Spraying Characteristics of Liquid Mulch and Their Effects on Headspace MITC Concentrations

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%. Dv0.5 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.