The research, published in the journal Plant and Soil, was led by Wenjia Yang, Xiaowei Ma, Jun Hou and Bing Cao of Yangtze University and the Beijing Academy of Agriculture and Forestry Sciences. The team set out to test a deceptively simple hypothesis. Controlled-release blended fertilizers, whose polymer coatings meter out nitrogen in step with crop demand, are known to raise yields and nitrogen use efficiency. But when these fertilizers are broadcast on the soil surface, as is common in practice, much of the nitrogen sits far from the densest concentration of rice roots, leaving it vulnerable to volatilization and runoff. Dense planting, the researchers reasoned, could close that gap by expanding the root absorption zone and intercepting nitrogen before it escapes.

To test the idea, the team ran a three-year field experiment with five treatments: a zero-nitrogen control, the conventional farmer’s practice, an optimized nitrogen practice, a single basal application of controlled-release blended fertilizer, and that same fertilizer combined with dense planting. Rather than tracking yield alone, they assembled an unusually complete environmental audit. They measured fluxes of methane and nitrous oxide from the paddies, ammonia volatilization from the floodwater, inorganic nitrogen in the soil and surface water, redox potential, dissolved oxygen, and the abundances of functional genes that govern the microbial nitrogen cycle. They then folded these measurements into greenhouse gas intensity, carbon footprint, and net ecosystem economic benefit calculations.

The yield results were striking. Compared with the farmer’s practice, the controlled-release fertilizer alone increased grain yield by 10 to 30 percent. Adding dense planting pushed yields a further 10 to 15 percent higher, and the mechanism behind the boost was a genuine synergy: the denser stands produced more effective panicles per unit area while each panicle also carried more grains. In other words, the combination did not simply cram more plants into the same space at the expense of individual performance; it lifted both components of the yield equation simultaneously, under a reduced nitrogen input regime.

The environmental accounting was equally revealing. The controlled-release fertilizer cut cumulative methane emissions by 18 to 35 percent, nitrous oxide emissions by 13 to 52 percent, and ammonia volatilization by 40 to 49 percent relative to conventional practice. These reductions trace back to the fertilizer’s core design. By releasing nitrogen gradually, the coating prevents the floodwater from becoming a concentrated nitrogen soup, which suppresses the microbial processes that generate nitrous oxide and the chemical equilibrium that drives ammonia into the air. The smoother nitrogen supply also alters the soil’s redox dynamics and dissolved oxygen profile, conditions that favor methanotrophs, the microbes that consume methane before it reaches the atmosphere.

Dense planting introduced one complication. Relative to the controlled-release fertilizer alone, the combined treatment showed a modest methane rebound of 13 to 14 percent, likely because denser canopies and root systems alter the carbon inputs and gas transport pathways in the flooded soil. The combination also did not further reduce nitrous oxide or ammonia losses beyond what the fertilizer achieved on its own. For a reader scanning only the emission columns, that might look like a step backward.

But the full accounting tells a different story. When emissions are expressed per unit of grain, the combined treatment held greenhouse gas intensity and carbon footprint at levels comparable to the fertilizer alone, because the yield gains diluted the per-kilogram emissions. The modest methane rebound was more than offset by the larger harvest. This yield-driven dilution is a critical insight for climate-smart agriculture: the goal is not simply to minimize absolute emissions from a field, but to minimize the emissions embedded in every bowl of rice produced.

The economics strengthened the case further. The combined treatment boosted net ecosystem economic benefits by 9 to 97 percent compared with the farmer’s practice, a range that reflects both higher grain output and savings on fertilizer and labor, since the controlled-release product requires only a single basal application rather than multiple split dressings. For smallholder farmers in central China, where rice paddies dominate the landscape and nitrogen overuse remains widespread, a practice that simultaneously raises income and cuts pollution addresses the two pressures that most often pull in opposite directions.

The study also connects to a broader scientific conversation about nitrogen-cycling microbes in paddy soils. By measuring the abundances of functional genes involved in nitrification and denitrification, the researchers could link treatment effects on nitrous oxide to shifts in the microbial communities that produce and consume this potent greenhouse gas, which has roughly 273 times the warming power of carbon dioxide over a century. Previous work by the same group and collaborators had shown that root-zone fertilization with controlled-release urea reduces nitrous oxide and ammonia losses under different irrigation regimes, and that combining controlled-release blended fertilizer with densification lowers ammonia volatilization. The new study extends that framework to a full greenhouse gas and carbon footprint assessment over multiple seasons, which is essential because single-year trials can be misleading in variable climates.

