A Review of Plastic Film Mulching on Water, Heat, Nitrogen Balance, and Crop Growth in Farmland in China

Abstract

Plastic film mulching has been widely used to improve crop yield and water use efficiency, although the effects of plastic film mulching on water, heat, nitrogen dynamics, and crop growth are rarely presented comprehensively. This study investigated a large number of studies in film mulching fields from the past 10 years (mostly from 2019 to 2023) and summarized the impact of plastic film mulching, progress in modeling with film mulching, and future research directions. The effects of plastic film mulching were intricate and were influenced by film mulching methods, irrigation systems, crop types, crop growth stages, etc. Overall, plastic film mulching showed a positive effect on improving soil water, temperature, and nitrogen status, enhancing crop transpiration and photosynthetic rates, and promoting crop growth and yield, although film mulching may have negative effects, such as increasing rainfall interception, blocking water entering the soil, and reducing net radiation income. The crop yield and water use efficiency could increase by 39.9–84.7% and 45.3–106.4% under various film mulching methods. Coupled models of soil water and heat transport and crop growth under plastic film mulching conditions have been established by considering the effects of plastic film mulching on the upper boundary conditions of soil water and heat, energy budget and distribution processes, and the exchange of latent and sensible heat between soil and atmosphere. The models have good applicability in film mulched farmland of maize, rice, and potato for different regions of China. Further development is needed for soil water, heat, nitrogen migration, and crop growth models under different plastic film mulching methods, and the acquisition of soil and crop indicators under plastic film mulching conditions based on big data support. The study will provide reference for the subsequent development and innovation of plastic film mulching technology.

1. Introduction

Plastic film is an important agricultural means of production in the process of agricultural development. As a pioneer in the global research and development of materials for plastic films, Japan began applying plastic films in 1951, and they were subsequently used in developed countries such as the United States, Italy, and France [1]. In 1977, the area of plastic film mulching in Japan accounted for 16.7% of the total dry land area [2]. China has a history of about 50 years in application of plastic film mulching technology since the introduction of plastic film mulching technology in 1978. In 1982, the area of plastic film mulching in China was only 117,000 hm², while the amount of plastic film used in China had reached 1.38 million tons so far, and the area of plastic film mulching had reached 17.63 million hm² [3]. In the past two to three decades, China has consistently ranked first in the world in terms of the area and usage of plastic film mulching. At present, the area and usage of plastic film mulching in China still grow at a rate of about 10% per year [4].

Since the advent of plastic film mulching technology, it has been widely used in the cultivation of vegetables, fruits, cash crops, and food crops in cold regions and water scarce regions [5]. In China, plastic film mulching technology has been demonstrated and promoted in the cultivation of more than 40 crop types in northwest China, northeast China, north China, and coastal areas, achieving significant effects in early maturity, high yield, and high quality [6]. Figure 1 and Figure 2 show that Xinjiang province is the region with the largest area and usage of plastic film in China, with an area and usage of 3.55 million hm² and 242,670 tons, respectively [3].

Plastic film mulching techniques can be divided into two categories. In terms of plastic film mulching materials, more than 10 full-specification, high-performance, and high-tech plastic film mulching materials have been developed [2]. Plastic films can be divided into transparent, white, black, and colored films according to the color of the films; they can be divided into biodegradable film, photodegradation film, reflective film, weeding film, and anti-aging film according to the purpose of the film. From the perspective of film mulching methods, plastic film mulching includes full-film mulching and partial-film mulching according to the mulching ratio; they can be divided into single-layer, double-layer, and multi-layer films according to the layer numbers mulched with film; according to the soil preparation method, they can be divided into flat bed, high bed, furrow bed, hole pit, low bed, high bed, row, and between rows with film mulching; and they can be divided into whole year, whole season, and some growth stage with film mulching according to the length of time.

Plastic film mulching has the advantages of holding soil moisture, regulating soil temperature, improving soil physical and chemical properties, and significantly increasing crop yield [7,8]. The emergence of plastic film accelerated the development of traditional agriculture towards modern agriculture. Plastic film mulching technology has been widely applied, and the impacts of plastic film mulching might exhibit differences in different agricultural management measures, such as different mulching methods, mulching materials, irrigation systems, etc. Therefore, the study aimed to summarize the effects of plastic film mulching in the soil–crop system, and propose the future development direction of plastic film mulching technology, which can provide reference for the subsequent development and innovation of plastic film mulching technology.

2. Film Mulching Effects on Water, Heat, and Nitrogen (N) Balance in Field and Crop Growth

2.1. Film Mulching Effects on Water Balance in Farmland

2.1.1. The Effect of Film Mulching on the Incoming Soil Water

Rainfall and irrigation are the main sources of soil water in farmland. Plastic film mulching could alter the infiltration processes of rainfall and irrigation when applied above plastic film. The effect of plastic film mulching on rainfall and irrigation infiltration is influenced by the method of plastic film mulching [9]. Under film mulching with conventional flat cultivation, water from rainfall and irrigation can enter the soil through the seedling holes, while some of the water falling on the plastic film surface evaporates before it reaches the seedling holes due to the impermeability of plastic films [10]. Therefore, rainfall and irrigation infiltration could be blocked under the conditions of film mulching with conventional flat cultivation [11]. In addition, the effect of film mulching on incoming soil water is generated through rainfall interception. In general, plastic film mulching promotes crop growth. The dense crop canopy under film mulching conditions could intercept more rainwater, resulting in a decrease in the amount of water entering the soil [6]. Therefore, the presence of plastic film mulching reduces the incoming soil water due to the dual effects of the vigorous crop canopy under plastic film mulching conditions on rainfall interception and the impermeability of the plastic film itself on water blocking, which is not conducive to soil water retention and was a negative effect, as shown in

Table 1. Positive and negative effects of film mulching on water, heat, nitrogen balance, and crop growth.

