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Assessment of phosphorus and zinc dynamics in a rice growing soil of North-Eastern India under contrasting moisture regimes

This study demonstrates that in North-Eastern Indian rice soils, continuous flooding and the combined application of 60 kg P and 25 kg Zn ha⁻¹ maximize grain yield by enhancing nutrient availability and uptake, with phosphorus acting as the primary productivity driver and zinc providing complementary benefits despite minor antagonistic interactions.

Original authors: Tathagata Karmakar, Rohit Kumar Choudhury, Dibyendu Mukhopadhyay, Sumana Balo, Sayan Debsingha, Ganesh Chandra Banik, Ranjan Bhattacharyya

Published 2026-09-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Tathagata Karmakar, Rohit Kumar Choudhury, Dibyendu Mukhopadhyay, Sumana Balo, Sayan Debsingha, Ganesh Chandra Banik, Ranjan Bhattacharyya

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Rice is the lifeblood for billions of people, a staple that fills bowls across the globe and anchors the food security of entire nations. In the low-lying, water-rich landscapes of eastern India, where the soil is often acidic and the rains are heavy, growing this crop is a delicate balancing act. Farmers here face a specific challenge: the soil is frequently missing two critical ingredients needed for healthy plants, phosphorus and zinc. Phosphorus acts as the engine of energy transfer within the plant, while zinc is a tiny but essential helper that keeps enzymes working and protects the plant from stress. When these nutrients are missing, the rice struggles to grow, and the harvest shrinks. But adding them is not as simple as pouring them on; the soil itself is a complex chemical environment where moisture, temperature, and the nutrients interact in ways that can either help the plant or lock the nutrients away, making them useless.

A team of researchers in West Bengal set out to untangle this web of interactions. They wanted to understand how different amounts of phosphorus and zinc, applied to the soil, would behave under two very different water conditions: one where the fields were kept completely flooded, as is traditional, and another where the soil was kept moist but not submerged, simulating a scenario where water is scarce. They grew two different varieties of rice, DRR Dhan 45 and IR 64, in controlled pots and in open fields over two years. By carefully measuring how much of each nutrient the plants absorbed, how much remained available in the soil, and how much grain was produced, they sought to find the perfect recipe for feeding rice in these specific, acidic soils.

The results painted a clear picture of how these nutrients behave. The study confirmed that adding phosphorus was the single most important factor for increasing the harvest. When the researchers applied higher doses of phosphorus, the plants took up more of it, and the grain yield climbed significantly, reaching a peak of 78 quintals per hectare when combined with the right amount of zinc. This high yield was achieved by the IR 64 variety when it was grown in continuously flooded fields with 60 kilograms of phosphorus and 25 kilograms of zinc per hectare. The researchers found that while phosphorus is the main driver of productivity, zinc plays a vital supporting role. Adding zinc did not hurt the plant's ability to use phosphorus; in fact, the two worked together well, provided the zinc was not in excess.

However, the relationship between the two nutrients in the soil was more complicated. The study revealed a subtle antagonism, or a kind of chemical friction, between them. When more phosphorus was added to the soil, the amount of zinc that could be easily extracted by the plant roots decreased slightly. Conversely, adding zinc caused a small drop in the amount of available phosphorus in the soil, though this did not stop the plants from absorbing enough phosphorus to grow well. This suggests that while the two nutrients compete for space in the soil solution, the competition is not strong enough to cause a deficiency if both are applied in balanced amounts. The researchers also observed that the water regime mattered deeply. Continuous flooding generally made nutrients more available to the plants, likely because the waterlogged conditions changed the chemical state of iron in the soil, releasing nutrients that were previously stuck. In contrast, when the soil was only kept at half its water-holding capacity, the nutrients were held more tightly by the soil particles, making them harder for the plants to reach.

To understand the mechanics behind these observations, the team conducted laboratory experiments to see how the soil particles grabbed onto phosphorus and zinc molecules. They found that the way phosphorus stuck to the soil was best described by a model that accounts for the energy of the bond, while zinc followed a different pattern where the energy of attachment changed as more zinc was added. Interestingly, they discovered that temperature played a role in this process. Warmer conditions, which mimic the peak summer heat, seemed to strengthen the bond between the soil and phosphorus, meaning the soil held onto it more tightly. This implies that in hotter climates, the soil might retain nutrients better, but it also means the plant has to work harder to pull them out. For zinc, the effect of temperature was more sensitive to the amount of water in the soil, showing that the interaction between heat and moisture is complex and specific to each nutrient.

Ultimately, the study concludes that for farmers in these acidic, rice-growing regions, the priority should be ensuring the soil has enough phosphorus, as this is the primary key to unlocking a good harvest. Zinc should be added to complement the phosphorus and prevent deficiency, but it does not need to be applied in massive quantities. The research suggests that the best strategy is a balanced approach: apply 60 kilograms of phosphorus and 25 kilograms of zinc per hectare, and maintain continuous flooding if possible. This combination allowed the IR 64 rice variety to reach its highest potential, proving that understanding the quiet, chemical dance between soil, water, and nutrients can lead to a more abundant and secure food supply. The findings offer a practical guide for managing these soils, showing that with the right balance of nutrients and water, the challenges of acidic soil can be overcome to sustain the rice production that so many depend on.