The authors conclude that controlled-release blended fertilizer combined with dense planting represents a promising strategy for sustainable rice production in the middle reaches of the Yangtze River, one of China’s most important rice belts. The findings arrive at a moment when rice systems worldwide face intensifying scrutiny: rice cultivation contributes a substantial share of agricultural methane, and global assessments consistently identify nitrogen management as a key lever for reducing the sector’s climate impact. If the yield-and-emissions synergy documented here holds across other rice regions, soil types, and cultivars, the humble act of planting rice a little closer together, with nitrogen delivered on the plant’s schedule rather than the farmer’s, could become one of the most cost-effective climate interventions in staple crop production. The next step, the researchers suggest, is verifying how the approach performs under the wider range of water management practices and soil conditions that define real-world rice farming across Asia.

Subject of Research: Effects of controlled-release fertilizer combined with dense planting on rice yield, greenhouse gas emissions, and carbon footprint

Article Title: Dense planting amplifies yield benefits of controlled-release urea while maintaining low environmental footprint in rice

Article References: Yang, W., Ma, X., Wang, X., Hou, J., Zou, G., & Cao, B. (2026). Dense planting amplifies yield benefits of controlled-release urea while maintaining low environmental footprint in rice. Plant and Soil. https://doi.org/10.1007/s11104-026-09125-y

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09125-y

Keywords: rice, controlled-release fertilizer, dense planting, greenhouse gas emissions, methane, nitrous oxide, ammonia volatilization, carbon footprint, nitrogen use efficiency, paddy field, sustainable agriculture, Yangtze River

The team set up a controlled pot experiment with three salinity levels: a non-saline control at 1.3 dS per meter, which reflects the natural background salinity of the soil they used, a moderate stress level of 6 dS per meter, and a severe stress level of 12 dS per meter. Across all three conditions, wheat plants received the same total amount of nitrogen, 287.50 milligrams per pot, delivered in three equal splits at sowing and at 25 and 50 days after sowing. The variable was the form of the urea itself: ordinary uncoated urea, polymer-coated urea, polymer-zinc-coated urea, and polymer-sulfur-coated urea, abbreviated PSCU. This design allowed the researchers to isolate exactly how the coating chemistry, rather than the nitrogen dose, shaped the plant’s response to salt.

The scale of the damage from salt alone was stark. Under high salinity with conventional urea, grain yield fell by roughly 35 percent and total crop biomass by about 23 percent, while shoot dry weight dropped by more than half. Behind those headline numbers lies a cascade of physiological failure. Salinity drove sodium ions into the plants, disrupted the uptake of potassium and other essential nutrients, and triggered electrolyte leakage as cell membranes lost their integrity. Photosynthetic pigments degraded, relative water content in the leaves declined, and the plants’ capacity to keep growing under what should have been adequate nutrition simply collapsed.

At the cellular level, the researchers traced much of this damage to reactive oxygen species. When salt stress disrupts photosynthesis and metabolism, plants accumulate these highly reactive molecules, which attack membranes, proteins, and DNA. One of the clearest signatures of severe stress in the experiment was the combination of elevated reactive oxygen species with reduced leaf water status and chlorophyll loss. Any fertilizer strategy that hopes to protect yields in salty soils must therefore do more than supply nitrogen; it must help the plant contain oxidative damage and maintain its internal water and ion balance.

All three coated urea formulations outperformed plain urea, but the differences among them were decisive. Polymer-sulfur-coated urea consistently delivered the strongest protection. Plants receiving PSCU showed reduced oxidative damage, better preserved chlorophyll, and the highest relative water content in their leaves. The coating also improved the uptake of nitrogen, phosphorus, and potassium, restoring a more favorable nutrient balance that salt stress had otherwise disrupted. In practical terms, the sulfur and polymer layers appear to slow the release of nitrogen and, in the case of sulfur, add a nutrient that plants can use to build amino acids and defensive compounds, giving stressed wheat a steadier and more useful supply of nutrition.