Film Mulching Effects	Positive Effect	Negative Effect
Water balance	Reduces soil water evaporation	Blocks water entering the soil under film mulching with conventional flat cultivation
		Increases rainfall interception by crop canopy
Heat balance	Suppresses sensible heat loss	Reduces net radiation income under transparent film mulching
Nitrogen balance	Improves nitrogen fertilizer utilization in drip irrigation	Blocks nitrogen fertilizer entering the soil in flood irrigation
Crop growth	Promotes crop growth and increase yield	Ripens early and reduces yield

.

In order to sufficiently utilize rainfall under plastic film mulching conditions, a ridge–furrow rainwater harvesting system with film mulching, where the ridge (R) was mulched with plastic film (M) while the furrow (F) was non-mulched, abbreviated as RFM, was proposed in arid and semi-arid areas [12]. This approach could promote water accumulation in the furrow from the ridge with the impermeable plastic film and accelerate the infiltration rate of rainfall and irrigation, and plastic film mulching on the ridge could reduce water evaporation losses [13]. Therefore, the RFM model could increase rainfall and irrigation infiltration and improve the utilization efficiency of input water in farmland [14]. The degree of rainfall utilization efficiency in the RFM model was affected by the width of the mulched ridge and the non-mulched furrow. Too large a ratio of mulched ridge width to non-mulched furrow width could cause a high amounts of rainwater and irrigation water to accumulate in the furrow, and soil water to quickly reach saturation state, thus generating surface runoff and reducing utilization efficiency of rainwater and irrigation water [8]. The underlying surface with a mulched ridge width of 70 cm and a non-mulched furrow width of 50 cm was considered to achieve maximum utilization efficiency for input water in farmland [8].

2.1.2. The Effect of Film Mulching on Soil Water Outcomes

Water consumption in farmland includes soil water evaporation, crop transpiration, deep percolation, and surface runoff. Deep percolation can be ignored in areas with deeper groundwater depths and no surface runoff occurred in flat areas, and soil water evaporation and crop transpiration were considered the most important factors of water consumption. Plastic film mulching had a significant impact on soil water evaporation and crop transpiration [15,16].

In general, plastic film mulching can reduce soil water evaporation (Table 1), as it isolates water exchange between the soil and the outside and increases the resistance to water vapor transfer, thereby weakening the loss of latent heat exchange [17]. Film mulching mainly reduces soil water evaporation during the day, while increasing soil water evaporation during the night [18]. This is because during the day, soil water evaporation is affected by soil surface resistance and soil water content due to the strong atmospheric evaporation capacity. The dual resistance effects formed by plastic film mulching and the “coating effect” of crop canopy make the soil surface resistance under film mulching conditions much higher than those under no mulching conditions, which greatly reduces soil water evaporation [18]. During the night, soil heat storage becomes the main energy source for soil evaporation, and film mulching increases soil temperature during the night, resulting in more soil water evaporation than no mulching [18].

Plastic film mulching promotes root growth, enhances the root water absorption rate, and uses more water for crop transpiration [19]. Crop transpiration in the middle and late stages of maize under film mulching conditions is 8.8% higher than under no mulching conditions [20]. Plastic film mulching can significantly enhance the transpiration of summer maize, and the total crop transpiration increases by 19.2–40.6% and 18.8–31.0% under full-film mulching and partial-film mulching conditions, respectively [21]. Film mulching significantly enhances transpiration due to the following aspects: on the one hand, film mulching promotes aboveground maize growth and increases the leaf area index (LAI) [22]; on the other hand, film mulching changes the soil surface temperature and relative humidity, improves the microclimate of near-surface farmland, and increases the stem flow rate of maize [21].

Plastic film mulching can reduce soil evaporation and increase crop transpiration, which indicates that film mulching reduces evapotranspiration at the early stage of crop growth and increases evapotranspiration at the middle and late stages of crop growth [20]. This is because crop plants are small in the early stage and soil water evaporation is the main part of water consumption. As crops grow, crop transpiration becomes the dominant factor in crop evapotranspiration. Although film mulching promotes more effective absorption and utilization of soil water by plants, the effect of film mulching on the total crop evapotranspiration for the whole growth stages is not certain [23], depending on the degree of the soil water evaporation decrease and crop transpiration increase [6]. More total crop evapotranspiration under film mulching conditions is caused by the dominant role of crop transpiration due to the higher proportion of crop transpiration in total water consumption of over 77% [20]. Plastic film mulching reduces the total crop evapotranspiration, as determined by shortening the length of the crop growth period [24].

