Controlled Drainage Mediates Cotton Yield at Reduced Nitrogen Rates by Improving Soil Nitrogen and Water Contents

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Controlled drainage (CD) can reduce outflow and improve soil productivity, whereas its impacts on soil nitrogen (N), moisture contents, and cotton (Gossypium hirsutum L.) yield at reduced N rates remain largely unknown. To address the question, a field experiment was conducted to investigate the effects of N fertilizer rates (280, 252, 224, and 196 kg N ha−1, designated N1, N0.9, N0.8, and N0.7, respectively) on soil moisture, nitrate N (NO3−–N), and ammonium N (NH4+–N) contents and yield of cotton under CD and free drainage (FD). Compared to FD, CD increased the soil moisture, NO3−–N, and NH4+–N contents in the 0–40 cm soil depths, leading to greater plant N uptake at each N rate. The decreased total N loss through runoff due to CD accounting for the enhanced soil N contents. Moreover, CD had greater soil NO3−–N content at the reduced N treatments, contributing to the enhanced plant N accumulation, number of bolls, and lint percentage, and thus obtains a relatively high cotton yield. Remarkably, cotton yield reduced significantly with the decrease in N rates under FD, but it was comparable between N1 and N0.9 or N0.8 under CD. These suggested that CD helps to maintain high soil N and moisture contents in the 0–40 cm soil depths at 10–20% reductions in N fertilizer rate and thus without compromising cotton yield. The results also provide scientific basis for drainage water management mediates cotton yield at reduced N fertilizer rates.