One of the most striking findings involved secondary metabolites, the specialized compounds plants synthesize to defend themselves. Under severe salt stress, plants fertilized with PSCU showed several-fold increases in flavonoids, total phenols, and tocopherol compared with plants on conventional urea. These molecules are not incidental byproducts; flavonoids and phenolics act as antioxidants that neutralize reactive oxygen species, while tocopherol protects lipid membranes from peroxidation. The coated fertilizer essentially armed the plants’ own chemical defenses, allowing them to mount a stronger antioxidant response precisely when salt stress demanded it most.

The benefits extended to how efficiently the crop used its nitrogen, a metric of enormous economic and environmental importance. The researchers evaluated multiple indices of nitrogen use efficiency, including partial factor productivity, nitrogen balance intensity, and uptake efficiency. PSCU maximized these indices across the salinity treatments, meaning that more of every unit of applied nitrogen ended up in the harvested plant material rather than lost or left stranded in the soil. Because nitrogen fertilizers are both a major farm expense and a source of nitrous oxide emissions and water pollution when used inefficiently, a coating that simultaneously boosts efficiency and stress tolerance addresses two problems at once.

The mechanism behind these gains likely involves the interplay between sulfur nutrition and controlled nitrogen release. Sulfur is a constituent of cysteine and glutathione, two cornerstones of the plant antioxidant system, and adequate sulfur supply is known to support the synthesis of stress-protective compounds. Meanwhile, the polymer layer moderates the dissolution of urea, reducing the sharp ammonia and nitrate pulses that uncoated urea produces and that can exacerbate ion toxicity in saline root zones. By smoothing nutrient availability over the growth period, the coating gives salt-stressed roots a more stable environment in which to function, and the plants respond with better membrane stability, water retention, and photosynthetic capacity.

The researchers are careful about the limits of their results. Even with PSCU, wheat under high salinity could not fully match the performance of plants grown in non-saline soil; the coating alleviated salt damage but did not erase it. That honesty matters, because no fertilizer formulation can substitute for drainage, leaching, salt-tolerant varieties, or sound irrigation management in severely affected fields. What the study does demonstrate is that, within the range of salinity tested, coating choice is not a minor detail but a first-order determinant of how much yield a farmer loses to salt. PSCU consistently outperformed both plain urea and the other coated formulations, suggesting that the specific chemistry of the coating, not just the act of coating, drives the benefit.

The implications reach well beyond one pot experiment. Much of the world’s irrigated wheat belt, from South Asia to the Middle East and North Africa, sits on soils with rising salt loads, and the region’s food security depends on squeezing more from every stressed hectare. Controlled-release fertilizers are already a growing market, and this study adds a compelling agronomic argument for formulations that pair polymer barriers with sulfur. Field-scale trials will be needed to confirm that the yield and efficiency gains hold under real farming conditions, variable soils, and different wheat varieties. But if they do, the message to farmers and fertilizer manufacturers alike is clear: when salt is in the soil, what wraps your urea may matter almost as much as the nitrogen inside it.

Subject of Research: Effects of sulfur and polymer coated urea fertilizers on salinity tolerance and nitrogen use efficiency in wheat

Article Title: Sulfur and polymer coating on urea enhances antioxidant defense, nutrient balance, secondary metabolites, and nitrogen use efficiency in salt‑stressed wheat

Article References: Aslam, M. T., Khan, I., Chattha, M. U., Ghafoor, S., Ismail, A. M., Alsunbul, M., Ur-Rahman, M. H., El Sabagh, A., Al-Ashkar, I., & Brestic, M. (2026). Sulfur and polymer coating on urea enhances antioxidant defense, nutrient balance, secondary metabolites, and nitrogen use efficiency in salt‑stressed wheat. BMC Plant Biology. https://doi.org/10.1186/s12870-026-09928-2

Image Credits: AI Generated

DOI: 10.1186/s12870-026-09928-2

Keywords: wheat, soil salinity, coated urea, nitrogen use efficiency, antioxidant defense, secondary metabolites, reactive oxygen species, nutrient uptake, controlled-release fertilizer, plant physiology, crop yield, sulfur nutrition

