Supplemental fertilizer application is critical for improving rice yield. Pneumatic deep fertilization effectively improves fertilizer utilization, but high-speed airflow may disturb the soil and affect the location of the fertilizer particles. An air–fertilizer separator was developed in this study to separate the fertilizer from the airflow before the two-phase flow rushes into the soil, and the airflow is directed away from the surface of the paddy soil. The structural and operating parameters of the air–fertilizer separator are determined in this paper. A quadratic orthogonal rotation combination experiment was conducted, taking structural parameters of the device as variables, and fertilizer injection speed, separation loss rate, and outlet airflow speed as performance indicators, to optimize the design parameters of the air–fertilizer separator. The variance analysis and surface response analysis of the experimental data are conducted, and the mathematical models between the indicators and the influencing factors are established. The optimal parameters were determined using multi-objective optimization, and the experimental verification was carried out. The optimal parameters for the air–fertilizer separator were obtained as an arc radius of the AFAST of 380 mm, central angle of arc trough of 45°, and depth of primary separation arc-trough of 12.5 mm. The validation experimental results show that the fertilizer injection speed is 21.45 m/s, the fertilizer separation loss rate is 10.22%, and the outlet airflow speed is 42.54 m/s. The experimental values are close to the predicted values, with errors of 1.2%, 1.7%, and 1.3%. The results of the study may provide a reference for the development of an air–fertilizer separator for pneumatic deep fertilization in paddy fields.

Liquid mulch, as a spray-applied film-forming material, has considerable potential for agricultural mulching and soil environmental regulation. However, how liquid mulch spraying parameters affect headspace methyl isothiocyanate (MITC) concentrations remains unclear. The objective of this study was to quantify the effects of nozzle type and spray pressure on the spraying characteristics of liquid mulch and headspace MITC concentration responses and to examine the relationships among measured spraying characteristics, calculated theoretical equivalent film thickness, and AUC₄–₂₄ under controlled laboratory conditions. Three standard flat-fan nozzles (VP110-03, VP110-04, and VP110-05) were evaluated at spray pressures of 0.3, 0.4, and 0.5 MPa using a full factorial design. Soil-column incubation experiments and atomization characterization tests were conducted to investigate the effects of different spraying parameters on headspace MITC concentrations, AUC₄–₂₄, droplet size, droplet velocity, and theoretical equivalent film thickness. The results showed that the AUC₄–₂₄ values of all liquid mulch treatments were lower than that of the unsprayed control, ranging from 1585.32 to 2073.42 mg·h·m⁻³, with relative reductions of 53.16–64.19%. Dv_0.5 showed a weak and nonsignificant negative correlation with AUC₄–₂₄ (r = −0.235, p = 0.543), while mean droplet velocity exhibited a moderate but nonsignificant negative correlation with AUC₄–₂₄ (r = −0.568, p = 0.111). Theoretical equivalent film thickness showed the strongest negative association with AUC₄–₂₄ (r = −0.691, p = 0.039). The piecewise model showed that the relative reduction in AUC₄–₂₄ increased with theoretical equivalent film thickness at lower thicknesses and approached a plateau of 60.64% above an estimated breakpoint of 0.190 mm. Under the present laboratory conditions, a theoretical equivalent film thickness of approximately 0.20 mm may be considered a preliminary reference value. These findings provide a laboratory basis for further evaluating nozzle–pressure combinations, liquid mulch application rates, and suitable theoretical equivalent film-thickness ranges under field conditions.