Nitrogen use efficiency of drip irrigated sugar beet as affected by sub-optimal levels of nitrogen and irrigation

Sugar beet has strong drought resistance and requires a large amount of nitrogen (N) during growth. Can deficit irrigation combined with low N supply of sugar beet improve water productivity (WP) and N use efficiency (NUE)? In this field study in 2020–2021, two irrigation regimes (W1, field capacity (FC) of the 0–60 cm soil layer was not lower than 70% from 45 days after emergence (DAE) to harvest period; W2, FC of the 0–60 cm soil layer was not lower than 50% in 45–90 DAE, but was consistent with that of W1 after 90 DAE) and three N application rates (0 (N0), 150 (N1), and 225 (N2) kg N ha−1) were designed. Then, their impacts on taproot yield (TY), water consumption (WC) and N uptake (NU) were explored. The results showed that there was no significant difference in TY between W2 and W1 treatments under the N1 and N2 levels. Under the N1 and N2 levels, the daily water consumption (DWC, 0.121 and 0.125 mm ℃d−1) and daily N uptake (DNU, 53.6 and 71.6 g N ℃d−1) of W2 treatment were significantly lower than the DWC (0.161 and 0.164 mm ℃d−1) and DNU (60.6 and 86.5 g N ℃d−1) of W1 treatment in 0–90 DAE. The W2N1 treatment had similar WP and higher NUE compared with the W1N2 treatment. In addition, the fertilizer-15N recovery rate in W2N1 treatment was 8.8% higher than that in W1N2 treatment, the fertilizer-15N loss rate was 6.6% lower than that in W1N2 treatment, but there was no difference in the fertilizer-15N residue between the two. This study clarified that implementing W2 deficit irrigation (50% FC at the canopy rapid growth stage (45–90 DAE)) and N1 nitrogen application rate (33% nitrogen reduction based on 225 kg N ha−1) could not reduce TY but improve WP and NUE.