2.1.3. The Effect of Film Mulching on the Soil Water Status

Soil water content is the result of incoming and outgoing farmland water. Under film mulching conditions, an increase in input water could increase soil water content, and vice versa. Correspondingly, an increase in water consumption under film mulching conditions could reduce soil water content, and vice versa. In general, film mulching can preserve soil water, and the water retention effect of film mulching is affected by mulching methods, irrigation systems, crop growth stages, etc. Full-film mulching has a stronger soil water retention effect than partial-film mulching, especially in arid areas where irrigation is the main source of incoming water [12]. This is because there is almost no difference in the incoming water between full-film mulching and partial-film mulching (due to low precipitation), while full-film mulching could reduce soil evaporation, thereby increasing soil moisture content. Compared with flood irrigation, the water retention effect under plastic film mulching with drip irrigation is mainly in shallow soil, and the impact on deep soil layers is relatively small [25]. Compared with deficit irrigation, plastic film mulching has a stronger effect on soil water retention under sufficient irrigation conditions [26]. This is because under sufficient irrigation conditions, crop transpiration with plastic film mulching is similar to that without mulching, while plastic film mulching can inhibit soil water evaporation, thereby increasing soil water storage. However, under deficient irrigation conditions, the irrigation amount is relatively small, the intensity of soil water evaporation is relatively weak, and there is little difference in the soil water evaporation between film mulching and no mulching, while the vigorous crop growth under film mulching conditions could consume more water, thereby reducing the soil water content [27]. The water retention effect of plastic film mulching is obvious at the early stage of crop growth, while at the middle stage, the water retention effect of plastic film covering can weaken or even disappear [6]. This is caused by the decrease in soil evaporation caused by film mulching at the early stages of crop growth and the increase in crop transpiration at the middle stages.

2.2. Film Mulching Effects on Heat Balance in Farmland

2.2.1. The Effect of Film Mulching on Heat Incoming in Farmland

Net radiation is the main source of heat for agricultural ecosystems. Net radiation is the difference between incoming net shortwave radiation and outgoing net longwave radiation of the underlying surface. Net shortwave radiation of the soil surface is downward, while net longwave radiation is upward. Plastic film mulching could affect shortwave and longwave radiation by altering the surface albedo, absorption, and soil temperature, indirectly affecting the net radiation [28]. Transparent plastic film mulching increases surface albedo, which reduces the energy income of solar radiation, meaning a decrease in downward net shortwave radiation [29]. Plastic film mulching can block the loss of net longwave radiation on the soil surface, and in addition, the plastic film itself emits the longwave radiation towards the soil surface, which reduces the net long radiation upward on the soil surface [24]. The total net radiation tends to decrease under transparent plastic film mulching conditions (Table 1) because net shortwave radiation is much larger than net longwave radiation. The net radiation is mainly determined by net shortwave radiation, which is influenced by the surface albedo. In farmland with vegetation cover, surface albedo is related to canopy coverage. Before crop emergence, the net radiation is related to the albedo of the plastic film [30]. As the canopy coverage increases, the net radiation is affected by both the albedo of the plastic film and the crop canopy [31]. When the soil surface is completely covered by the crop canopy, the albedo of crop canopy plays a dominant role in net radiation. Therefore, the surface albedo, crop canopy albedo, and canopy coverage have a combined impact on the net radiation throughout the entire growth period of crops [32]. Research shows there is a significant difference in net radiation between film mulching and no mulching during the early stages of maize growth; it is 22.1% lower under film mulching conditions than under no mulching conditions [24]. As the canopy coverage increases, the difference decreases and the net radiation under film mulching becomes consistent with no mulching conditions [24]. This indicates that the impact of plastic film mulching on net radiation is influenced by the canopy coverage.

2.2.2. The Effect of Film Mulching on Heat Emitted in Farmland

In agricultural ecosystems, net radiation is consumed by latent heat exchange, sensible heat exchange, and soil heat flux. The impact of plastic film mulching on latent heat flux is related to its impact on crop evapotranspiration, reducing latent heat loss during the early stage of crop growth while increasing latent heat loss during the middle and late stages of crop growth. Plastic film mulching might increase or decrease the total latent heat consumption, which has been explained in detail when discussing the film mulching effects on water balance in farmland in this study. Plastic film mulching blocks sensible heat exchange between the soil surface and the atmosphere (Table 1), which, in turn, performs heat exchange through heat conduction between the soil surface and the plastic film [33]. The thermal conductivity between the soil surface and the film is much smaller than the turbulent diffusion coefficient, resulting in a decrease in the sensible heat flux under film mulching conditions, with a reduction rate of more than 50% [34]. The different optical properties of plastic films also affected the direction of heat conduction [35]. Under transparent film mulching conditions, the high transmittance allowed the soil surface to directly receive solar radiation, and heat conduction occurred from the soil surface to the film surface [35]. Under black film mulching conditions, a large amount of solar radiation was first absorbed due to the high absorption of black film for solar radiation; then, the radiation energy was transferred to the soil through heat conduction [35]. For soil heat flux, during the day, the soil surface received heat and exchanged it with the underlying soil, while during the night, the heat dissipated from the soil surface. Plastic film could inhibit both the heat gain on the soil surface during the day and the heat loss on the soil surface during the night [36]. Plastic film mulching reduced soil heat flux and the reduction effect weakened with the increase in canopy coverage in maize fields of northwest arid China [24]. However, plastic film mulching increased soil heat flux throughout the entire growth period in the potato fields of northwest arid China [37]. Therefore, plastic film mulching might increase or decrease soil heat flux, which is associated with crop types.

