3.1. Effect of Different Mulch Film Treatments on Crop Physiological Characteristics
Compared to the bare soil, the three mulch films greatly affected the emergence rate and growth progression of the maize and cotton (
Table 2). The emergence rate of both crops was similar in the three mulch film treatments, all of which were higher than in the bare soil (
p < 0.05). The emergence rate of the maize and cotton was in the following order: A = B = C > D. The average emergence rate of the maize and cotton under the three types of mulch film treatments was 56.1% and 62.4% higher, respectively, than that under the bare soil. For maize, the emergence times in treatments A and B were both 0.7 d earlier than in treatment C, and the emergence time was significantly shorter in the three mulch film treatments than that under bare soil (
p < 0.05). For cotton, the emergence time under the different treatments was consistent with the pattern observed in maize. Treatment A had the shortest emergence time of 10.8 d, while the B and C treatments had similar emergence times of 12.1 d and 12.8 d, respectively, both of which were significantly earlier than treatment D at 16.5 d (
p < 0.05). In terms of the time taken from sowing to harvesting the maize, treatment A was shortest at 129.7 d. Treatments B and C took 135.0 d and 143.7 d, respectively. Treatment D took the longest time of 145 d. In terms of the time taken from cotton sowing to boll opening, the difference in time was consistent with that of maize: treatment A took the shortest time of 111.5 d, while treatments B and C took 112.3 d and 113.1 d, respectively. Treatment D took 117.2 d. As plant growth progressed, when comparing treatment A with B and C, the differences in the time required for growth became increasingly significant. Furthermore, the time needed for growth to progress in B and C became increasingly similar to D, particularly in treatment C. It is possible that, in the early stages, the biodegradable mulch films were intact and had a similar effect as the common plastic film, but as the biodegradable mulch film gradually degraded, the moisture conservation function decreased, and thus the effect became increasingly similar to that of the bare soil.
Similarly, compared to the bare soil, the three mulch films also had a large impact on the physiological characteristics of the maize and cotton (
Table 3 and
Table 4). During the entire growth process, treatments A, B, and C significantly increased the plant height, stalk diameter, and leaf area of the maize compared to D (
p < 0.05). The difference was not significant among treatments A, B, and C. For cotton, the physiological characteristics under the four treatments exhibited a similar pattern as in maize. In each stage, plant height, leaf area, boll number per plant, and boll weight of cotton (boll-opening stage only) did not differ among treatments A, B, and C, all of which were significantly greater than in treatment D (
p < 0.05).
For maize, we found that treatments A, B, and C increased plant height by 41.7%, 35.4%, and 33.3% at the jointing stage, 9.5%, 5.1%, and 5.1% at the heading stage, 7.4%, 8.0%, and 9.1% at the harvest stage, respectively, compared to treatment D, suggesting that film mulching had the greatest impact on maize plant height at the jointing stage. Treatments A, B, and C increased stalk diameter by 17.9%, 12.8%, and 9.0% at the jointing stage, 14.7%, 14.0%, and 14.0% at the heading stage, 15.5%, 14.1%, and 15.2% at the harvest stage, respectively, compared to treatment D. This indicated that mulch films had a similar effect on maize stalk diameter at each stage. Treatments A, B, and C increased leaf area by 45.0%, 40.0%, and 30.0% at the jointing stage, 9.4%, 7.1%, and 7.1% at the heading stage, 9.9%, 8.6%, and 9.9% at the harvest stage, respectively, compared to treatment D, indicating that film mulching had the most significant impact on maize leaf area during the jointing stage.
For cotton, we found that treatments A, B, and C increased plant height by 22.5%, 16.8%, and 15.8% at the flower bud stage, 11.1%, 9.9%, and 9.6% at the flower stage, 10.6%, 9.5%, and 10.0% at the boll-opening stage, respectively, compared to treatment D, suggesting that film mulching affected cotton plant height most significantly at the flower bud stage. Treatments A, B, and C increased leaf area by 30.3%, 25.4%, and 23.8% at the flower bud stage, 34.4%, 33.0%, and 32.1% at the flower stage, 24.7%, 24.3%, and 25.9% at the boll-opening stage, respectively, compared to treatment D, indicating that film mulching affected cotton leaf area most significantly at the flower bud stage. Treatments A, B, and C increased the boll number per plant by 14.1%, 11.0%, and 12.0% at the boll-opening stage, and increased boll weight by 13.1%, 12.8%, and 12.9%, respectively, compared to treatment D, indicating that the mulch film treatments had a significant effect on increasing cotton yield.
For maize, treatments A, B, and C, on average, increased plant height by 17.2% and increased stalk diameter and leaf area by 14.1% and 18.5%, respectively, compared with treatment D. This suggested that the mulch film treatments affected the physiological characteristics of the maize in the following order: leaf area > plant height > stalk diameter. In terms of cotton, treatments A, B, and C, on average, increased plant height by 12.7%, increased leaf area by 28.2%, and increased the boll number per plant by 5.0% and the boll weight by 12.9%, compared to treatment D. This suggested that the mulch film treatments affected the physiological characteristics of the cotton in the following order: leaf area > boll weight > plant height > boll number per plant.
