Multi-objective optimization of water and nitrogen regimes for drip-fertigated sugar beet in a desert climate

The scarcity of water resources and low water-nitrogen use efficiency largely restrict the sustainable development of sugar beet (Beta vulgaris L.) in desert climates. A three-year field experiment was conducted to investigate the responses of taproot yield, sucrose content, sugar yield, irrigation water use efficiency, nitrogen partial factor productivity, soil nitrate nitrogen residue and economic benefit of drip-fertigated sugar beet under plastic mulch to varying water and nitrogen supplies. There were three irrigation levels (W0.6: 0.60 ETc, W0.8: 0.80 ETc and W1.0: 1.00 ETc in 2019 and 2020; and W0.4: 0.40 ETc, W0.6: 0.60 ETc and W0.8: 0.80 ETc in 2021, where ETc is the crop evapotranspiration) and six nitrogen (N) application rates (N25: 25 kg N ha−1, N60: 60 kg N ha−1, N120: 120 kg N ha−1, N240: 240 kg N ha−1, N360: 360 kg N ha−1 and N480: 480 kg N ha−1). The combined evaluation model based on the overall difference was further used to explore the optimal combination of irrigation amount and N rate for multi-objective optimization. The results showed that there was no significant difference in sucrose content among the irrigation treatments in the same year. Taproot yield, sugar yield, irrigation water use efficiency of both taproot yield and sugar yield increased and then generally stabilized with the increasing N rate under the same irrigation amount. Nitrogen partial factor productivity of both taproot yield and sugar yield decreased with the increase of N rate. There was no significant difference in soil nitrate nitrogen residue among the irrigation treatments under N25, N60 and N120, but increasing N rate increased the soil nitrate nitrogen residue. Net return first increased and then decreased as N rate increased under the same irrigation amount. Based on the combined evaluation model, the multi-target optimization was obtained under W0.6N120. The results indicated that the combination of irrigation amount of 0.60 ETc and N rate of 120 kg N ha−1 was benifical for the sustainable production of drip-fertigated sugar beet in desert climates.