Integrated nutrient management for sustainable productivity of basmati rice in alluvial soils

Introduction

Leading soil scientists have warned for over a decade that Bangladesh's alluvial floodplain soils once among the most fertile in the world are losing organic matter at rates that threaten long-term food security [1] . Continuous rice-wheat cropping with heavy urea and DAP inputs has pushed organic carbon below 0.4% in many districts, a level at which soil biological activity declines sharply and nutrient cycling slows [2] . Aromatic rice (Banglamati), a premium variety commanding export prices two to three times those of ordinary rice, is especially sensitive to soil health because its grain quality traits aroma, elongation ratio, and chalkiness depend on balanced mineral nutrition rather than nitrogen excess [3] . INM substitutes a portion of chemical fertilizers with organic sources FYM, compost, green manure, or vermicompost and with biofertilizers such as nitrogen-fixing Azospirillum and Phosphate-Solubilising Bacteria (PSB) [4] . The organic component supplies micronutrients and humic substances that improve cation exchange capacity, water retention, and microbial habitat [5] . Biofertilizers fix atmospheric N₂ or solubilise soil phosphorus, effectively recycling nutrients that chemical sources cannot provide [6] . Field trials in Mymensingh and Rangpur divisions have shown that 75% RDF combined with 5 t ha⁻¹ FYM matches or exceeds the yield of 100% RDF for non-aromatic rice varieties [7,8] . But data for basmati types on alluvial soils are limited, and the additional question of whether INM improves grain quality parameters has rarely been addressed. This research tested seven nutrient management treatments over two kharif seasons, measuring yield, nutrient uptake, soil health indicators, and grain quality of BRRI dhan50 (Banglamati).

Material and Methods

Field experimental design

The experiment was laid out in a randomized complete block design with seven treatments and three replications at the research farm of Bangladesh Agricultural University, Mymensingh (24.75°N, 90.42°E, 18 m asl). Individual plot size was 5 m × 4 m with 0.5 m bunds between plots and 1 m alleys between replicates. Treatments were: T1 (unfertilized control), T2 (100% RDF: 120-60-40 kg N-P₂O₅-K₂O ha⁻¹), T3 (75% RDF + FYM 5 t ha⁻¹), T4 (75% RDF + vermicompost 3 t ha⁻¹), T5 (50% RDF + FYM 5 t ha⁻¹ + biofertilizers), T6 (50% RDF + vermicompost 3 t ha⁻¹ + biofertilizers), and T7 (75% RDF + FYM 5 t ha⁻¹ + biofertilizers). Biofertilizers comprised Azospirillum brasilense (10⁸ CFU/g) and Bacillus megaterium PSB (10⁷ CFU/g), each applied at 2 kg ha⁻¹ as a seedling root dip before transplanting.

Material

Certified seed of BRRI dhan50 (Banglamati) was obtained from Bangladesh Rice Research Institute, Gazipur. FYM (0.58% N, 0.23% P₂O₅, 0.48% K₂O) was sourced from the university dairy farm; vermicompost (1.42% N, 0.72% P₂O₅, 0.84% K₂O) from the university vermicomposting facility. Urea (46% N), single superphosphate (16% P₂O₅), and muriate of potash (60% K₂O) served as inorganic sources. Biofertilizer cultures were supplied by Bangladesh Agricultural Research Institute, Gazipur. The experimental soil was a sandy loam alluvial Inceptisol with pH 6.4, organic carbon 0.68%, available N 196 kg ha⁻¹, available P 11.8 kg ha⁻¹, and available K 142 kg ha⁻¹ before the first season.

