Evaluation of slow-release nitrogen fertilizers on growth, yield and nitrogen use efficiency in aerobic rice production systems

Introduction

Nitrogen use efficiency (NUE) is a term that captures how much of the nitrogen applied to a field actually ends up in the harvested grain. In flooded rice worldwide, NUE averages about 33%, meaning two-thirds of the applied N is lost through volatilisation, denitrification, or leaching [1] . Aerobic rice-grown under non-flooded, saturated-to-moist soil conditions-was developed partly to save water, but it also shifts the dominant N-loss pathway from denitrification to leaching, because the soil stays aerobic and nitrate is mobile [2] . Slow-release nitrogen fertilisers (SRFs) address this problem by matching nutrient supply to crop demand. Polymer-coated urea (PCU) releases N through diffusion controlled by coating thickness and soil temperature [3] . Sulphur-coated nitrogen (SCN) relies on microbial degradation of the sulphur shell, while neem-coated urea (NCU) uses neem oil as a nitrification inhibitor that slows the conversion of ammonium to nitrate [4,5] . All three have been tested in irrigated lowland rice in Asia, but their performance in high-latitude aerobic systems-where cooler soil temperatures slow both crop uptake and fertiliser release-is largely unknown [6] . This research aimed to: (a) compare the growth and yield responses of aerobic rice to four N sources, (b) quantify agronomic and recovery NUE for each source, and (c) determine whether a single basal SRF application can replace the conventional three-split urea schedule.

Soil Characterisation

Composite topsoil samples (0-20 cm) taken before sowing showed pH 6.4, organic carbon 2.87%, total N 0.21%, available P (Olsen) 18.3 mg/kg, exchangeable K 0.28 cmol/kg, and bulk density 1.24 g/cm³. Texture was classified as silt loam. Mineral N (NH₄⁺ + NO₃⁻) at sowing was 14.6 mg/kg, indicating moderate background fertility typical of southern Finnish agricultural soils [7] .

Materials and Methods

Materials

The experiment was conducted at the Viikki Experimental Farm, University of Helsinki (60°13′ N, 25°01′ E), during the 2023 growing season (May-September). Rice cultivar Koshihikari (short-grain japonica, 130-day maturity under Finnish photoperiod) was direct-seeded. Four N sources were tested: prilled urea (PU, 46% N), polymer-coated urea (PCU, 42% N, Agrium ESN), sulphur-coated nitrogen (SCN, 39% N, Sulphur Institute standard), and neem-coated urea (NCU, 43% N, prepared by coating PU with 5% neem oil). All were applied at 120 kg N/ha as a single basal dose incorporated to 5 cm depth [8,9] .

Methods

A randomised complete block design with five treatments and four replications gave 20 plots (4 m × 3 m each). The control received no N; all plots received 40-40 kg P₂O₅-K₂O/ha. Sowing was on 15 May 2023 at 80 kg/ha seed rate in 25 cm rows. Soil was maintained near saturation by sprinkler irrigation triggered when tensiometers at 15 cm depth read -15 kPa. Plant height, tiller count, and SPAD readings were recorded at 30-day intervals. Grain yield was harvested from 6 m² net plots at 14% moisture. Agronomic NUE (AEN) was calculated as (yield_fertilised-yield_control)/N applied; recovery NUE (REN) as (N uptake_fertilised-N uptake_control)/N applied × 100. Data were analysed by ANOVA in R; means separated by Tukey's HSD at p < 0.05 [10] .

Seasonal Weather

Mean air temperatures during May-September 2023 were 11.4, 16.7, 19.8, 18.1, and 12.3 °C, respectively. Cumulative rainfall was 318 mm and an additional 147 mm was applied as irrigation. Growing degree days (base 10 °C) summed to 1,287, which was adequate for Koshihikari to reach physiological maturity by 22 September [11] . 1). PCU and SCN were statistically similar to NCU for yield but lower for recovery efficiency.

Results

Discussion

The 49% higher recovery NUE with neem-coated urea versus prilled urea confirms that nitrification inhibition is effective even in cool boreal soils where microbial activity is already slower than in the tropics [4,5] . The neem oil coating likely extended the ammonium phase by 10-15 days, keeping more N in a form less prone to leaching in the aerobic soil profile. PCU performed similarly for yield but slightly lower for recovery, possibly because its polymer shell releases N primarily in response to temperature, and the cool Finnish May (11.4 °C) delayed early-season release beyond the period of peak tillering demand [3,12] . The single basal dose of SRFs simplified field operations and eliminated two top-dressing passes required by the conventional urea schedule. From an economic standpoint, the higher per-unit cost of NCU and PCU may be offset by labour savings and the yield premium, though a formal cost-benefit analysis was beyond the scope of this research [13] . A second season's data would strengthen confidence in these rankings.

Conclusion

Neem-coated urea applied as a single basal dose at 120 kg N/ha produced the highest grain yield (6.52 t/ha) and recovery NUE (51.8%) in aerobic rice at Helsinki. Polymer-coated and sulphurcoated formulations also outperformed prilled urea significantly. Single-application slow-release N can replace three-split urea schedules in high-latitude aerobic rice without yield penalty, while reducing apparent N losses by up to 26%. These sources deserve wider testing across Nordic and Baltic rice trials.

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