Did manure improve saline water irrigation threshold of winter wheat? A 3-year field investigation

Winter wheat production has been constrained by groundwater shortage in North China Plain. Adoption of a suitable saline water irrigation program and fertilizer management can save water and improve yield. A three-year field experiment was conducted in Bohai lowland plain to identify the effects of fertilizer application, saline water irrigation quantity and irrigation water salinity on wheat yield, water productivity and soil properties. Manure and chemical fertilizer were used as basal fertilizer. Saline water irrigation quantities were 80 mm (irrigation at jointing stage) and 160 mm (irrigation at jointing and grain-filling stages, 80 mm per irrigation). Irrigation water salinity levels were 2.0, 6.0 and 8.8 dS m−1. Results showed that greater irrigation quantities improved grain yield by 15.1% regardless of irrigation water salinity with chemical fertilizer in dry year, as shown by the increase in 1000-kernel weight. Manure increased grain yield compared with chemical fertilizer by 15.3%, 37.5% and 12.3% in three seasons 2017–2020 mainly by increasing spike number, regardless of saline water irrigation quantity or irrigation water salinity. Increases in grain yield and aboveground biomass led to higher water productivity. Saline water irrigation threshold increased from 6.0 to 8.8 dS m−1 with deficit irrigation and manure fertilizer due to the stable high yield. Irrigation water salinity of 6.0 and 8.8 dS m−1 decreased organic matter (0–20 cm) by 19.4% and 13.1% compared with salinity of 2.0 dS m−1. Manure increased soil organic matter (0–20 cm) by 20.5% and soil water depletion (0–160 cm) by 68.6% compared with chemical fertilizer, and reduced soil EC by 18.3% and NO3-N by 62.2% at 0–160 cm soil depth. Heatmaps and entropy-TOPSIS analysis showed that the optimum field management for high-yield and water-saving was obtained with the combination of manure fertilizer and saline water irrigation quantity of 80 mm with salinity 8.8 dS m−1.