Research progress of heat and mass transfer in film mulching cultivation system: materials, theory and application

Mulching cultivation regulates the root-zone microenvironment by modifying the exchange of radiation, heat, water vapor, and gases at the soil–atmosphere interface. Its effectiveness is governed by the selective transport properties of the covering material. As agriculture transitions from conventional polyethylene films—which persist in the environment and cause plastic pollution—to biodegradable alternatives such as polylactic acid, polybutylene adipate terephthalate, plant fiber mulch, and liquid mulch, a critical knowledge gap persists: the heat and mass transfer characteristics of these emerging materials remain poorly quantified. This gap impedes both the rational design of novel materials and the optimization of field application strategies.This review systematically evaluates the heat and mass transfer performance of representative mulch films, focusing on four key parameters: light transmittance, thermal conductivity, oxygen permeability, and water vapor permeability. Substantial differences exist among material types. Polyethylene films offer high transmittance of ranging from 84% to 90%, and moderate water vapor barrier performance, ranging from 0.015 to 0.053 g·mm/(m²·d). Polylactic acid films achieve even higher transmittance—ranging from 88% to 98%—but exhibit significantly elevated water vapor permeability, between 1 and 5 g·mm/(m²·d), which severely compromises moisture retention in dryland farming. Polybutylene adipate terephthalate films provide good flexibility, yet their gas barrier properties are poor, with water vapor permeability ranging from 6 to 15 g·mm/(m²·d), and these properties degrade further upon hydrolysis. Plant fiber films, characterized by porous structures, deliver excellent thermal insulation with thermal conductivity values between 0.04 and 0.07 W/(m·K) and strong weed suppression, but they show very high vapor permeability, reaching 896 to 2489 g·mm/(m²·d). Liquid mulch films, applied as sprayable emulsions, provide moderate moisture retention through the formation of a continuous surface layer. Notably, standardized testing protocols for optical and mass transfer properties are absent across all material categories. Wavelength ranges, weighting functions, and measurement conditions vary considerably among studies, rendering cross-study comparisons largely invalid.In theoretical development, soil heat and mass transfer models have evolved from early static coefficient approaches—built upon Fourier‘s law of heat conduction, Fick’s law of diffusion, and Darcy's law of fluid flow—to dynamic coupled frameworks, most notably the Richards equation for unsaturated flow and the Philip–De Vries model, which unified liquid water and water vapor transport under non-isothermal conditions. Subsequent refinements introduced thermal–hydraulic–mechanical multi-field coupling and advanced numerical techniques including finite element and finite difference methods. However, the majority of existing models still treat the mulch layer as a passive, inert barrier with invariant optical and transport properties. This simplification fails to capture the time-dependent behavior of biodegradable materials, whose transmittance, permeability, and thermal properties evolve markedly over the cropping season as degradation progresses. Although several specialized models have been developed to simulate the sub-film microenvironment—accounting for convective air movement, condensation–evaporation phase changes, and biological activity from roots and soil microorganisms—these efforts remain fragmented and lack a cohesive theoretical foundation.In terms of practical application, current numerical approaches fall into three distinct categories. First, rigorous partial differential equation solvers, exemplified by the HYDRUS series, deliver high physical accuracy and numerical precision, yet require substantial computational resources and extensive parameter calibration. Second, soil–water–atmosphere–plant system models, such as the soil–water–atmosphere–plant model and the Denitrification–Decomposition model, integrate multiple biophysical processes and support scenario-based management analysis, though they represent mulch effects in a highly simplified manner. Third, empirical and semi-empirical models offer computational efficiency and are well-suited for regional-scale assessments, but their extrapolation capability is inherently limited. Case studies illustrate successful applications of each category—from assessing the regional climatic feedback of large-scale mulching, to optimizing nitrogen fertilization under biodegradable film coverage, to establishing simple coverage-to-moisture response relationships for rapid on-farm decision support. Nevertheless, a persistent bottleneck remains: simulation outputs are rarely translated into actionable agronomic guidance, such as site-specific film selection, optimal timing for film application and removal, or integrated water and nutrient management.To overcome these challenges, future research should prioritize three interconnected directions. The first is the establishment of a comprehensive and openly accessible performance database covering all mulch film types—including optical, thermal, mass transfer, and biodegradation metrics—together with standardized testing protocols, with particular emphasis on documenting the dynamic evolution of material properties during field service. The second is the development of multi-field, multi-phase, and multi-scale theoretical models that explicitly integrate degradation-induced changes in film properties and resolve the coupled convection–phase change–biological feedback processes operating within the sub-film microenvironment. The third is the deeper integration of physics-based simulations with data-driven approaches, such as machine learning, to preserve physical interpretability while substantially reducing computational cost, and to translate model predictions into actionable management strategies—specifically, answering which film to select, when to apply it, and when to remove it under given climatic, soil, and cropping conditions. Addressing these interrelated deficiencies will collectively advance film mulching technology toward greater precision, intelligence, and long-term sustainability.