Effect of slow release urea sources and nitrogen levels on growth and productivity of rice (Oryza sativa L.)

Introduction

Rice (Oryza sativa L.) is one of the most important staple food crops globally, providing food security to more than 60% of the world's population, particularly in Asia. India remains the largest rice-producing country with an estimated area of 51.42 million hectares and production of 151 million tonnes in 2024-25, achieving an average productivity of 3.3 t ha⁻¹. In Telangana, rice occupies about 4.81 million hectares, contributing 17.09 million tonnes with a productivity of 3.20 t ha⁻¹ (DA&FW, 2024-25) [4] . The growing global population, projected to reach 9.8 billion by 2050, demands enhanced rice productivity from existing agricultural land while maintaining environmental sustainability. Nitrogen is the most critical nutrient limiting rice productivity, with conventional urea being the predominant source of nitrogen used by farmers. However, conventional urea application is associated with significant nitrogen losses of 30-70% through various pathways including ammonia volatilization, denitrification, and leaching, resulting in poor nitrogen use efficiency (NUE) of only 31% in irrigated rice systems in Asia (Cameron et al. 2013 and Cassman et al. 2022) [2,3] . These losses not only reduce crop productivity but also contribute to environmental pollution, groundwater contamination, and greenhouse gas emissions. Low nitrogen use efficiency and losses through different pathways due to high solubility of conventional fertilizers necessitate the development of slow-release or controlled-release fertilizers. There are broadly two types: coated conventional fertilizers or inherently lowsolubility fertilizers. Conventional nitrogen fertilizers can be coated with permeable or semipermeable substances, such as sulfur, oilseed cakes, polymers, or resins, to control nitrogen release (Trenkel M. E. (2010) [9] . The applied nitrogen through slow-release fertilizers releases nitrogen slowly over a more extended period during the crop growth cycle, mainly governed by the urease enzyme and microbial processes.Among the known soil urease inhibitors, the chemically synthesized N-(n-butyl) thiophosphoric triamide (NBPT) is currently the most efficient compound. In the presence of soil microbiota, NBPT is converted to its oxoanalogue called N-(n-butyl) phosphoric triamide (oxo-NBPT) which exhibits high capacity for inhibiting soil urease. However, synthetic inhibitors have been reported to cause some adverse effects on plant structure, creating the need for natural alternatives. Natural urease inhibitors derived from plant sources are increasingly recognized as eco-friendly alternatives to synthetic compounds. Substances such as allicin (from garlic), natural oils like cedar wood oil and eucalyptus oil have demonstrated inhibitory effects on urease enzyme activity. Although their inhibition capacity may be slightly lower than synthetic compounds, these natural inhibitors enhance soil microbial activity, improve soil health, and offer long-term sustainability. Recent studies have highlighted the effectiveness of various slow-release nitrogen sources in improving the growth and productivity of rice. The application of STCR-based neemcoated urea has been reported to significantly enhance dry matter accumulation and grain yield compared to conventional nitrogen fertilizers (Sowmya et al., 2025) [7] . Similarly, urease inhibitors such as allicin and NBPT have shown superior performance over traditional neem-coated urea, even at reduced nitrogen levels, indicating improved nitrogen use efficiency and reduced losses (Surekha et al., 2024) [8] . Additionally, the use of silicon-coated urea has been shown to enhance yield and economic returns, particularly in wet direct-seeded rice cultivation (Aruna et al. 2023) [1] . Collectively, these findings suggest that slow-release and inhibitor-coated urea formulations play a crucial role in enhancing nitrogen use efficiency and maintaining rice productivity under various management conditions. Despite extensive research on individual urease inhibitors, comprehensive comparative studies evaluating multiple slowrelease nitrogen sources at different application rates under uniform experimental conditions are limited. Furthermore, cedar wood oil and eucalyptus oil as natural coating materials for urea have received limited attention in field studies. Therefore, the present study was conducted to evaluate the efficacy of various slow-release urea sources on the growth, yield, and nitrogen use efficiency of rice under irrigated conditions.