The study, published in the Journal of Agriculture and Food Research, was carried out in Pingluo County, Ningxia, a warm temperate arid zone receiving a mere 177 millimeters of rain per year. Roughly 30 percent of the cultivated land in this region is moderately saline-alkali, part of the roughly 99 million hectares of such soil across China, of which more than 40 percent lies in the arid and semi-arid northwest. With per capita arable land in China below 40 percent of the global average, and maize covering about 43 million hectares and supplying more than 40 percent of total grain production, the stakes of squeezing more grain from degraded soils could hardly be higher. The research team, led by Yueqi Li and Jili Liu, set out to test whether matching nitrogen release to crop demand could rewire the entire physiology of the plant, not merely feed it more efficiently.

The experimental design was elegantly straightforward. Six treatments were compared in a randomized block layout with three replicates each: a no-nitrogen control, a conventional urea-only treatment, and four treatments in which 25, 50, 75, or 100 percent of the nitrogen was supplied as polyurethane-coated urea with a 60-day release period. Every treatment received identical total nutrients, 300 kilograms of nitrogen, 180 kilograms of phosphorus pentoxide, and 90 kilograms of potassium oxide per hectare, so any difference in performance could be attributed purely to the timing and pattern of nitrogen delivery. The maize cultivar Xianyu 1225 was sown at a high density of 90,000 plants per hectare, and all plots received the same 2,700 cubic meters of irrigation water, applied in equal installments at the six-leaf, twelve-leaf, and grain-filling stages.

The 50 percent substitution treatment, labeled T50, emerged as the clear winner across nearly every metric the team measured. Compared with conventional urea alone, it raised the leaf chlorophyll index by 11.65 percent and the net photosynthetic rate by 18.43 percent, while lowering intercellular carbon dioxide concentration, a sign that carbon was being fixed more vigorously. More dramatically, the activities of four key nitrogen-metabolism enzymes surged: nitrate reductase by 64.95 percent, glutamate dehydrogenase by 36.63 percent, glutamine synthetase by 22.07 percent, and glutamate synthetase by 60.41 percent. These enzymes form the biochemical pipeline that converts soil nitrate into amino acids, and their heightened activity indicates that the slow-release nitrogen was not merely present in the soil but actively being assimilated into plant metabolism throughout the season.

The concept that unifies the paper’s findings is the source-flow-sink framework, a classical model of crop yield formation. The source comprises the leaves that capture sunlight and build sugars; the flow is the vascular transport system that moves stored assimilates from stems and leaves into the developing ears; and the sink is the grain itself, whose capacity and activity determine how much of that assimilate can actually be packed into harvestable kernels. Salt stress sabotages all three links simultaneously. High pH and osmotic pressure impair root uptake of water and nutrients, reactive oxygen species accelerate chloroplast breakdown and leaf senescence, and poor assimilate supply starves the grain-filling process, shrinking both grain number and grain weight. The controlled-release fertilizer, the researchers found, repaired each link in sequence.

On the flow side, the results were particularly emphatic. Dry matter translocation, the movement of carbohydrates stored in stems and leaves into the grain after flowering, rose by 76.06 percent under T50 compared with urea alone, and the contribution of these pre-anthesis reserves to final grain yield climbed by nearly 18 percent. Grain filling, modeled with a logistic growth curve that fit the data with coefficients of determination above 0.98, was faster and heavier: the maximum filling rate increased by 32.48 percent, the average filling rate by 49.01 percent, and the peak filling date arrived nine days earlier. Ear weight, kernels per ear, and hundred-grain weight all rose by roughly 10 to 19 percent, confirming that the sink had genuinely expanded rather than simply being fed longer.

Why did 50 percent substitution outperform both lower and higher ratios? The authors argue that the answer lies in matching the nitrogen supply curve to the crop’s demand curve. Conventional urea floods the soil with available nitrogen early, promoting lush vegetative growth that later collapses into premature senescence when the supply runs out just as the grain-filling demand peaks. Pure controlled-release fertilizer, by contrast, releases nitrogen too slowly at the start, failing to support the rapid canopy expansion between the six-leaf and twelve-leaf stages and limiting the photosynthetic machinery that must be built early. The 50 percent blend, with conventional urea topdressed at two early stages and coated urea releasing steadily through mid-season, delivers what the authors describe as an early-stage support, mid-stage stability, late-stage sustainability pattern, sustaining leaf function precisely when salt stress would otherwise cut photosynthesis short.

