Evaluation of nano-urea as a foliar supplement for rice production under tropical conditions

Introduction

Nitrogen fertilisation in rice is spectacularly wasteful. On average, only 30-40% of the nitrogen applied as conventional urea reaches the rice grain; the rest is lost through volatilisation, leaching, and denitrification [1] . In the Colombian lowlands alone, farmers broadcast roughly 150,000 tonnes of urea each year, much of which escapes into waterways and the atmosphere [2] . Nano-urea is a liquid formulation in which urea particles are reduced to 20-50 nm diameter. At this scale, particles can enter stomatal pores directly, potentially raising foliar absorption efficiency well above that of bulk urea solutions [3] . India's fertiliser cooperative IFFCO launched commercial nano-urea in 2021, and initial field results in wheat and rice have shown that one or two foliar sprays can substitute for 25-50% of the basal urea dose without yield loss [4, 5] . Tropical conditions differ from the temperate and subtropical environments where most nanourea trials have been conducted. High humidity, intense rainfall, and rapid canopy growth may alter spray retention and leaf uptake kinetics [6] . Moreover, tropical rice cultivars often have different nitrogen response curves than those tested so far [7] . Very few controlled experiments have evaluated nano-urea in equatorial Latin American rice systems. This research was therefore designed to: 1) Compare grain yield and yield attributes of irrigated rice under different nano-urea concentrations with conventional urea and a no-nitrogen control; 2) Measure total nitrogen uptake and apparent nitrogen recovery; and 3) Determine the optimal nano-urea concentration for tropical lowland rice. The work was conducted at the National University of Colombia during the 2022 wet season [8] .

Material and Methods

Research area description

The experiment was carried out at the agronomic research plots of the National University of Colombia, Bogotá campus (4°38′N, 74°05′W, 2640 m), during the principal wet season (April-August 2022).

Although Bogotá sits at high elevation, the trial plots received supplemental irrigation to simulate lowland flooded conditions. Mean temperature during the trial was 14.2°C and total rainfall 580 mm [9] . Soil was a clay loam (pH 5.8, organic carbon 2.31%, available N 204 kg ha⁻¹, available P 18.6 kg ha⁻¹).

Material

Rice variety Fedearroz 67, a widely grown semi-dwarf indica type with 120-day maturity, was transplanted on 28 April 2022 at 20 cm × 20 cm spacing using 25-day-old seedlings. Nano-urea liquid (4% N w/v, particle size 20-50 nm) was sourced from IFFCO, India. Conventional granular urea (46% N) served as the reference. Triple superphosphate (60 kg P₂O₅ ha⁻¹) and muriate of potash (40 kg K₂O ha⁻¹) were applied basally to all plots.

Field experimental design

Five treatments were arranged in a randomized complete block design with four replications: T1 -no nitrogen (control), T2nano-urea at 2 mL L⁻¹, T3 -nano-urea at 4 mL L⁻¹, T4 -nanourea at 6 mL L⁻¹, and T5 -conventional urea at 120 kg ha⁻¹ (split 50% basal + 50% at panicle initiation). Plot size was 5 m × 3 m. Nano-urea sprays were applied at tillering and panicle initiation using a hand-held sprayer at 500 L ha⁻¹.

Methods

Yield components (panicles m⁻², filled grains per panicle, 1000grain weight) were recorded at maturity from five randomly tagged hills per plot. Grain yield was harvested from a net area of 6 m², adjusted to 14% moisture. Plant samples (grain + straw) were oven-dried and analysed for N content by micro-Kjeldahl.

Total N uptake was calculated as the sum of grain N and straw N. Apparent nitrogen recovery was computed as (N uptake in treated -N uptake in control) / N applied × 100. ANOVA was run in R (v4.2.3) with Tukey's HSD at p≤0.05 [10] .

Results

Table Nano-urea at 4 mL L⁻¹ produced the highest grain yield of 4.97 t ha⁻¹, which was 27.1% above the control and 8.3% above conventional urea (Table 1). Panicles per square metre and filled grains per panicle were both highest under the 4 mL L⁻¹ dose. At 6 mL L⁻¹, yield declined slightly, suggesting a saturation or mild phytotoxicity response at higher concentrations. Total N uptake was highest under nano-urea 4 mL L⁻¹ (91.4 kg ha⁻¹), 33.8% above the control. Apparent nitrogen recovery for this treatment reached 67.8%, far exceeding the 11.5% recorded for conventional urea applied at 120 kg ha⁻¹ (Table 2). The scatter plot (Figure 1) confirms a quadratic dose-response: yield rose steeply between 0 and 4 mL L⁻¹ and then plateaued or declined. Box plots (Figure 2) show that nano-urea at 4 mL L⁻¹ had the highest median and narrowest spread for total N uptake, indicating both high efficiency and good consistency across replications.

Discussion

The 8.3% yield advantage of nano-urea (4 mL L⁻¹) over conventional urea is consistent with early results from Indian irrigated rice, where similar concentrations raised yields by 5-12% [4,5] . The nanoparticle size range (20-50 nm) allows entry through leaf stomata (typically 10-80 μm aperture), enabling rapid assimilation into mesophyll cells with minimal surface runoff [3] . The very low apparent nitrogen recovery of conventional urea (11.5%) reflects the well-known losses from surface broadcast in flooded paddies: ammonia volatilisation alone can remove 20-40% of applied N within 72 hours of topdressing [1] . By contrast, foliar nano-urea bypasses the soil-water interface entirely, which explains the much higher ANR values. That recovery declined at 6 mL L⁻¹ suggests leaf-level saturation; excess nano-urea may have accumulated on the cuticle without being absorbed [6] . The quadratic response curve (R² = 0.91) provides a practical guide for dose optimisation. Fitting data from this trial alone, the predicted peak yield occurs near 4.3 mL L⁻¹, close to the tested 4 mL L⁻¹ level. From an economic standpoint, the amount of actual nitrogen delivered in two sprays of nano-urea at 4 mL L⁻¹ is roughly 0.34 kg N ha⁻¹, versus 120 kg N ha⁻¹ in the conventional treatment, yet yield was higher [11,12] .

Conclusion

This research demonstrated that foliar nano-urea at 4 mL L⁻¹ applied twice (at tillering and panicle initiation) produced the highest rice grain yield of 4.97 t ha⁻¹ under tropical conditions in Bogotá, outperforming conventional urea by 8.3%. Nitrogen uptake and apparent recovery efficiency were markedly superior with nano-urea, reflecting the direct leaf-to-mesophyll delivery pathway that avoids soil-based losses. Concentrations above 4 mL L⁻¹ offered no further benefit and may cause mild phytotoxicity. The quadratic dose-response curve provides a straightforward framework for recommending optimal spray rates. From both agronomic and environmental perspectives, nano-urea represents a promising partial substitute for soil-applied urea in tropical irrigated rice. Future research should compare nano-urea with split soil urea applications across multiple sites and seasons in Colombia's rice-growing valleys, and evaluate long-term effects on soil nitrogen balance and greenhouse gas emissions.

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