Materials and Methods

An experiment was conducted during the kharif, seasons of 2023 and 2024 at the research farm of ICAR-Indian Institute of Rice Research (ICAR-IIRR), Rajendranagar, Hyderabad, Telangana, to evaluate the effect of different urease inhibitor-coated urea sources and nitrogen levels on growth, yield, and nitrogen use efficiency of rice. The site is situated at 17°19' N latitude and 78°23' E longitude with an altitude of 542.3 m above mean sea level, representing the Southern Telangana agro-climatic zone and falling under the semi-arid tropics as per Troll's classification. The experimental soil was clay loam in texture, slightly alkaline in reaction (pH 8.05), low in available nitrogen (201.2 kg ha⁻¹) and phosphorus (24.0 kg ha⁻¹), and high in available potassium (404.2 kg ha⁻¹), with 0.55% organic carbon and electrical conductivity of 0.6 dS m⁻¹. The experiment was laid out in a randomized block design with sixteen treatments replicated three times, comprising a total of forty-eight plots, each measuring 5.0 × 4.0 m. The treatments consisted of five different urease inhibitor-coated urea sources applied at three nitrogen levels (100%, 75%, and 50% of the recommended dose of nitrogen RDN) along with a control (no nitrogen application). The treatments were as follows: T₁, 100% RDN through neemcoated urea (NCU); T₂, 100% RDN through allicin-coated urea @ 2000 ppm; T₃, 100% RDN through eucalyptus-coated urea @ 2000 ppm; T₄, 100% RDN through NBPT-coated urea @ 1000 ppm; T₅, 100% RDN through cedar wood oil (CWO)-coated urea @ 2000 ppm; T₆, 75% RDN through NCU; T₇, 75% RDN through allicin-coated urea @ 2000 ppm; T₈, 75% RDN through eucalyptus-coated urea @ 2000 ppm; T₉, 75% RDN through NBPT-coated urea @ 1000 ppm; T₁₀, 75% RDN through CWOcoated urea @ 2000 ppm; T₁₁, 50% RDN through NCU; T₁₂, 50% RDN through allicin-coated urea @ 2000 ppm; T₁₃, 50% RDN through eucalyptus-coated urea @ 2000 ppm; T₁₄, 50% RDN through NBPT-coated urea @ 1000 ppm; T₁₅, 50% RDN through CWO-coated urea @ 2000 ppm; and T₁₆, control (no nitrogen application). The recommended fertilizer dose was 120:60:40 kg ha⁻¹ of N: P₂O₅:K₂O. The short-duration rice variety DRR Dhan-64 (115-120 days; yield potential 6.0-6.5 t ha⁻¹) was used for the study. Seeds were soaked for 24 hours, incubated, and sown in raised nursery beds, and 20-25-day-old seedlings were transplanted at 20 × 15 cm spacing with 2-3 seedlings per hill. Phosphorus (60 kg P₂O₅ ha⁻¹) was applied as basal through single super phosphate, potassium (40 kg K₂O ha⁻¹) through muriate of potash in two splits 75% as basal and 25% at 75 days after transplanting and nitrogen as per treatment through coated urea in three splits (50% basal, 25% at tillering, and 25% at panicle emergence). A water depth of 2-5 cm was maintained during the crop period with drainage at maximum tillering and before fertilizer application, and continuous flooding was followed from panicle initiation to 21 days after flowering. Pretilachlor 50% EC @ 1250 ml ha⁻¹ was applied as pre-emergence herbicide followed by one hand weeding at 30 days after transplanting, and Chlorpyriphos @ 2 ml l⁻¹ was sprayed at 45 days after transplanting for stem borer control. Observations on tiller number, dry matter accumulation, and leaf area were recorded at major growth stages, while grain and straw yields were measured from the net plot area (16 m²) after drying, threshing, and cleaning. Data were analyzed using ANOVA for randomized block design, and treatment means were compared using the LSD test at 5% significance level (Gomez and Gomez, 1984) [5] .