The team backed these physiological interpretations with a battery of statistical tools. Mantel tests confirmed significant correlations between the substitution ratio and source traits such as leaf area index, leaf area duration, and chlorophyll content. Random forest modeling ranked dry matter translocation rate, stomatal conductance, and glutamate synthetase activity as the top contributors to grain yield, while grain weight per ear and mean filling rate dominated predictions of nitrogen use efficiency and irrigation water productivity. A partial least squares structural equation model then tied the whole story together: the substitution ratio acted positively on source, flow, and sink characteristics, with the strongest path coefficient, 0.71, linking it to sink traits, and sink characteristics in turn showed the strongest association with grain yield at 0.76. The model also revealed a negative association between the substitution ratio and the source-sink ratio, indicating a better balance between leaf supply and grain demand.

The efficiency gains carry real economic and environmental weight. Agronomic efficiency of nitrogen, the yield gained per kilogram of fertilizer applied, jumped by 72.79 percent under the optimal treatment, while nitrogen recovery efficiency rose by 68.39 percent and apparent recovery by 40.40 percent. Because irrigation volumes were identical across treatments, the 18.06 percent gain in irrigation water productivity reflects purely the yield increase, meaning every cubic meter of scarce irrigation water in this arid basin produced more grain. The authors caution, however, that their conclusions rest on two years of data from a single site, one maize cultivar, and a warm temperate continental climate, and that nitrogen loss pathways such as ammonia volatilization and nitrate leaching were inferred indirectly from plant uptake rather than measured directly in the soil.

Even with those caveats, the implications are considerable for a country wrestling with food security on degraded land. Controlled-release fertilizers remain more expensive than commodity urea, but a 50 percent substitution strategy halves the added cost while capturing most of the agronomic benefit, and the yield and efficiency gains documented here suggest the premium could pay for itself in saline-alkali regions. As China pushes to bring its vast salt-affected reserves into productive agriculture, the study offers a physiologically grounded template: rather than simply applying more fertilizer, farmers may get far more from every kilogram by controlling when the nitrogen arrives. The humble polymer coating, it turns out, does not just slow a chemical reaction. It re-orchestrates the entire source-flow-sink economy of the maize plant, turning hostile soil into a stage for one of the more quietly impressive yield breakthroughs of recent agronomy.

Subject of Research: Controlled-release fertilizer substitution effects on maize source-sink physiology, yield, and water-nitrogen use efficiency in saline-alkali soil

Article Title: Controlled-release fertilizer substitution boosts maize yield and water-nitrogen use efficiency via optimizing source-sink characteristics in saline-alkali soil

Article References: Controlled-release fertilizer substitution boosts maize yield and water-nitrogen use efficiency via optimizing source-sink characteristics in saline-alkali soil. (n.d.). https://doi.org/10.1016/j.jafr.2026.103312

Image Credits: AI Generated

DOI: 10.1016/j.jafr.2026.103312

Keywords: maize, controlled-release fertilizer, saline-alkali soil, nitrogen use efficiency, source-sink relationship, grain filling, dry matter translocation, irrigation water productivity, Ningxia, urea, photosynthesis, soil salinity

The review, conducted by researchers at the Bangladesh Rice Research Institute and collaborators, followed the PRISMA 2020 reporting standard and the Synthesis Without Meta-analysis guideline, screening 240 records from five databases down to a final evidence base spanning ten rice-producing countries. The authors deliberately avoided a formal statistical meta-analysis because the underlying trials differed too widely in design, agro-ecology, and reporting quality, opting instead for a structured, semi-quantitative synthesis with explicit confidence ratings for each technology.

The headline numbers are striking. Site-specific nutrient management, including the Nutrient Expert decision-support tool developed by IFDC and CIMMYT, improved rice yields by 6 to 18 percent and nitrogen use efficiency by 20 to 117 percent over typical farmer practice across 143 on-farm trials. In Nepal’s Terai region, a four-year multi-site trial recorded yield gains exceeding 2 tonnes per hectare, roughly doubling farm profitability, while applications across India’s Indo-Gangetic Plains cut the global warming potential of rice by about 2.5 percent and of wheat by 12 to 20 percent.