2.2.3. The Effect of Film Mulching on the Soil Temperature

Soil temperature, as an index of soil heat, is determined by the soil heat budget, consisting of the solar radiation balance, soil heat balance, and soil thermal properties. The impact of plastic film mulching on soil temperature is influenced by the film color due to the differences in the energy budget under different colors of plastic film [37]. In general, during the night, different colored films increase soil temperature, which is caused by the decrease in net longwave radiation loss on the soil surface at night [38]. During the day, the soil temperature is influenced by the optical properties of the film. Transparent plastic films can reduce latent and sensible heat losses, thus increasing soil temperature during the day [33]. However, the effect of black film on soil temperature during the day is related to the degree of contact between the film and the soil surface. When the black film is in strong contact with the soil surface, a large portion of the heat absorbed by the black film from the solar radiation can be transferred from the film surface to the soil surface, resulting in higher soil temperatures than without mulching during the day [39]. When the black film is not in close contact with the soil surface, the small thermal conductivity between the film surface and the soil surface transfers a small portion of the solar radiation on the soil surface from the black film surface, resulting in a relatively lower soil surface temperature than without mulching [40]. This indicates that the warming mechanisms of transparent film and black film are different, and the difference in warming effect depends on the thermal conductivity between the soil surface and film surface under black film conditions and the transmission of solar radiation under a transparent film. Compared with no mulching, the effective accumulated soil temperature at 5 cm soil depths increases by 7.9–10.2% under transparent film and by 4.1–4.7% under black film [41].

Transparent film mulching increases soil temperature at the early stage of crop growth, while the warming effect of transparent film reduces as the maize grows [15]. Therefore, there is no significant difference in soil temperature between plastic film mulching and no mulching at the middle and late stages of maize growth. Research has shown that the soil warming effect of plastic film mulching at the early stages of maize growth in arid areas of northwest China is significant; in one study, the soil temperature at a 20 cm depth increased by 4.4 °C compared with no mulching [15]. However, in the rainfed areas of North China, this warming effect is not significant, only increasing by 0.6 °C [42]. Further research in Northwest China showed that the warming effect of film mulching on soil temperature at the early stage of soybean growth firstly increased and then decreased [43], which was because the warming effect of the plastic film could increase with the increase in the air temperature, and the shading effect of the leaves could weaken the warming effect of the plastic film with the continuous growth of crops. The maximum increase in soil temperature at a 20 cm soil depth under film mulching conditions was 1.56 °C compared with no mulching [43]. At the middle and late stages of soybean growth, film mulching could reduce soil temperature and the reduction effect first increased and then decreased [43], which was because the reduction effect of the plastic film could increase with crop canopy growth and it could be slowed to a certain extent when the leaves began to fall off. The maximum decrease in soil temperature at a 20 cm soil depth under film mulching conditions was 0.93 °C compared with no mulching [43]. Research on potato fields in North China showed that the transparent film significantly increased soil temperature throughout the entire growth period by 1.8 °C, with increases of 3.8 °C at the early stages and 1.0 °C at the mature stage [44]. Therefore, the impact of plastic film on soil temperature is influenced by climate conditions, crop types, and crop growth stages.

2.3. Effect of Plastic Film Mulching on Soil N Balance

2.3.1. The Effect of Plastic Film Mulching on the Incoming N by Ground

The demand of soil nitrogen for crop growth is high, and the nitrogen supply capacity of the soil determines the plant growth status [45,46]. Fertilization is the most important source of soil nitrogen in crop ecosystems. Under film mulching with traditional border irrigation, some nitrogen fertilizer could be blocked to the outer surface of the film, reducing fertilization efficiency due to the presence of plastic film (Table 1) [47]. With the development of drip irrigation technology under film, irrigation water and fertilizer can be directly transported to the soil through drip irrigation systems, reducing the ineffective loss of nitrogen and effectively utilizing fertilizers (Table 1) [48], which solves the problem of low fertilization efficiency under film mulching with border irrigation.

2.3.2. The Effect of Plastic Film Mulching on N Emitted by the Ground

The main pathways of nitrogen loss in farmland are NH₃ volatilization, N₂O emissions, and N leaching. Film mulching inhibits the gas flow between the soil and the atmosphere and changes the pathway of NH₃ volatilization, reducing NH₃ volatilization and the loss of ammonium nitrogen [49]. Soil water content and soil temperature are important physicochemical properties that could affect NH₃ volatilization, N₂O emissions, and N leaching [50]. Film mulching can indirectly affect NH₃ volatilization, N₂O emissions, and N leaching by influencing soil water and heat conditions. Appropriate soil water content and soil temperature under film mulching conditions could promote urea hydrolysis and organic nitrogen mineralization, increase the concentrations of NO₃-N and NH₄-N in the soil, and accelerate the conversion of NH₄⁺ to NH₃, which increased the volatilization rate of NH₃ and the risk of N leaching [51]. Plastic film mulching could improve the soil water and heat conditions and enhance the activities of nitrification and denitrification bacteria and enzymes, thus increasing N₂O emission [52]. Compared with no mulching, more inorganic nitrogen under film mulching conditions could be absorbed by the roots of crops, which increases the nitrogen utilization efficiency and reduces the content of NO₃-N in shallow soil, which indicates that the amount of NO₃-N leaching is reduced and N₂O emissions are not significantly increased, and maybe even reduced [53].

The impacts of film mulching on NH₃ volatilization, N₂O emissions, and N leaching are also related to the growth stages of crops. RFM not only reduces NH₃ volatilization and delays nitrification before overwintering for winter wheat; it also suppresses the N leaching caused by rainfall, so that more effective nitrogen is retained in the root zone at the later growth stage of winter wheat [49]. NH₃ volatilization under film mulching conditions is greater than that without film mulching after the regreening stage for winter wheat [49]. However, for maize, film mulching improves the soil hydrothermal conditions and increases N₂O emission flux throughout the entire growth period. The average leaching amount of nitrate nitrogen for the entire maize growth periods under film mulching conditions (2.83 g/kg) was higher than that of no mulching (2.63 g/kg) [54]. For each growth stage, plastic film mulching reduced the amount of nitrate nitrogen leaching at the seedling stage, while increasing the amount of nitrate nitrogen leaching at the heading and jointing stages [54].