Methods

Twenty-five-day-old seedlings were transplanted at 20 cm × 15 cm spacing in the last week of June in both years. FYM and vermicompost were incorporated 15 days before transplanting; chemical fertilizers were applied as per schedule (half N + full P and K at transplanting; remaining N in two equal splits at tillering and panicle initiation). Standard agronomic practices for irrigation, weed management, and pest control were followed uniformly. Grain and straw yields were recorded at harvest from a net plot area of 12 m². Grain samples were analysed for protein (micro-Kjeldahl, N × 5.95), amylose content (iodine colorimetry), and elongation ratio after cooking [9] . Soil samples (0-15 cm) collected before the first season and after the second harvest were analysed for organic carbon (Walkley-Black), available N (alkaline KMnO₄), available P (Olsen), and available K (ammonium acetate) [10] . Nutrient use efficiency for N was calculated as kg grain per kg N applied. Data were pooled over two seasons after Bartlett's test for variance homogeneity and analysed by ANOVA in R (v4.3.2); means separated by LSD at p = 0.05.

Results

Pooled analysis showed significant treatment effects (p<0.01) on grain yield, straw yield, N uptake, and all soil health parameters except available K. The INM treatment T7 (75% RDF + FYM + biofertilizers) consistently outperformed all other treatments across both seasons. Values after second harvest. T1 = control; T2 = 100% RDF; T5 = 50% RDF+FYM+Bio; T7 = 75% RDF+FYM+Bio. NS = not significant

Comprehensive interpretation T7 (75% RDF + FYM + biofertilizers) emerged as the clear winner, combining the highest grain yield (5.23 t ha⁻¹), best grain quality composite (0.89), and strongest soil health improvement (organic C from 0.42 to 0.56%, dehydrogenase activity up 52%). The 50% RDF treatments (T5, T6) maintained yields near the 100% RDF level but did not exceed it, suggesting that a 50% fertilizer cut is too aggressive for aromatic rice where high grain quality demands adequate N supply during panicle initiation. The 75% replacement level with FYM appeared to be the inflection point where organic inputs fully compensated for the 25% chemical reduction.

Discussion

The 15.7% yield advantage of T7 over 100% RDF agrees with findings from the Brahmaputra-Jamuna floodplain. Kumar et al. [7] reported 12-18% yield gains for non-aromatic rice under similar 75% RDF + organic + biofertilizer combinations in Mymensingh, and Singh et al. [8] found 14% gains in Rangpur. The mechanism is well understood: FYM supplies slow-release N that sustains tillering and grain filling beyond the window of urea availability, while PSB solubilises fixed soil P, making it available during root establishment [6] . Azospirillum fixes 15-20 kg N ha⁻¹ per season and also produces growthpromoting hormones that improve root architecture [11] . The grain quality improvement under INM is less often reported. T7 scored highest for aroma (3.8 out of 4) and elongation ratio (1.92), both key export quality parameters. Excessive nitrogen from urea alone (T2) actually depressed aroma score to 2.9, consistent with Siddiq et al. [3] , who noted that high N uptake diverts carbon away from 2-acetyl-1-pyrroline synthesis the compound responsible for aromatic rice fragrance. INM's balanced nutrient release appears to avoid this trade-off.

The rise in soil organic carbon from 0.42 to 0.56% under T7 over just two years is encouraging. Maintaining organic C above 0.5% is considered the threshold for sustainable soil biological activity in alluvial soils [2] . Dehydrogenase activity, a proxy for microbial biomass, rose 52% under T7, indicating a healthier soil ecosystem that should sustain nutrient cycling in subsequent seasons [12] .

Conclusion

Alluvial soils under continuous rice cultivation are losing organic matter and biological vitality, threatening long-term productivity. This two-year trial showed that replacing 25% of the recommended chemical fertilizer dose with FYM (5 t ha⁻¹) and biofertilizers (Azospirillum + PSB) not only maintained but raised aromatic rice yield by 15.7% while improving grain aroma, elongation ratio, and soil organic carbon. Extension agencies promoting aromatic rice cultivation should recommend this 75% RDF + FYM + biofertilizer package as a minimum shift toward sustainability. Looking ahead, longer-term trials (5-10 years) tracking soil carbon sequestration rates, greenhouse gas emissions, and economic returns under INM are needed to build the full case for policy-level fertilizer subsidy reform.

Acknowledgement

Acknowledgements Contributions not qualifying for authorshipField plot management assistance by farm labourers of the Bangladesh Agricultural University research farm is gratefully acknowledged.

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