Integrated nutrient management, which blends organic inputs such as farmyard manure, compost, and green manures with balanced inorganic fertilization, emerged as the strongest long-term foundation for soil health. Across trials in Bangladesh, India, China, the Philippines, and Vietnam, it raised grain yields by 8 to 28 percent and soil organic carbon by 14 to 60 percent over periods of five to twenty years. In Bangladesh, where paddy organic matter averages below 1.5 percent against a target of 2.5 percent for optimal harvests, the most effective strategy combined 2 to 3 tonnes per hectare of poultry manure with 50 to 70 percent of the recommended inorganic fertilizer rate.

On the nitrogen side, controlled-release urea, whose polymer coatings synchronize nitrogen release with crop demand, curbed ammonia volatilization losses by 23 to 62 percent and cut runoff nitrogen losses by 8 to 58 percent. In double-rice systems it reduced annual methane emissions by roughly 19.7 percent and nitrous oxide by 35.2 percent, trimming global warming potential by around 21 percent. Nano-enabled formulations promise nitrogen use efficiency gains of 18 to 30 percent, but the authors caution that field-scale safety and ecotoxicology data remain insufficient for policy recommendations.

Water management delivered some of the most compelling climate results. Alternate wetting and drying irrigation, which lets fields dry to 15 centimeters below the surface between floodings using a simple PVC tube costing one to three dollars, reduced global warming potential by 20 to 36 percent, mainly through lower methane emissions, with generally no yield penalty. A landmark three-year multi-site experiment across Vietnam, Indonesia, Thailand, and the Philippines confirmed GWP reductions of 22 to 36 percent, with yields actually rising by 12 percent at the Thai site. National assessments for the Philippines suggest nationwide adoption could avoid roughly 5.6 megatonnes of carbon dioxide equivalent per year.

The review is careful, however, not to oversell AWD. The same intermittent aeration that suppresses methanogenic archaea can stimulate coupled nitrification and denitrification, raising nitrous oxide, a gas with far higher per-molecule warming power. The net climate benefit therefore depends on the severity of drying, soil texture, and especially how nitrogen is applied. Mild ‘safe’ AWD paired with deep-placed or controlled-release nitrogen tends to preserve the methane savings, while aggressive drying under high broadcast nitrogen can erode them.

That insight points to what the authors identify as the most powerful combination in the entire evidence base: urea deep placement plus alternate wetting and drying. Placing large urea briquettes 7 to 10 centimeters into the anaerobic soil zone keeps nitrogen in its plant-available ammonium form, blocking the volatilization and denitrification pathways that can strip away 30 to 50 percent of broadcast fertilizer. In Bangladesh, deep placement alone raised yields by 10 to 21 percent while saving 25 to 33 percent of nitrogen and cutting GWP by 9 to 21 percent. At a Bangladesh Rice Research Institute farm in Satkhira, the combined approach lifted yields by about 28 percent and nitrogen recovery by 167 percent over broadcast urea, without the additional nitrous oxide emissions often feared under aerobic conditions.

Not every technology earns an unqualified endorsement. Biochar, which can reduce methane emissions by 10 to 43 percent, rests on the weakest and most geographically concentrated evidence, dominated by Chinese trials. Controlled-release fertilizers remain too costly for most South Asian smallholders without targeted subsidies. The evidence base itself skews heavily toward Bangladesh, India, and China, leaving countries such as Pakistan, Myanmar, Cambodia, and Malaysia largely unrepresented, and most organic amendment trials run only three to five years, too short to capture full soil carbon trajectories.

The policy implications are nonetheless concrete. For Bangladesh, the authors recommend immediate scale-up of AWD and deep placement, both already validated and certifiable under the Verra VM0006 carbon methodology, alongside continued digital extension through the Nutrient Expert platform. India is advised to adopt national site-specific nutrient management mandates for irrigated rice in the Indo-Gangetic Plains, where groundwater depletion and residue burning create co-benefit opportunities. The central message is that no single technology optimizes all four sustainability outcomes at once; the largest gains come from locally tailored combinations, with each recommendation’s confidence level traveling with it into the field.

