(a) Discuss the primary pathways of gaseous nitrogen losses from soil and the factors affecting nitrogen losses. (20 marks) (b)…

(a) Gaseous nitrogen losses from soil and the factors affecting them

Nitrogen leaves soil as gas by three routes.

Ammonia volatilisation. Ammonium turns to NH₃ gas (NH₄⁺ ⇌ NH₃ + H⁺); with urea, urease first hydrolyses it to ammonium carbonate, raising the pH around the granule. Losses can reach 20–40% from surface-broadcast urea on moist, warm, calcareous or alkaline soils and in flooded rice with floodwater pH above 8.

Denitrification. Facultative anaerobic bacteria (Pseudomonas, Paracoccus, Bacillus) reduce nitrate in steps, NO₃⁻ → NO₂⁻ → NO → N₂O → N₂, when oxygen is short and organic carbon is available; it is large in puddled rice, waterlogged and heavy clay soils. N₂O is a greenhouse gas with about 273 times the warming potential of CO₂ (100 years).

Nitrification-linked and chemodenitrification losses. Nitrification releases N₂O and NO, and nitrite in acidic soils reacts chemically to give NO and N₂.

Factors. Soil pH (high favours NH₃), temperature (losses rise with warmth), moisture and aeration (waterlogging favours denitrification), organic carbon and residue, texture and CEC (clay holds NH₄⁺, but heavy soils stay wet), fertiliser source, rate, placement and timing, rainfall or irrigation, and crop cover. Mitigation includes neem-coated urea, urea deep placement or urea super granules, split doses, urease inhibitors (NBPT), nitrification inhibitors (DCD, nitrapyrin) and alternate wetting and drying in rice.

(b) Groundnut: nutrients, pegging, irrigation, weeds and cropping systems

(i) Nutrients. Groundnut fixes much of its own nitrogen through Bradyrhizobium, so only a starter dose of 20–25 kg N/ha is needed. Phosphorus (about 40 kg P₂O₅/ha) supports roots and nodulation; potassium improves pod filling and oil; sulphur aids oil and protein; and calcium, given as gypsum at about 500 kg/ha at flowering, is essential in the pegging zone, since deficiency causes "pops" (empty pods). Among micronutrients, boron prevents hollow heart, zinc sulphate corrects zinc deficiency, iron (0.5% ferrous sulphate spray) cures chlorosis in calcareous soils, and molybdenum supports nodule nitrogenase.

(ii) Pegging. After fertilisation the ovary base elongates into a gynophore (peg), which grows down and pushes into the soil, where the tip swells into a pod in darkness. Loose friable soil, moisture and calcium are needed; crusting and dry topsoil cause peg failure.

(iii) Irrigation. Critical stages are flowering (about 25–30 days after sowing), pegging (40–45 DAS) and pod development (60–70 DAS); stress at these stages lowers pod number, kernel filling and shelling percentage.

(iv) Weeds. The critical period is 20–45 DAS. Use pendimethalin 1.0 kg a.i./ha pre-emergence, imazethapyr at 75–100 g a.i./ha at 15–20 DAS, quizalofop for grasses, and hand weeding and earthing up at 20–25 DAS and 40–45 DAS, avoiding disturbance after pegging.

(v) Cropping systems. Gujarat: groundnut–wheat or summer groundnut; Tamil Nadu: rice–groundnut and groundnut–sorghum; Andhra Pradesh and Karnataka: groundnut + pigeonpea (about 7:1 or 8:2) intercrop in rainfed areas and rice-fallow summer groundnut; Maharashtra: groundnut–sunflower.

(c) GIS-based soil mapping and yield mapping

A Geographic Information System stores, analyses and displays spatial data. Soil mapping combines GPS-tagged soil samples with satellite imagery (Sentinel-2, LISS-IV), a digital elevation model and geology, and applies geostatistical interpolation (kriging, IDW) or digital soil mapping, based on soil-landscape relationships, to produce maps of pH, organic carbon and nutrient status; NBSS&LUP soil maps and the Soil Health Card programme use this.

Yield mapping records crop yield at each location with a yield monitor and GPS on a combine, or estimates it from NDVI of satellites and drones, to show spatial variability. Together they define management zones for variable-rate fertiliser, irrigation and amendments, raising efficiency and cutting cost. The software used includes ArcGIS and QGIS. GPS only fixes position; GIS analyses it.