2.3.3. The Effect of Plastic Film Mulching on the N Content

Soil NO₃-N and NH₄-N are the two main forms of soil nitrogen. Soil NO₃-N is the most important nutrient for crop growth; it is easily soluble in soil water, and its movement and distribution in soil profiles are mainly influenced by soil water movement. Film mulching can significantly increase the contents of soil NO₃-N, soil NH₄-N, total nitrogen and alkaline nitrogen, increasing soil fertility in flat land and the sloping farmland of semi-arid areas [55]. Film mulching significantly inhibits soil evaporation, increases soil water content and NO₃-N concentration, and increases nitrogen absorption by crop roots and their transport to stems, leaves, and fruits [56,57]. Film mulching increases the content of soil NO₃-N at a 0–20 cm soil depth by 24.8–43.5%, reduces soil NO₃-N at a 40–100 cm soil depth by about 12.1%, and exhibits no significant effects on the soil NO₃-N at a 20–40 cm soil depth in soybean fields [43]. The content of soil NH₄-N at 0–100 cm soil depths under film mulching conditions shows a significant increasing trend with an average increase of 33.7% compared with no mulching [43]. Before overwintering for winter wheat, the content of soil NH₄-N under RFM conditions was observed to be 166.7% higher than that under no mulching, and the content of soil NO₃-N under RFM conditions was lower than that under no mulching [49]. After turning green for winter wheat, the difference in the content of soil NH₄-N between RFM and no mulching was small, and the content of soil NO₃-N was higher under RFM than that under no mulching [49]. It has been suggested that RFM has the effect of inhibiting or delaying nitrification before winter wheat overwintering, and exhibits a nitrogen retention effect after turning green. Polyethylene plastic film and biodegradable film significantly reduce the NO₃-N distribution and the average contents of soil NO₃-N at 0–100 cm soil depths under polyethylene plastic film mulching, biodegradable film mulching, and no film mulching were 0.078, 0.077, and 0.11 mg/cm³, respectively [58]. It can be seen that the impact of plastic film mulching on soil nitrogen is influenced by crop type, crop growth period, climate conditions, etc.

2.4. Effect of Plastic Film Mulching on Crop Growth

2.4.1. The Effect of Plastic Film Mulching on Crop Transpiration and the Photosynthetic Rate

Transpiration is the main process by which crops absorb and transport water, and is one of the most important components in the crop growth and development process. The leaf temperature of crops can be reduced through crop transpiration, and photosynthesis and dry matter accumulation are further promoted [59]. Photosynthesis provides material and energy for crop growth, and is the foundation for crop yield [60]. Film mulching could preserve soil water and increase soil temperature and increase the photosynthetic and transpiration rates of crops, thereby achieving the goal of increasing crop yield and improving irrigation water efficiency [61]. However, the photosynthetic and transpiration processes of crops are extremely complex physiological processes, and are affected by light, air temperature, humidity, crop types, and mulching methods, resulting in differences in film mulching impacts on crop physiological indicators and crop yield. Film mulching can promote the opening of stomata in maize plants and increase the photosynthesis rate, transpiration rate, and chlorophyll content [62]. Film mulching significantly increases the photosynthetic rate of cotton at the flowering and boll stage by 116.4% [63]. During the tasseling period of millet, film mulching increases the photosynthetic rate and water use efficiency (WUE) by 9.80–28.50% and 3.01–18.10%, respectively [64]. A study on the photosynthetic physiological response characteristics of spring maize showed that the yield, WUE, and photosynthetic rate of spring maize under plastic film mulching with drip irrigation were 20.90–22.40%, 13.60–21.60%, and 12.90–22.80% higher than those of no mulching, respectively [65]. Film mulching could only increase the chlorophyll and photosynthetic rate under sufficient water and nitrogen conditions, while reducing the chlorophyll and photosynthetic rate under insufficient water and nitrogen conditions [66]. Both RFM and flat film mulching can increase the chlorophyll content, stomatal conductance, transpiration rate, and photosynthetic rate during the jointing and flowering stages of maize, increasing the photosynthetic rate by 12.4–52.9% and 20.2–45.1% at the jointing and flowering stages of maize, respectively. However, during the filling period of maize, RFM can reduce the photosynthetic rate and transpiration rate by 24.0%; flat film mulching can increase the photosynthetic rate by 12.9% [62]. The effect of plastic film mulching on photosynthesis and the transpiration rate is also influenced by tillage methods. It can increase the photosynthetic rate of winter wheat and reduce the transpiration rate during the filling period under tillage condition, while under no tillage conditions, it reduces the photosynthetic and transpiration rates of winter wheat during the filling period [67].