Subject of Research: Smart soil fertility management practices for sustainable rice production in South and Southeast Asia

Article Title: Smart soil fertility management for improving rice yield, nitrogen use efficiency, and greenhouse gas mitigation across South and Southeast Asia

Article References: Islam, S. M. M., Islam, M. N., Naher, U. A., Rahman, F., Hossain, A. T. M. S., Milu, M. K. H., Islam, A., Jahan, A., Khan, M. H., Gaihre, Y. K., & Islam, M. R. (2026). Smart soil fertility management for improving rice yield, nitrogen use efficiency, and greenhouse gas mitigation across South and Southeast Asia. Discover Soil, 3(1), Article 164. https://doi.org/10.1007/s44378-026-00316-8

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00316-8

Keywords: rice, soil fertility, nitrogen use efficiency, greenhouse gas emissions, alternate wetting and drying, urea deep placement, site-specific nutrient management, integrated nutrient management, controlled-release fertilizer, soil organic carbon, methane, climate-smart agriculture

The research team, led by Amanda S. Giroto, set out to build a controlled-release urea fertilizer from carboxymethyl cellulose, or CMC, a renewable, low-toxicity derivative of cellulose that is abundant, highly hydrophilic, and capable of forming films and hydrogels. CMC has long been viewed as a promising scaffold for agricultural delivery systems, but it carries a fundamental weakness: its affinity for water is so strong that, unless the polymer network is stabilized, it swells excessively or dissolves outright, releasing its payload by disintegration rather than by regulated diffusion. The Brazilian group tackled this problem with a dual strategy, crosslinking the polymer chains with citric acid, a cheap and food-safe polycarboxylic acid that forms ester bonds with cellulose hydroxyl groups, and dispersing bamboo-derived biochar throughout the matrix as a physical barrier and adsorptive phase.

The experimental design was deliberately systematic. A 2 percent aqueous CMC solution was prepared, urea was added at 50 percent of the polymer mass, and biochar was incorporated at a fixed 1 percent of total composite mass. Citric acid was then introduced at four levels, 0, 10, 20, and 30 percent by weight relative to CMC, and the mixtures were heated to 80 degrees Celsius for two hours to drive the crosslinking reaction before being cast into self-supporting films and dried. The resulting formulations, labeled CMC/U, CMC/U/BC/CA0, CA10, CA20, and CA30, were probed with X-ray diffraction, infrared spectroscopy, electron microscopy, and thermal analysis, then subjected to swelling tests, water-retention trials in sandy soil, aqueous urea-release experiments, and a 42-day soil incubation tracking ammonia volatilization and ammonium formation.

The structural characterization revealed formulation-dependent changes in the crystalline organization and thermal environment of urea within the matrices. Electron microscopy showed that the non-crosslinked material possessed a smooth surface with well-defined urea crystals, while biochar incorporation produced a rougher, more heterogeneous morphology dotted with porous features. The 10 percent citric acid formulation displayed a relatively cohesive structure, the 20 percent version was the most compact and homogeneous, and the 30 percent sample turned granular and fragmented. Thermal analysis showed that the decomposition temperature of the polymer backbone shifted progressively upward with increasing citric acid content, reaching roughly 284 degrees Celsius at the highest loading, evidence of a more thermally stabilized network. Differential scanning calorimetry, meanwhile, showed that the sharp melting endotherm of pure urea at about 136 degrees Celsius was broadened, reduced, or suppressed in the composites, indicating that urea molecules were dispersed and interacting within the polymeric network rather than sitting as free crystalline granules.

The swelling experiments delivered the study’s most striking numbers. Without citric acid, the composites rapidly lost their structural integrity in water, forming a viscous gelatinous mass that could not even be weighed reliably. In contrast, all crosslinked formulations remained intact throughout immersion, and the 10 percent citric acid composite swelled to approximately 7500 percent of its dry mass within an hour, compared with about 4800 percent at 20 percent crosslinker and 2800 percent at 30 percent. Counterintuitively, the highest swelling did not translate into the fastest nutrient release. In water, pristine urea dissolved almost instantaneously, releasing its entire payload within the first four hours, whereas the CA10 composite had surrendered only about 18 percent of its urea at that point and roughly 80 percent by the end of the eight-day test. The 20 and 30 percent formulations released faster, with the CA20 sample reaching up to 90 percent.

