Defining the “Point of No Return”: Thickness-Dependent Fragmentation Controls Nonlinear Microplastics Emissions from Agricultural Mulch Films

Plastic mulching is a major source of microplastics in agricultural soils, yet field-based emission rates and their controlling factors remain poorly constrained. Here, we carry out a 12 month field study to quantify microplastic emissions from six widely used low-density polyethylene films as a function of thickness (0.005 mm to 0.014 mm) and color. We identify a thickness-dependent pattern featuring an oxidation-dominated initial induction period with negligible physical breakdown, followed by a rapid surge in film fragmentation. Thinner films reach this "surge phase" 2-4 months earlier, doubling the cumulative emissions of their thicker counterparts. While white films exhibit greater physical damage (e.g., cracking and shredding) at equivalent oxidation levels, black films oxidize faster; this trade-off results in comparable long-term emissions across colors. By linking the carbonyl index to mechanical failure and fragmentation kinetics, we define a thickness-dependent "point of no return" (critical carbonyl indices of 0.24 and 0.36 for white and black films)─the threshold where films become unrecoverable and transition into persistent microplastic sources. Our model demonstrates that optimizing the minimum film thickness to match specific crop cultivation periods can reduce microplastics emissions by over 90% with negligible economic impact. These findings provide a quantitative framework to mitigate agricultural microplastic pollution.