Results

Significant variations were observed in the growth and yield parameters of rice as influenced by different slow-release urea fertilizers during both years of experimentation. The number of tillers m⁻² increased markedly with the application of 100% RDN through cedar wood oil (CWO)coated urea @ 2000 ppm, which recorded the highest values of 350 and 359 m⁻² during kharif 2023 and 2024, respectively, with a pooled mean of 355 m⁻². This treatment was followed by 100% RDN through NBPT-coated urea @ 1000 ppm (pooled 327 m⁻²) and 75% RDN through CWO-coated urea @ 2000 ppm (pooled 326 m⁻²), while the lowest number of tillers (pooled 173 m⁻²) was observed in the control (no application of urea fertilizer). A similar trend was recorded at all successive growth stages, with 100% RDN through CWO-coated urea @ 2000 ppm maintaining the maximum number of tillers at panicle emergence (441 m⁻²), flowering (418 m⁻²), and maturity (386 m⁻²). Dry matter accumulation followed a similar pattern, wherein 100% RDN through CWO-coated urea @ 2000 ppm recorded the maximum values (2649 and 2567 kg ha⁻¹ at tillering; 13685 and 12953 kg ha⁻¹ at harvest, with a pooled mean of 13319 kg ha⁻¹). These values were statistically comparable with 100% RDN through NBPT-coated urea @ 1000 ppm (pooled 13115 kg ha⁻¹) and 75% RDN through CWO-coated urea @ 2000 ppm (pooled 13169 kg ha⁻¹), whereas the minimum dry matter accumulation (pooled 6934 kg ha⁻¹) was recorded in the control. Leaf area was also significantly influenced by the treatments. The highest leaf area (772 and 759 cm² hill⁻¹ during 2023 and 2024, with a pooled mean of 765 cm² hill⁻¹) was observed under 100% RDN through CWO-coated urea @ 2000 ppm, followed by 100% RDN through NBPT-coated urea @ 1000 ppm (pooled 716 cm² hill⁻¹) and 75% RDN through CWO-coated urea @ 2000 ppm (pooled 736 cm² hill⁻¹). The control recorded the lowest leaf area (pooled 460 cm² hill⁻¹). Grain and straw yields showed similar responses to slow-release nitrogen sources. The treatment receiving 100% RDN through CWO-coated urea @ 2000 ppm produced the highest grain yield (6722 and 6560 kg ha⁻¹ during 2023 and 2024, respectively, with a pooled mean of 6641 kg ha⁻¹). This was statistically on par with 75% RDN through CWO-coated urea @ 2000 ppm (pooled 6363 kg ha⁻¹) and 100% RDN through NBPT-coated urea @ 1000 ppm (pooled 6348 kg ha⁻¹), while the control recorded the lowest yield (pooled 2847 kg ha⁻¹). Compared with 100% RDN through neem-coated urea, grain yield improved by 12.8% under 100% RDN through CWO-coated urea @ 2000 ppm and by 15.1% under 75% RDN through CWO-coated urea @ 2000 ppm. A similar trend was recorded for straw yield, where 100% RDN through CWO-coated urea @ 2000 ppm recorded the maximum (8206 and 8073 kg ha⁻¹ during 2023 and 2024, respectively, with a pooled mean of 8139 kg ha⁻¹), followed by 100% RDN through NBPT-coated urea @ 1000 ppm (pooled 7956 kg ha⁻¹) and 75% RDN through CWO-coated urea @ 2000 ppm (pooled 7930 kg ha⁻¹). The control recorded the minimum straw yield (pooled 3973 kg ha⁻¹). The relative improvement in straw yield over neem-coated urea was 10.6% at 100% RDN and 15.5% at 75% RDN through CWO-coated urea @ 2000 ppm. The superior performance of CWO-and NBPT-coated urea treatments can be attributed to their ability to synchronize nitrogen release with crop demand, thereby minimizing losses and ensuring continuous nutrient availability throughout the crop growth period. Cedar wood oil, rich in bioactive compounds such as cedrol, α-cedrene, and β-cedrene, forms a hydrophobic coating around urea granules, reducing urea hydrolysis and volatilization while moderating urease activity. The sustained nitrogen supplies improved tiller initiation, leaf expansion, photosynthetic efficiency, and assimilate translocation, resulting in higher dry matter accumulation and yield. The ability of 75% RDN through CWO-coated urea to produce yields comparable to 100% RDN treatments indicates a potential 25% nitrogen saving without yield penalty, contributing to economic and environmental sustainability. These results agree with Liu et al. (2013) [6] , Surekha et al. (2024) [8] , and Yang et al. (2021) [10] , who also reported improved nitrogen use efficiency and yield with slow-release and urease inhibitor-coated urea sources. Furthermore, the enhanced performance of organic coatings such as CWO, allicin, and eucalyptus suggests that bioactive components in these materials may not only regulate nitrogen release but also impart additional biochemical benefits that enhance nutrient utilization and crop productivity.

