Nitrogen use efficiency and vegetative performance of maize treated with ammonium-chitosan nanofertilizer under tropical savanna conditions

Introduction

Nitrogen use efficiency is defined as the proportion of applied nitrogen that is absorbed and utilized by the target crop for productive biomass synthesis, reflecting the net balance between nitrogen supply, uptake, assimilation, and loss processes within the soil-plant system [1] . In tropical savanna environments, nitrogen use efficiency rarely exceeds 35-40% under conventional fertilization practices due to the combined effects of high soil temperatures accelerating urea hydrolysis and ammonia volatilization, intense rainfall events promoting nitrate leaching, and microbial immobilization competing with plant uptake [2] . These losses represent both economic waste and environmental degradation through groundwater contamination and nitrous oxide emissions [3] . Chitosan-based nanofertilizer systems have emerged as a promising technology for improving nitrogen delivery efficiency through controlled-release mechanisms that modulate nitrogen availability according to plant demand [4] . The encapsulation of ammonium ions within chitosan nanoparticle matrices provides physical protection against volatilization while enabling sustained release through diffusion and polymer degradation [5] . This research assessed the agronomic nitrogen use efficiency and temporal growth dynamics of maize treated with ammonium-chitosan nanoparticles under tropical savanna conditions characteristic of southwestern Nigeria.

Material and Methods

The research was conducted at the open-field experimental station of the Ibadan Institute of Agronomic Research, Ibadan (7.38°N, 3.94°E), Nigeria, during the early rainy season (May-August 2023). Mean temperature was 28.4 °C, cumulative rainfall 842 mm, and relative humidity 76-88%. Maize hybrid SAMMAZ-52 was grown in 10-liter containers with ferralitic Alfisol (pH 5.8, OM 1.8%, available N 24 mg/kg). NH4-CS-NP was synthesized by ionic gelation of chitosan (0.5% in acetic acid) with STPP-NH4Cl solution. Particle characterization: 198 ± 18 nm diameter, +32.4 ± 2.8 mV zeta potential, 66.8% encapsulation efficiency. Seven treatments with four replications in CRD: control, urea (150 mg N/kg soil), CS-NP 50, CS-NP 100, NH4-CS-NP 50, NH4-CS-NP 100, and NH4-CS-NP 150 mg/L. Applications commenced at V3 stage (soil drench, 250 mL/container) at 10-day intervals. Growth The temporal chlorophyll analysis revealed a critical distinction between treatment groups: while urea-treated plants showed chlorophyll decline after day 35 reflecting nitrogen depletion, the NH4-CS-NP 100 mg/L treatment maintained increasing chlorophyll levels through day 49, demonstrating the sustained nitrogen release advantage of the nanocarrier system under tropical conditions [5] .

The overall results establish that NH4-CS-NP at 100 mg/L delivers a 43.0% improvement in nitrogen use efficiency over conventional urea (52.6% versus 36.8%) while simultaneously enhancing all vegetative parameters under tropical savanna conditions. The sustained chlorophyll accumulation pattern provides direct evidence that the controlled-release mechanism maintains nitrogen availability throughout the critical growth period when conventional fertilizer nitrogen has been substantially lost to the environment [6] .

Discussion

This research demonstrated that ammonium-loaded chitosan nanoparticles at 100 mg/L significantly improved nitrogen use efficiency and vegetative performance of maize under tropical savanna conditions, with NUE reaching 52.6% compared to 36.8% for conventional urea [1] . These results align with previous reports of enhanced nitrogen delivery through chitosan nanocarrier systems in various cereal crops [3] . However, the magnitude of NUE improvement observed here (43.0% over urea) exceeds the 20-30% improvements typically reported in temperate studies, likely reflecting the greater scope for reducing the higher baseline nitrogen losses characteristic of tropical environments through controlled-release technology [4] . The sustained chlorophyll response through day 49, contrasting with urea chlorophyll decline after day 35, provides a temporal biomarker confirming effective controlled release under fieldrelevant tropical conditions [6] . These findings suggest that chitosan nanofertilizer technology may offer proportionally greater benefits in tropical production systems where nitrogen loss pathways are most active.

Conclusion

In the context of improving nitrogen management in tropical maize production, this research evaluated ammonium-loaded chitosan nanoparticles as controlled-release nitrogen carriers under savanna conditions. Three principal results emerged: nitrogen use efficiency reached 52.6% at the optimal 100 mg/L concentration representing a 43.0% improvement over urea; plant height and biomass were enhanced by 61.2% and 72.8% respectively; and chlorophyll dynamics confirmed sustained nitrogen availability through day 49 compared to urea nitrogen depletion by day 35. Synthesizing these findings, chitosan nanocarriers offer a viable technology for reducing the disproportionate nitrogen losses in tropical agriculture while maintaining superior crop nutrition. Adoption of this technology at the 100 mg/L concentration is recommended for tropical maize systems where nitrogen volatilization and leaching represent primary efficiency constraints.

Acknowledgement

AcknowledgementsThe Ibadan Institute of Agronomic Research is gratefully acknowledged for providing field station access, laboratory facilities, and climate monitoring infrastructure. Technical staff contributions in nanoparticle preparation, field management, and laboratory analyses were indispensable to this research. Financial support was provided by the Tertiary Education Trust Fund (TETFund) through Institutional Research.

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