Rice (Oryza satiava) straw mulch, irrigation and nitrogen effects on productivity and nitrogen-use efficiency of Bt cotton (Gossypium hirsutum) in subtropical environment

A 2-years (2015 and 2016) field study was carried out at Ludhiana, Punjab, to examine the combined effects of residue mulch, irrigation frequency and nitrogen levels on productivity and nitrogen-use efficiency of Bt cotton (Gossypium hersutum L.) in a subtropical environment. Combination of mulch rates, viz. 0 (M₀) and 6 t/ha (M₆), and irrigation regimes, viz. irrigation water to pan evaporation ratio of 0.2 (I_0.20), 0.3 (I_.3) and 0.4 (I_0.4), in main plots; and 4 Nitrogen (N) rates, viz. 0 (N₀), 50 (N₅₀), 100 (N₁₀₀) and 150 (N₁₅₀) kg/ha, in subplots were randomized with 3 repli cations. Mean seed-cotton yield was greater in mulch (3.8 t/ha) than in no-mulch (2.9 t/ha) treatment. Interaction effects of irrigation and N on seed cotton N uptake were significant. Mean seed cotton N uptake increased from 61.4 kg/ha in N₁₅₀ to 80.4 kg/ha in N_0.4 in Iregime; while in I_0.2, uptake increased from 44.9 kg/ha in N₀ to 58.9 kg/ha in N₁₅₀ in crop-growing season of 2015. Mulching enhanced this interaction by causing greaters uptake increase in I_0.2N₁₅₀ than in I_0.4N₁₅₀ regime. Comparable seed-cotton nitrogen uptake in I_0.2M₆N₀ (51.2 kg/ha) and I_0.4M₀N₁₅₀ (65.3 kg/ha) indicated that same N uptake could be realized with less water and nitrogen with mulching. Similar trend in crop-growing season of 2016 was observed. Similarly, nitrogen-use efficiency (5.8 and 6.1 kg seed cotton/kg N applied) was higher under rice-straw mulch than no-mulch treatment. Frequent irrigation regimes resulted in significantly higher seed-cotton yield (3.4 and 3.9 t/ha), total nitrogen uptake (142.4 and 150.3 kg/ha) over restricted irrigation regimes respectively. Among nitrogen rates, the highest nitrogen-use efficiency was observed in N₅₀ level. Pooled analysis showed that M₆I_0.2 treatment had 10.7% higher seed-cotton yield than M₀I_0.04. Higher seed-cotton yield (3.4 t/ha) was recorded with M₆N₀ treatment than M₀N₁₀₀ (3.1 t/ha). Results indicated that the rice-straw mulching saved 1 irrigation and N responses restricted up to 100 kg N/ha. With increase in N fertilization 9.6% gain in yield was noted in N₀ over N₁₀₀ with mulching and it saved 50 kg N/ha with yield grain in Bt cotton.