Conclusion

It was concluded that the application of 100% RDN through cedar wood oil (CWO)-coated urea proved to be the most effective treatment, resulting in significantly higher growth and yield parameters of rice. This treatment was statistically comparable with 100% RDN through NBPT-coated urea and 75% RDN through CWO-coated urea, which produced yields on par with full nitrogen doses, indicating a 25% saving in nitrogen without yield reduction. These treatments enhanced tiller production, dry matter accumulation, and grain and straw yields by ensuring synchronized nitrogen release, reduced nutrient losses, and improved nitrogen use efficiency. Hence, the use of CWO-and NBPT-coated urea can be recommended as efficient and sustainable alternatives to conventional urea for achieving higher productivity and better nitrogen utilization in rice cultivation.

References

1. Aruna K, Tuti MD, Bhanu Rekha K, Surekha K, Mahender Kumar R. Effect of varying levels and sources of slowrelease urea fertilizers on yield, post-harvest soil nutrient status and economics of wet direct-seeded rice (Oryza sativa L.). Int J Environ Climate Change. 2023;13(9):34673474.
2. Cameron KC, Di HJ, Moir JL. Nitrogen losses from the soil/plant system: A review. Ann Appl Biol. 2013;162:145173.
3. Cassman KG, Dobermann A, Walters DT. Agroecosystems, nitrogen-use efficiency, and nitrogen management. AMBIO J Hum Environ. 2002;31(2):132-140.
4. Department of Agriculture and Farmers Welfare (DA&FW). Agricultural Statistics at a Glance 2024-25. New Delhi: Ministry of Agriculture and Farmers Welfare, Government of India; 2025.
5. Gomez KA, Gomez AA. Statistical procedures for agricultural research. New York: John Wiley and Sons Publishers; 1984. p.357-423.
6. Liu X, Zhang Y, Han W, Tang A, Shen J, Cui Z, et al. Enhanced nitrogen deposition over China. Nature. 2013;494:459-462.
7. Sowmya P, Madhavi A, Jayasree G, Sreelatha D, Triveni S. Comparative effects of neem-coated urea, agrotain urea and nano nitrogen on nutrient uptake of paddy crop. J Exp Agric Int. 2025;47(1):237-246.
8. Surekha K, Manasa V, Gobinath R, Rao DVKN, Mahender Kumar R, Brajendra P. Assessment of genotypic variability for nitrogen use efficiency (NUE) and improving NUE through urease inhibitors in irrigated rice. J Rice Res. 2024;17(1):81-90.
9. Trenkel ME. Slow-and controlled-release and stabilized fertilizers: An option for enhancing nutrient use efficiency in agriculture. 2nd ed. Paris: International Fertilizer Industry Association (IFA); 2010. p.1-160.
10. Yang X, Geng J, Liu Q, Lu X, Zhou W, Hou P. Controlledrelease urea improved rice yields by providing nitrogen in synchrony with crop demand. J Sci Food Agric. 2021;101(9):3820-3830.