flour. The choice of biopolymer matters: fully synthetic polyacrylamide-based superabsorbents persist in soil and can generate toxic degradation by-products, whereas starch-grafted matrices biodegrade into benign compounds. Polymerization was carried out under a nitrogen atmosphere with ammonium persulfate as initiator and methylene bis-acrylamide as crosslinker, producing dry granules that were milled and sieved to a uniform two-millimeter size.

Characterization confirmed that the chemistry worked as designed. Fourier transform infrared spectroscopy showed that the characteristic silicate and starch peaks of the raw materials weakened or vanished after polymerization, while new bands near 1552 and 2321 wavenumbers signaled the formation of carbon-nitrogen and carbon-carbon linkages. X-ray diffraction revealed that bentonite’s diagnostic peak at a diffraction angle of 6.4 degrees disappeared entirely in the finished composites, indicating that the clay’s silicate layers had been exfoliated and dispersed throughout the polymer matrix, a hallmark of successful nanohybridization that improves barrier properties and mechanical strength. Scanning electron microscopy showed smooth, compact surfaces before loading and a rough, crystalline texture afterward, direct visual evidence that urea had been deposited across the polymer network.

The composites proved to be exceptional sponges. When one gram of dry material was immersed in water for 24 hours, the maize flour version absorbed roughly 68 grams of water and the mango kernel version about 62 grams, thanks to three-dimensional crosslinked networks of hydrophilic polymers studded with hydroxyl groups. This enormous swelling capacity is what allows the granules to soak up a urea solution, along with suspended microbial cultures, and then release both gradually in soil. The maize flour matrix absorbed more water than its mango kernel counterpart, likely because maize starch is richer in free hydroxyl groups, while mango kernel flour contains more fiber, protein, and phenolics that restrict swelling.

A critical question was whether the beneficial bacteria could survive inside a nitrogen-saturated polymer matrix. The researchers embedded Lactococcus lactis, a phosphorus solubilizer, and Pseudomonas aeruginosa, a zinc solubilizer, into the urea-loaded granules and tracked colony-forming units over 180 days. Both organisms remained viable throughout the six-month period. Lactococcus lactis held nearly steady, dropping only from 34 to 30 colony-forming units per plate, while Pseudomonas aeruginosa declined from 40 to 18. The team attributes this resilience to the composite’s porous structure and moisture retention: hydrogen bonding between water molecules and the polymer’s hydroxyl groups keeps the interior humid even when the exterior feels dry, and the nanoclay creates a protective microenvironment known to support bacterial survival.

The field trial, run during the 2021-22 kharif and rabi seasons on alluvial sandy loam at the IARI research farm, compared ten treatments in a randomized complete block design. The key comparison pitted 75 percent of the recommended nitrogen dose delivered through microbe-loaded composites against 100 percent of the recommended dose applied as prilled urea. Across maize and wheat, the composite treatments maintained higher and more sustained soil ammonium and nitrate levels, peaking around 60 days after sowing when crop demand is greatest. Notably, ammonium concentrations stayed significantly higher under the composite treatments than under full-dose urea, suggesting that the slow-release matrix delayed nitrification and kept nitrogen in a less leachable, less emission-prone form for longer.

The greenhouse gas results were striking. Using closed chambers and gas chromatography with electron capture detection, the team measured nitrous oxide fluxes weekly through the first 50 days of each crop. Full-dose urea produced average fluxes of 39.9 grams per hectare per day in wheat and 16.2 in maize, with peaks exceeding 120 and 35 grams per hectare per day respectively. The mango kernel composite with both bacterial cultures cut cumulative emissions by 17.2 percent in wheat and 21.4 percent in maize, while the maize flour composite performed even better, reducing emissions by 28.7 percent in wheat and 24.2 percent in maize. The mechanism is largely one of synchronization: by metering nitrogen out slowly and delivering 25 percent less of it, the composites shrink the pulses of excess mineral nitrogen that fuel nitrifying and denitrifying microbes, without starving the crop.

The solubilizing bacteria added a micronutrient bonus. Plots receiving composites with both cultures showed significantly higher phosphorus and zinc content in the grain and stover of both crops than plots receiving the composites alone. Phosphorus-solubilizing bacteria convert insoluble phosphates into plant-available forms through acidification and chelation, supporting ATP synthesis and root growth, while zinc solubilizers release zinc ions needed for enzymatic activity, protein synthesis, auxin regulation, and nitrogen metabolism. The result is a form of biofortification: more phosphorus and zinc sequestered in the edible grain, a meaningful consideration in regions where zinc deficiency affects human health.

Intriguingly, wheat emitted more nitrous oxide than maize despite growing in cooler winter conditions. The researchers link this asymmetry to irrigation. In the hot summer maize season, when temperatures reach 40 to 45 degrees Celsius, irrigation water vanishes within a day, allowing some applied nitrogen to leach away and leaving the soil too dry for maximal microbial activity. In winter, dew and cooler temperatures mean fields stay moist for five to seven days after irrigation, creating the anaerobic microsites in which denitrifying microbes thrive. The finding underscores that fertilizer technology interacts intimately with water management, and that emission mitigation strategies must account for both.

The authors are candid about limitations. The study covered a single site, one soil type, one climatic regime, and one maize-wheat rotation, and longer-term, multi-location validation is needed. The use of Pseudomonas aeruginosa, an opportunistic pathogen in clinical settings, also raises biosafety questions that will need addressing before widespread deployment. Still, the concept of a single granule that carries fertilizer, feeds beneficial microbes a moist refuge, and releases nutrients on the crop’s schedule rather than the soil’s is a compelling vision of climate-smart agriculture. If scaled economically, such composites could help cereal farmers grow more food with less nitrogen, fewer micronutrient deficiencies, and a substantially smaller greenhouse gas footprint.

Subject of Research: Slow-release urea nanoclay biopolymer composites with beneficial microbes for nutrient uptake and greenhouse gas mitigation in cereal cropping

Article Title: Urea-loaded nanoclay biopolymer composites integrated with beneficial microbes enhance nutrient uptake and mitigate greenhouse gas emissions

Article References: Kumar, A., Manjaiah, K. M., Roy, D., Saini, R., Bhatia, A., & Chauhan, A. P. S. (2026). Urea-loaded nanoclay biopolymer composites integrated with beneficial microbes enhance nutrient uptake and mitigate greenhouse gas emissions. Discover Soil, 3(1), Article 175. https://doi.org/10.1007/s44378-026-00334-6

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00334-6

Keywords: nitrogen use efficiency, nanoclay biopolymer composite, urea, nitrous oxide, greenhouse gas emissions, plant growth-promoting rhizobacteria, phosphorus solubilizing bacteria, zinc solubilizing bacteria, slow-release fertilizer, maize-wheat cropping system, biofortification, climate-smart agriculture