2.4.2. The Effect of Plastic Film Mulching on Crop Biomass and Yield

Film mulching increases the leaf area index (LAI) during the early and middle stages and reduces the LAI during the late stage because of the rapid senescence of leaves, which is caused by the higher soil temperature and soil water content [68]. Film mulching increases the aboveground biomass of crops by 13.7–34.5%, as well as the underground biomass. The number of ears for maize under film mulching conditions was increased by 11.2–53.4%, and by 2.6–24.7% for the weight of 100 grains, which was conducive to improving yield by 30.1–92.3% [62]. In semi-arid areas, plastic film mulching can significantly increase the soil water content and soil temperature, ensure crop water supply, and enhance light from the soil surface to the canopy, thereby promoting the photosynthetic capacity of crop leaves (photosynthetic rate (P_n) and photosynthetic efficiency (IPAR)) and ultimately significantly improving crop productivity [69]. Film mulching significantly improves the yield factor and biomass yield, thereby increasing the grain yield, which is related to the IPAR and nutrient absorption [62]. The impact of plastic film mulching on crop yield is influenced by the soil water and heat conditions, and film mulching can significantly increase crop yield under sufficient rainfall conditions due to the provision of suitable soil water conditions [70]. Full-film mulching demonstrates a significant increase in yield compared with partial-film mulching, because full-film mulching can provide more suitable hydrothermal conditions than partial-film mulching, and improvements in hydrothermal conditions induced by partial-film mulching are limited [62]. In terms of research within China, the crop yield and water use efficiency could increase by 39.9–84.7% and 45.3–106.4% under various film mulching methods, including full-/partial-film mulching with a ridge or without a ridge [1]. However, film mulching might also lead to early crop maturity due to excessive soil temperature increases, posing a risk of reducing the yield [71], as shown in Table 1.

3. Development of a Crop Model under Film Mulching Conditions

The crop model, also known as the crop growth simulation model, is a computer program based on crop growth and development theory that can quantitatively describe crop growth and development stages, organ formation, dry matter accumulation and distribution, and yield. It could also reflect the impact of environmental factors on crop growth and development. Crop models are powerful tools for predicting crop growth and yield under various agricultural management measures such as film mulching, irrigation systems, and water and fertilizer management. However, most crop models do not involve plastic film mulching; crop models under plastic film mulching conditions have been developed by considering the impact of plastic film mulching on farmland water and heat flux based on existing crop models.

3.1. Crop Model under Film Mulching Conditions Based on an Existing Model

For current mainstream crop models, such as SWAP (Soil–Water–Atmosphere–Plant) [72], WAVES (the WAter Vegetation Energy and Solute model) [73], DNDC (denitrification and decomposition) [74], and HYDRUS [75], soil water movement, heat flow, and solute transport processes can be described using Richards, heat conduction, and convection diffusion equations, respectively, under given initial and boundary conditions. Plastic film mulching, as a barrier layer, can regulate rainfall and inhibit soil evaporation, thus affecting the soil moisture status along the soil profile. Plastic film mulching can regulate soil surface temperature, thus improving soil temperature conditions along soil profile. Plastic film mulching mainly changes the water and temperature distribution characteristics of the soil profile by altering the soil water and heat transport boundary conditions (rainfall, soil evaporation, and soil surface temperature), and then changing the crop growth status.

The influence of film mulching on the upper boundary of soil water movement was generally investigated by integrating the impact of film mulching on soil evaporation and rainfall into a soil water module to simulate the process of soil water movement under film mulching conditions. Soil evaporation under film mulching conditions was generally considered to decrease linearly [76] or in a power–function relationship with the film mulching ratio [6], as shown in Equation (1).

$$E_{\mathrm{m}}={(1-{\delta }_{\mathrm{m}})}^{\mathrm{a}}\cdot E$$

(1)

where E and E_m are soil evaporation under the conditions of no mulching and film mulching, respectively (mm); δ_m is the film mulching ratio, and means the ratio of the actual mulching area of plastic film to the land area; and a is a constant—when a = 1, it means that the relationship between soil evaporation under film mulching conditions (E_m) and the mulching ratio (δ_m) is linear, and when a ≠ 1, it is in a power–function relationship between E_m and δ_m.

Under the conditions of flat film mulching, the amount of rainfall infiltrated into the soil was influenced by the interception effect of the film, which could be expressed by Equation (2) [6].

$$P_{\mathrm{m}}=(1-{\Delta }_m)\cdot P$$

(2)

where P and P_m are the rainfall infiltration amounts (mm) under the conditions of no mulching and film mulching, respectively; Δ_m is the retention rate of the plastic film.

In the soil temperature module, the influence of film mulching on the upper boundary of soil heat flow was determined by introducing parameters such as film thickness and film thermal conductivity to quantify the thermal effect of film mulching, as shown in Equations (3) and (4) [77].

$$T_{\mathrm{mtop}}=T_{\mathrm{top}}+\frac{dH}{{\mathrm{C}}_{\mathrm{soil}}\mathrm{d}V}$$

(3)

$$dH=\left(T_{\mathrm{m}}-T_{\mathrm{top}}\right)\cdot D_{\mathrm{m}}\cdot {TC}_{\mathrm{m}}\cdot dt$$

(4)

where T_top and T_mtop are the soil surface temperature under no mulching and film mulching, respectively (°C); T_m is the film surface temperature (°C) and can be estimated by the air temperature, such as T_m = T_a or T_m = (T_a + T_max)/2; T_a and T_max are the average air temperature and maximum temperature, respectively; dH is the heat flux from the film surface to the soil surface (J); C_soil is the soil specific heat capacity (J cm⁻³°C⁻¹); V means the soil volume, with dV set to 1 cm³; D_m is the film thickness (cm); TC_m is the thermal conductivity (J cm⁻¹°C⁻¹d⁻¹); and t is the unit of time (d).

