Objectives Investigating the effects of foliar spraying of deep eutectic solvents (DES) on winter wheat yield, nitrogen fate will provide a scientific basis for green upgrading and application of traditional liquid foliar fertilizer.

Methods DES were first prepared by mixing choline chloride, ethylene glycol, glucose, and citric acid in ratio of 4∶4∶1∶1 (in mole) in 60℃ for 30 min. and then standing for further 30 min. Commercially available foliar fertilizers containing amino acids and foliar fertilizers containing alginate were also selected for comparison. Winter wheat (Jimai 22) was used as the test crop to carry out a field soil column cultivation trial, using the isotope ¹⁵N urea as a base application of nitrogen fertilizer. Five treatments were setup, as: no nitrogen fertilizer input control (CK), N application plus foliar spraying water (W), containing amino acid (Y1), containing alginate spraying (Y2), and DES spraying (D). Except CK, 0.15 g/(kg, dry soil) of ¹⁵N-urea (¹⁵N 10.09%), and 0.2 g/(kg, dry soil) of P₂O₅ and K₂O were basal applied into top 30 cm soil layer of each cultural column. The SPAD values were monitored before each foliar application. After harvest, wheat ear number, grain number per ear, shoot biomass and grain yield were investigated, the N content (¹⁵N abundance) was tested. The soil samples were collected in 0−90 cm deep, with one sample every 20 cm deep for the determination of ¹⁵N abundance.

Results Compared with W, foliar application (Y1, Y2 and D treatments) significantly increased the flag leaf SPAD values of wheat at late filling stage by 17.14%−32.24%, with significantly higher effect in D treatment; increased wheat grain yield by 17.48%−21.11%, and D treatment also achieved highest yield effect through increased spike number; foliar application significantly increased the total above-ground N uptake by 10.00%−18.33%, D and Y2 also significantly increased fertilizer N uptake of grains by 9.43% and 7.55%, respectively. Foliar application increased the N apparent recovery by 7.98, 8.25 and 14.53 points, D also significantly higher than the Y1 and Y2 treatments by 6.55 and 6.28 percentage points; Compared with W, N fertilizer agronomic efficiency was significantly increased by 34.07%, 31.09% and 41.20%, with optimal D enhancement. The fertilizer N use efficiency of D was significantly increased by 4.19 percentage points compared to Y1, which was significantly different from Y1; compared with Y1 and Y2, the fertilizer N residual rate of D was significantly higher by 7.94 and 8.95 percentage points and the fertilizer N loss rate was significantly lower by 12.13 and 10.73 percentage points.

Conclusion Foliar spraying of DES can significantly increase the chlorophyll content of wheat leaves in the late grouting, delay the senescence of wheat flag leaves, improve wheat grain yield by increasing the number of spike grains, promote the uptake of fertilizer N and soil N by wheat grains, improve the efficiency rate of fertilizer N, and reduce the loss rate of fertilizer N.