This non-linear relationship between crosslinker content and release behavior is one of the paper’s central insights. The authors argue that beyond an optimal point, additional citric acid restricts polymer-chain mobility, increasing matrix rigidity and morphological heterogeneity in ways that can open preferential diffusion pathways and accelerate transport rather than slow it. Kinetic modeling reinforced the complexity of the mechanism. All composites fitted the Higuchi diffusion model well, but the Peppas–Sahlin analysis showed that urea release involved simultaneous diffusional transport and polymer-chain relaxation, with the relaxational contribution growing from roughly a quarter to nearly 40 percent of the modeled release over the first eight hours. The researchers are careful to note that the data do not support describing the process as purely Fickian diffusion; instead, release emerges from the coupled interplay of hydration, structural reorganization, and solute transport.

Water retention, a critical co-benefit for drought-prone agriculture, also improved. In sandy soil amended with the composites and held at 40 degrees Celsius, the non-crosslinked biochar formulation retained about 59 percent of its initial water after 120 hours, compared with 47 percent for unamended sand, although this difference fell short of statistical significance. At 24 hours, significant differences among treatments were detected, and the biochar-containing matrices showed a consistent tendency toward better long-term moisture preservation, a property the authors attribute to the hydrogel’s water-holding capacity combined with biochar’s porosity.

Perhaps the most consequential results came from the soil incubation. Over 42 days under conditions deliberately chosen to accelerate urea hydrolysis and nitrogen loss, every composite reduced measured ammonia volatilization relative to pure urea. Pure urea lost the most nitrogen as ammonia, 3.74 milligrams, while the uncrosslinked CMC/urea composite lost the least, 2.29 milligrams, a statistically significant reduction. Ammonium dynamics shifted as well: in the first week, the composites sustained higher exchangeable ammonium pools than pristine urea, suggesting that the matrices kept nitrogen in a plant-available mineral form for longer rather than letting it escape as gas. A partial nitrogen recovery accounting, based on the two measured pools, ranged from 56.4 to 72.4 percent of the applied nitrogen, with the remainder classified conservatively as unaccounted because nitrate and matrix-associated nitrogen were not directly quantified.

The practical implications are considerable. Because CMC, citric acid, and biochar are all inexpensive, renewable, and biodegradable, the proposed system sidesteps the central criticism of commercial controlled-release fertilizers, many of which rely on non-biodegradable synthetic coatings that persist in soil. The fabrication route is equally appealing: a simple aqueous process, a single heating step at 80 degrees Celsius, and air drying, with no exotic reagents or energy-intensive procedures. The authors frame the work as demonstrating that one biodegradable polymer matrix, properly architected, can efficiently regulate nutrient release through a simple, scalable, and sustainable strategy for advanced nitrogen fertilizers.

The deeper lesson, however, is about optimization philosophy. Increasing crosslinker content did not monotonically improve performance, and the best formulation was the one that balanced swelling, structural integrity, and diffusion-pathway continuity rather than the most heavily crosslinked one. As agriculture confronts the twin pressures of feeding a growing population and cutting the nitrogen pollution that warms the climate and degrades waterways, studies like this one suggest that the smartest fertilizers of the coming decade may be built not from petrochemical shells but from engineered plant polysaccharides, kitchen-safe acids, and charcoal, tuned with precision at the molecular scale.

Subject of Research: Development of citric acid-crosslinked carboxymethyl cellulose and biochar composites for controlled-release urea fertilizer

Article Title: Controlled urea release from CMC-based composites: effects of citric acid crosslinking and biochar incorporation

Article References: Giroto, A. S., Prado, T. R., Yashima, A., Valle, S. F., Alves, B. L., & Gonçalves, M. (2026). Controlled urea release from CMC-based composites: effects of citric acid crosslinking and biochar incorporation. Polymer Bulletin, 83(11), Article 636. https://doi.org/10.1007/s00289-026-06685-y

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06685-y

Keywords: controlled-release fertilizer, carboxymethyl cellulose, urea, biochar, citric acid crosslinking, nitrogen use efficiency, ammonia volatilization, biodegradable polymer, hydrogel, soil water retention, release kinetics, sustainable agriculture