While there were differences among the three mulch films, these differences were not statistically significant. During the jointing stage and the heading stage of maize, the plant height, stalk diameter, and leaf area were in the following order: A = B = C (no significant difference, p > 0.05). At the mature stage, the plant height, stalk diameter, and leaf area did not differ significantly among the mulch treatments. For cotton, the plant height and leaf area at the flower bud stage and the flowering stage did not differ significantly. Similarly, at the boll-opening stage, the plant height, leaf area, boll number per plant, and boll weight did not differ significantly between the mulch treatments.
3.2. Effect of Different Mulch Films on Crop Biomass
The maize biomass under different treatments during the experiment is shown in
Figure 4. The results showed that: firstly, in the three years, during the entire growth period, treatment A increased the maize biomass by 27% compared to treatment D; B increased maize biomass by 21% compared to treatment D; and C increased maize biomass by 20.1% compared to treatment D. This indicated that, under all three mulch film treatments, maize biomass was significantly higher than the bare soil treatment. Of these treatments, the common plastic film increased the maize biomass most significantly, while the two biodegradable mulch films did not differ significantly from each other.
Secondly, during the three years, treatment A increased the maize biomass by 7.0%, 8.5%, 29.3%, 29.6%, and 32.5% in the bud stage, seedling stage, jointing stage, heading stage, and harvest stage, respectively, compared to treatment D. Treatment B increased the maize biomass by 3.5%, 3.9%, 23.8%, 23.6%, and 25.0% in the bud stage, seedling stage, jointing stage, heading stage, and harvest stage, respectively, compared to treatment D. Compared to treatment D, treatment C increased the maize biomass by −0.1%, 2.4%, 22.6%, 23.4%, and 24.4% in the bud stage, seedling stage, jointing stage, heading stage, and harvest stage, respectively. This showed that the changes in maize biomass during the growth period fitted a logistic-shaped growth curve, with the film mulching having a greater impact on maize biomass in the later growth stages than in the early growth stages.
Figure 5 shows cotton biomass under the different treatments during the experiment. Firstly, in the three treatment years during the entire growth period of cotton, cotton biomass increased by 20.7%, 15.8%, and 12.1% in treatment A, treatment B, and treatment C, respectively, indicating that cotton biomass was significantly higher in all three mulch film treatments compared to the bare soil.
Secondly, during the three years, treatment A increased cotton biomass by 16.9%, 12.8%, 11.7%, 30.2%, and 32.1% in the bud stage, seedling stage, flower bud stage, boll stage, and boll-opening stage, respectively, compared to treatment D. Treatment B increased cotton biomass by 9.9%, 7.7%, 7.6%, 25.2%, and 28.6% in the bud stage, seedling stage, flower bud stage, boll stage, and boll-opening stage, respectively, compared to treatment D. In comparison to treatment D, treatment C increased cotton biomass by 2.8%, 4.8%, 4.5%, 22.2%, and 26.0% in the bud stage, seedling stage, flower bud stage, boll stage, and boll-opening stage, respectively. The results showed that the changes in cotton biomass during the growth period fitted a logistic-shaped growth curve, with the effect of mulch films being greater in the later growth stages than in the early stages.
The interannual variations in cotton biomass showed that the mulch film treatments had a similar effect on biomass accumulation in cotton and maize. Specifically, during the entire growth period, the three mulch film treatments and the unmulched treatment all exhibited a logistic-shaped growth curve, with biomass being relatively low before the end of May and rapidly increasing thereafter, and then changing slowly after September. All three mulch film treatments significantly increased cotton biomass (p < 0.05), but there was no significant difference among the three treatments, although the differences gradually increased during the later growth stages.
The final crop yield during the experiment is shown in
Figure 6. As shown in
Figure 6, the mulch film treatments significantly increased the yield of maize and cotton (
p < 0.05), but there was no significant difference among the three mulch film treatments (
p > 0.05). In terms of maize yield, treatment A had the highest average yield of 6473.6 kg/ha, which was 76.2% higher than in treatment D, and the yield of treatment B was 69.4% higher than treatment D. In treatment C, the yield was 72.6% higher than that in treatment D. Treatments A, B, and C did not differ significantly in average yield over the three years (
p > 0.05). Of these treatments, treatment A had the highest average yield, followed by treatment C and then treatment B. The variations in cotton yield followed a similar pattern as in maize. Treatment A was associated with the highest cotton yield of 6575.2 kg/ha, which was 71.9% higher than the yield in treatment D, while the yield of treatment B was 65.2% higher than treatment D. In treatment C, the yield was 69.2% higher than in treatment D. Treatments A, B, and C did not differ significantly in terms of the average cotton yield over the three years (
p > 0.05). Overall, the different mulch film treatments increased maize yield by 72.7% on average and increased the cotton yield by 68.8% on average. This indicated that the mulch films significantly increased the yield of maize and cotton at the experimental site, and the increase in maize yield was higher than that of cotton.