Technical Summary: Assessment of Phosphorus and Zinc Dynamics in a Rice Growing Soil of North-Eastern India Under Contrasting Moisture Regimes

Problem Statement
Rice cultivation in the acidic Terai soils of North-Eastern India is frequently constrained by deficiencies in phosphorus (P) and zinc (Zn). While P is essential for energy transfer and growth, and Zn is critical for enzyme activation and protein synthesis, their availability is highly sensitive to soil chemistry and moisture regimes. In flooded rice ecosystems, the reduction of ferric iron (Fe³⁺) to ferrous iron (Fe²⁺) typically increases P availability but may decrease Zn availability due to the formation of insoluble compounds. Conversely, excessive P application can antagonize Zn uptake. Despite extensive research on individual nutrient behaviors, there is a limited understanding of the simultaneous interactions between P and Zn fertilization, their adsorption thermodynamics, and their combined impact on yield under contrasting moisture regimes (continuous flooding vs. moisture stress) in these specific acidic soils.

Methodology
The study employed a multi-faceted approach combining pot experiments, field trials, and batch sorption studies conducted over two consecutive years (2023–2024 and 2024–2025) at Uttar Banga Krishi Viswavidyalaya, Cooch Behar, West Bengal.

Experimental Design: Two rice cultivars, DRR Dhan 45 (V1) and IR 64 (V2), were grown under two moisture regimes: continuous flooding (W1) and 50% of maximum water-holding capacity (W2).
Nutrient Treatments: Graded levels of P (0, 45, and 60 kg ha⁻¹) and Zn (0, 12.5, and 25 kg ha⁻¹) were applied in a factorial arrangement. Nitrogen and Potassium were applied at recommended doses.
Measurements: Soil and plant samples were analyzed at maximum tillering (85 days after transplanting, DAT) and maturity (115 DAT) for available P, DTPA-extractable Zn, and nutrient uptake. Grain yield was recorded in the field trial.
Sorption and Thermodynamics: Batch incubation studies were conducted at 25°C and 35°C under W1 and W2 moisture conditions. Adsorption data were fitted to four isotherm models (Freundlich, Langmuir, Temkin, and Dubinin–Radushkevich) to determine binding energies and adsorption mechanisms. Thermodynamic parameters (Gibbs free energy, $\Delta G$ ) were calculated to assess reaction spontaneity.
Statistical Analysis: Data were analyzed using ANOVA, and yield prediction models were developed using multiple linear regression and Partial Dependence Plots (PDP) to evaluate the relative influence of nutrient variables.

Key Results

Nutrient Dynamics and Uptake:
- Phosphorus: P application significantly increased soil available P, P uptake, and grain yield. P uptake was positively correlated with soil P availability ( $r = 0.76^{**}$ ).
- Zinc: Zn fertilization significantly increased soil Zn availability and Zn uptake.
- P-Zn Interaction: A clear antagonistic effect was observed where increased P application reduced soil Zn availability and Zn uptake. Conversely, Zn application caused only a marginal decline in soil P availability and did not negatively affect P uptake or yield; in fact, Zn application slightly enhanced P uptake during the vegetative stage (85 DAT).
- Moisture Regime: Continuous flooding (W1) generally enhanced P availability and uptake compared to moisture stress (W2). However, Zn availability was also higher under W1, attributed to the competition of Fe²⁺ and Mn²⁺ for exchange sites, which limited Zn retention.
Sorption Characteristics:
- Model Fit: P adsorption was best described by the Dubinin–Radushkevich (D-R) model ( $R^2 = 0.98$ ), while Zn adsorption followed the Temkin isotherm ( $R^2 = 0.92$ ).
- Thermodynamics: Thermodynamic analysis indicated that Zn adsorption was spontaneous ( $\Delta G < 0$ ), whereas P adsorption was non-spontaneous under the experimental conditions.
- Temperature and Moisture Effects: Continuous flooding reduced binding energy, thereby enhancing nutrient availability. Increased temperature (from 25°C to 35°C) strengthened adsorption, particularly for P, indicating better nutrient retention under warmer conditions. The magnitude of sorption parameters for P increased with temperature, suggesting an endothermic process.
Yield Performance:
- The highest grain yield (78 q ha⁻¹) was achieved with the combined application of 60 kg P ha⁻¹ and 25 kg Zn ha⁻¹ under continuous flooding for the cultivar IR 64 (Treatment P2Zn2V2W1).
- Regression analysis revealed that grain yield was primarily driven by P uptake, which explained 72.80% of the yield variation. While Zn nutrition was necessary to prevent deficiency, additional Zn beyond adequate levels contributed minimally to further yield increases.

Key Contributions

Mechanistic Insight: The study elucidates the mechanisms governing P and Zn retention in acidic Terai soils, identifying D-R and Temkin models as the most appropriate descriptors for their respective adsorption behaviors.
Interaction Clarification: It quantifies the P-Zn antagonism, demonstrating that while high P reduces Zn availability, balanced Zn application does not significantly hinder P utilization, supporting a complementary rather than strictly antagonistic relationship when managed correctly.
Moisture-Temperature Coupling: The research highlights the complex interaction between moisture regimes and temperature on nutrient sorption, showing that flooding reduces binding energy (increasing availability) while higher temperatures increase adsorption strength.
Yield Prediction: Through machine learning and regression approaches, the study establishes that P nutrition is the principal driver of productivity in these soils, with Zn acting as a complementary factor.

Significance and Claims
The paper claims that balanced P and Zn fertilization is a viable strategy to sustain rice productivity in the acidic soils of North-Eastern India, provided that moisture regimes are managed effectively. The findings suggest that P management should be the primary focus for yield optimization, supported by adequate (but not excessive) Zn nutrition to mitigate antagonism and prevent deficiency. The study provides a mechanistic basis for optimizing fertilization strategies under changing climate conditions, specifically addressing how varying moisture and temperature regimes alter nutrient bioavailability. The authors conclude that the treatment combination P2Zn2V2W1 (60 kg P ha⁻¹, 25 kg Zn ha⁻¹, cultivar IR 64, continuous flooding) represents the optimum strategy for this specific agro-ecological zone.

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