In crop models, it is generally believed that the soil surface temperature is consistent with air temperature; thus, air temperature can be used to simulate crop physiological processes, including phenology development stages, photosynthesis, and respiration. However, under plastic film mulching conditions, the soil surface temperature is significantly higher than the air temperature, which could cause great errors in describing the crop phenology development process using air temperature. The soil temperature at a 5 cm soil depth can be used to characterize phenology stages to achieve good simulation results for the crop growth process under plastic film mulching conditions. Taking the SWAP model as an example, crop growth and development stages are described using Equations (5) and (6) [6].

$$D_{\mathrm{s}}(JD+1)=D_{\mathrm{s}}(JD)+\frac{T_{\mathrm{eff}}}{T_{\mathrm{sum},i}}$$

(5)

$$T_{\mathrm{eff}}=\left\{\begin{array}{ll}0\hfill & \hfill (T_{\mathrm{a}}<T_{\mathrm{b}})\hfill \\ T_{\mathrm{a}}\le T_{\mathrm{b}}\hfill & \hfill (T_{\mathrm{b}}\le T_{\mathrm{a}}\le T_0)\hfill \\ T_0\le T_{\mathrm{b}}\hfill & \hfill (T_{\mathrm{a}}>T_0)\hfill \end{array}\right.$$

(6)

where D_s(JD + 1) and D_s(JD) are the crop development stage when DoY is JD + 1 and JD, respectively; DoY is the day of the year, which means the number of days after January 1st; T_eff is the effective temperature (°C); T_sum,i is the temperature sum (°C) required to complete either the vegetative or the reproductive stage and is given by the user; T_b is the base point temperature (°C); and T₀ is the optimum temperature (°C).

For crop models lacking a soil temperature module, such as the AquaCrop model, in order to accurately describe the impact of temperature increase under film mulching conditions on crop phenology, the compensation effect of film mulching on air temperature was considered to realize the impact of soil temperature on crop growth and development. The compensation can be expressed through Equations (7) and (8) [71].

$$K=\frac{T_{\mathrm{cum}-\mathrm{a}-\mathrm{FM}}-T_{\mathrm{cum}-\mathrm{a}-\mathrm{NM}}}{T_{\mathrm{cum}-\mathrm{s}-\mathrm{FM}}-T_{\mathrm{cum}-\mathrm{s}-\mathrm{NM}-\mathrm{FM}}}$$

(7)

$$\Delta T=\frac{K\times (T_{\mathrm{s}-\mathrm{FM}}-T_{\mathrm{s}-\mathrm{NM}})\times (T_{\mathrm{a}}-T_{\mathrm{b}})}{T_{\mathrm{s}-\mathrm{NM}}–T_{\mathrm{b}}}$$

(8)

where K is the compensation coefficient of the increasing effect of film mulching on air temperature; T_cum-a-FM and T_cum-a-NM are the effective air accumulation temperature under film mulching and no mulching conditions, respectively (°C); T_cum-s-FM is the effective soil accumulation temperature at a 5 cm soil depth under film mulching conditions (°C); T_cum-s-NM-FM is the effective soil accumulation temperature at a 5 cm soil depth under no mulching conditions during the same period as under the film mulching (°C); ΔT is the compensation value for the soil temperature increase under film mulching conditions to air temperature (°C); and T_s-FM and T_s-NM are the daily average soil temperatures at 5 cm soil depths under film mulching and no mulching conditions (°C).

Crop models under plastic film mulching conditions have been established by integrating the above process, including the impacts of film mulching on the boundary conditions of soil water and heat transport, and crop growth and development stages into existing models such as SWAP, AquaCrop, DNDC (denitrification–decomposition), and WHCNS (water–heat–carbon–nitrogen system) models [68,69,70]. The models have been verified, obtaining good applicability in soil water and heat dynamics and crop growth processes under film mulching conditions in maize fields in northwest and northeast China, and in rice fields in southern China [76,77].

3.2. Crop Model under Film Mulching Conditions Based on Energy Budget

Compared with the above model, where the boundary conditions of soil water and heat transport are directly given by the user, the models where they are determined by the energy exchange processes estimated from meteorological data are more rational. The water and heat transfer module of the models represents a relatively comprehensive soil–plant–atmosphere continuum (SPAC) water and heat transfer model established by combining soil water and heat dynamics, micrometeorology, and energy balance principles. It consists of energy balance and water and heat transfer processes above the surface, as well as soil water and heat transfer processes below the surface. The former provides the upper boundary conditions of soil water and heat transport for the latter.

The model under plastic film mulching conditions has been modified based on existing models. Considering the water and heat transfer from film surface to reference surface, the Shuttleworth Wallace (SW) model under film mulching conditions, named the MSW model, was established by introducing the resistance of film mulching and mulching ratio [78]. The surface albedo affects the water and energy cycle in the atmosphere, land, and sea by directly affecting the distribution of solar radiation between the surface and the atmosphere, thus influencing biological–chemical processes such as photosynthesis, the respiration of plants, and water and heat transfer processes of the underlying surface [79]. Under partial mulching conditions, as the crop canopy changes continuously, the surface albedo of the underlying surface is influenced by the canopy coverage and film mulching ratio, resulting in three types of reflective surfaces: canopy, film, and soil [32]. In response to the diversity of surface albedo and the complexity of surface radiation transfer in farmland with film mulching, a dynamic parameterization theoretical model for surface reflectance in film-covered farmland, the MICA model was proposed by establishing suitable dynamic parameterization formulas for the albedos of soil surface, film surface, and crop canopy surface [32]. Subsequently, the MICA model was embedded into the MSW model to construct the SWIM model for water and heat transfer in mulched farmland, which enhances the accuracy of energy transfer and distribution, water and heat flux, and other processes under film mulching conditions [80].

