Introduction
Waterlogged paddy soils are characterized by a distinct stratification consisting of a thin aerobic surface layer overlying an extensive anaerobic (reduced) zone. In India, Nitrogen Use Efficiency (NUE) in submerged lowland rice is critically low, hovering between 30% and 35%, leading to substantial financial losses, soil health degradation, and environmental pollution.
Mechanisms of Nitrogen Loss in Waterlogged Soils
Due to the distinct redox conditions in flooded soils, nitrogen undergoes multiple transformation pathways leading to significant losses:
- Denitrification: Ammonium (NH₄⁺) applied to the surface is oxidized to nitrate (NO₃⁻) in the thin aerobic layer. This NO₃⁻ subsequently diffuses into the underlying anaerobic zone, where facultative anaerobic bacteria (such as Pseudomonas and Bacillus) reduce it into gaseous nitrous oxide (N₂O) and elemental nitrogen (N₂), which escape into the atmosphere. This represents the primary loss pathway in submerged soils.
- Leaching Losses: Nitrate (NO₃⁻) ions carry a negative charge and are not adsorbed onto the negatively charged soil clay colloids. In porous soils with high percolation rates, NO₃⁻ readily leaches beyond the active root zone into the groundwater, contributing to aquifer contamination.
- Ammonia Volatilization: In floodwaters with elevated pH (often driven by algal photosynthetic consumption of CO₂) or under high temperatures, ammonium (NH₄⁺) is converted into gaseous ammonia (NH₃) and lost directly to the atmosphere.
- Immobilization: When crop residues having a wide C:N ratio (>30:1) decompose in the soil, microorganisms assimilate the available mineral nitrogen into microbial biomass. Although this is a temporary biological tie-up rather than a permanent loss, it creates transient nitrogen deficiency during peak crop growth phases.
Practices to Enhance Nitrogen Use Efficiency (NUE)
Improving NUE in submerged rice requires synchronizing nitrogen availability with plant demand and physically blocking transformation pathways that cause loss:
- Advanced Fertilizer Formulations:
- Slow and Controlled-Release Fertilizers: Mandatory Neem-Coated Urea acts as a natural nitrification inhibitor, retarding the oxidation of NH₄⁺ to NO₃⁻. The introduction of Sulphur-coated urea (Urea Gold) further regulates release rates while supplying secondary nutrients.
- Precision Subsurface Placement:
- Deep Placement of Urea Super Granules (USG): Point-placing USG or briquettes at a depth of 5–10 cm directly into the reduced zone prevents ammonium from diffusing into the aerobic surface layer, thereby minimizing nitrification-denitrification and volatilization losses.
- Site-Specific Nutrient Management (SSNM):
- Diagnostic Tools: Utilizing the Leaf Color Chart (LCC), SPAD chlorophyll meters, and optical sensors (e.g., GreenSeeker) allows real-time nitrogen top-dressing based on actual crop physiological status rather than rigid calendar schedules.
- Agronomic and Biological Interventions:
- Split Application: Applying nitrogen in 3 to 4 split doses synchronized with critical growth stages—such as active tillering and panicle initiation—avoids high basal concentrations susceptible to early-season losses.
- Integrated Nutrient Management (INM): Combining chemical fertilizers with biofertilizers such as Azolla and Blue-Green Algae (BGA) provides a steady, biologically mediated nitrogen supply.
Conclusion
Enhancing NUE in submerged rice systems is imperative for achieving sustainable intensification, lowering the fiscal burden of chemical fertilizer subsidies under schemes like PM-PRANAM, and mitigating agricultural greenhouse gas emissions. A coordinated shift toward slow-release formulations, precision placement tools, and sensor-based management ensures high productivity alongside environmental safety.