Rational Water and Nitrogen Management Improves Root Growth, Increases Yield and Maintains Water Use Efficiency of Cotton under Mulch Drip Irrigation

Abstract

There is a need to optimize water-nitrogen (N) applications to increase seed cotton yield and water use efficiency (WUE) under a mulch drip irrigation system. This study evaluated the effects of four water regimes [moderate drip irrigation from the third-leaf to the boll-opening stage (W₁), deficit drip irrigation from the third-leaf to the flowering stage and sufficient drip irrigation thereafter (W₂), pre-sowing and moderate drip irrigation from the third-leaf to the boll-opening stage (W₃), pre-sowing and deficit drip irrigation from the third-leaf to the flowering stage and sufficient drip irrigation thereafter (W₄)] and N fertilizer at a rate of 520 kg ha^-1 in two dressing ratios [7:3 (N₁), 2:8 (N₂)] on cotton root morpho-physiological attributes, yield, WUE and the relationship between root distribution and dry matter production. Previous investigations have shown a strong correlation between root activity and water consumption in the 40-120 cm soil layer. The W₃ and especially W₄ treatments significantly increased root length density (RLD), root volume density (RVD), root mass density (RMD), and root activity in the 40-120 cm soil layer. Cotton RLD, RVD, RMD was decreased by 13.1, 13.3, and 20.8%, respectively, in N₂ compared with N₁ at 70 days after planting (DAP) in the 0-40 cm soil layer. However, root activity in the 40-120 cm soil layer at 140 DAP was 31.6% higher in N₂ than that in N₁. Total RMD, RLD and root activity in the 40-120 cm soil were significantly and positively correlated with shoot dry weight. RLD and root activity in the 40-120 cm soil layer was highest in the W₄N₂ treatments. Therefore increased water consumption in the deep soil layers resulted in increased shoot dry weight, seed cotton yield and WUE. Our data can be used to develop a water-N management strategy for optimal cotton yield and high WUE.