The models applicable to plastic film mulching conditions have been developed based on water and heat transfer in the SPAC system. Under plastic film mulching conditions, numerical simulation models for energy balance and heat transfer under different optical properties of plastic film and mulching ratios are usually established by considering the effects of plastic film on longwave and shortwave radiation of the soil surface, energy distribution, and aerodynamic resistance, revealing the energy distribution and heat transfer process of soil surface under plastic film mulching conditions [35,81]. On this basis, researchers have considered the impact of crops and divided the agroecosystem into four layers: soil, mulch, crops, and atmosphere. The impact of film mulching on surface energy reflection and water vapor exchange between the underlying surface and the atmosphere has been quantified, and the dynamic transfer processes of water, heat, and CO₂ in soil–mulch–plant–atmosphere systems (SMPAS) under film mulching conditions have been described in detail [81,82,83]. Furthermore, photosynthetic effective radiation transmittance and canopy coverage have been introduced, fully considering the energy transfer process in the dynamic canopy, mulch, and soil to construct a heat transfer model for the drip irrigation of potatoes under film mulching conditions [33]. The above models have been validated to be applicable for the simulation of water and heat transfer and crop growth dynamics in farmland under the conditions of different film mulching ratios [35], as well as under plastic film with different colors [33]. The energy transfer process in the SMPAS under plastic film mulching is shown in Figure 3.

Construction concepts for the two abovementioned types of crop models under film mulching conditions are summarized in

Table 2. Construction ideas for crop models under film mulching conditions.

Crop Models	Crop Models Where the Boundary Conditions of Soil Water and Heat Transport Were Directly Given by the User		Crop Models Based on Energy Exchange Processes
Top boundary condition of soil water movement	Soil water evaporation	Equation (1)	Determined by energy balance with film mulching	Figure 3
	Rainfall interception	Equation (2)
Top boundary condition of soil heat flow	Soil surface temperature	Equations (3) and (4)
Crop growth	Crop growth and development stages controlled by soil temperature at a 5 cm depth	Equations (5) and (6)	Crop growth and development stages controlled by soil temperature at a 5 cm depth	Equations (5) and (6)

.

4. Future Research Trend in Plastic Film Mulching

4.1. Acquisition of Soil and Crop Indicators under Plastic Film Mulching Based on Big Data Support

The effects of plastic film mulching have been concluded to be relatively mature, although related research has mostly been based on the point scale in field. Soil and crop data monitored at the point scale might have inaccuracies in reflecting the status of soil water, heat, nitrogen, and crop growth for the entire farmland. Further remote sensing recognition under plastic film mulching and the acquisition of crop phenotype data using multi-scale sensors and optical imaging technology are needed. This can provide big data decision-making support for crop cultivation under plastic film mulching and the development of precision agriculture.

4.2. Exploration of New Mechanism under Film Mulching Conditions

Comparative studies have been conducted on the effects of mulched ridge-non mulched furrow and flat plastic film mulching, full-film mulching, and partial-film mulching on soil water, heat, and nitrogen migration and crop growth. Other mulching methods, such as plastic film mulching with different colors and different purposes, have a significant impact on the dynamic changes in soil water, heat, nitrogen, and crop growth due to their differences in energy budget and distribution. Therefore, the migration mechanisms of soil water, heat, and nitrogen, and their regulation mechanisms with crop growth, need further exploration for specific film mulching methods.

4.3. Establishment of New Models under Film Mulching Conditions

Models under plastic film mulching conditions have been developed, all aiming to develop a single film mulching method. The energy budget and distribution, the water cycle process of farmland, and the transport processes of soil water, heat, and nitrogen, as well as the crop growth process under the conditions of different film materials (polyethylene film and degradable film), film colors (transparent film, black film, silver-gray film, etc.), and the proportions of mulched ridge width and non-mulched furrow width, cannot be reflected in the model. In order to further develop film mulching technology and expand the application of different film mulching methods, future research should focus on establishing crop models under different film mulching methods.

Contemporary models only consider one-dimensional vertical movement in the processes of soil water, heat, and nitrogen transport. However, with the widespread application of drip irrigation with film mulching, soil water, heat, and nitrogen also exhibit uneven distribution in the horizontal direction. The two-dimensional HYDRUS model can simulate soil water, heat, and nitrogen transport processes under plastic film mulching conditions by setting different upper boundary conditions; however, it was not able to simulate the crop growth process and ignored the impact of crop dynamic growth on the soil water, heat, and nitrogen transport processes. Therefore, it is necessary to consider the horizontal differences in soil water, heat, and nitrogen transport processes, further establishing a two-dimensional coupling model of soil water, heat, and nitrogen transport and crop growth under film mulching conditions.

5. Conclusions

The study summarizes the effects of plastic film mulching on soil water, heat, and nitrogen balance, as well as on crop physiology and growth processes, through the analysis of previous studies. It is concluded that plastic film mulching can improve soil water, temperature, and nitrogen status, and enhance crop transpiration and photosynthetic rates, promoting crop growth and yield. However, these effects of plastic film mulching are influenced by film mulching methods, irrigation systems, crop types, crop growth stages, etc. The study has also reviewed the methods for constructing crop models under plastic film mulching conditions. The crop models under film mulching conditions were established by considering the impact of plastic film mulching on the upper boundary conditions of soil water and heat, and crop growth and development stages, based on existing models. A method for constructing a mechanistic model under plastic film mulching conditions that considered energy budget and distribution processes, and the exchange of latent and sensible heat between soil and atmosphere, was proposed. Subsequently, this study proposes future research focus in the field of plastic film mulching, providing reference for the further development of plastic film mulching technology.