Response of maize root to residual plastic film and root distribution model in Hetao Irrigation District of Inner Mongolia

Abstract: Plastic film residues have posed a great threat to crop root growth and distribution in soil. The accumulation of agricultural wastes in the soil can also hinder the water and fertilizer migration in the farmland. Therefore, it is of great significance to clarify the effects of different Residual Plastic Film (RPF) amounts in soil on root growth and distribution. In this study, five levels of agricultural film residues were set at the Jiuzhuang Agricultural Comprehensive Water-Saving Experimental Station in Hetao Irrigation District from 2019 to 2020, including the control group, CK (0 kg/hm2), T1 (150 kg/hm2, plastic film covering for about 18 years), T2 (300 kg/hm2, plastic film covering for about 45 years), T3 (450 kg/hm2, plastic film covering for about 72 years), and T4 (600 kg/hm2, plastic film covering for about 100 years). A systematic investigation was also made on the effects of various RPF amounts on the Root Length Density (RLD) and the distribution of diameter classes for the maize roots. The RPF amount was then introduced to establish a root distribution model suitable for the farmland with the agricultural film residues. The results showed that the RPF effectively reduced the dense range of maize roots in the horizontal distribution, where the degree of reduction increased with the increase of RPF. The distribution of roots in the horizontal direction was divided into the main and the lateral root zone. The effect of RPF on the root system in the lateral root zone was greater than that in the main root zone, indicating a significant indigenous level (P＜0.05). Specifically, the RLD decreased sharply, when the RPF amount was 300 kg/hm2 (T2), 75.98% lower than that in the 0 kg/hm2 (CK) treatment. In the vertical distribution, the root system showed a significant downward trend with the increase of RPF. The maize root system was more affected by the RPF in the 0-30 cm soil layer than in the 30-80 cm soil layer. Furthermore, the RLD decreased by 50.02%, when the RPF reached 450 kg/hm2. In addition, the RPF treatment reduced the coarse root ratio of maize (d＞2 mm), decreased by 29.25% on average. The proportion of fine roots (d≤2 mm) increased, with an average increase of 4.80%. A relative root length density (RPF-NRLD, Residual Plastic Film-Normalized Root Length Density) distribution model considering the RPF content presented a higher accuracy, in which the determination coefficient (R2) was 0.961, the Root Mean Square Error (RMSE) was 0.282, and the Mean Relative Error (MRE) was 18.87%, thereby to accurately simulate the distribution of maize roots under different residual film amounts. Meanwhile, the RPF-NRLD distribution model was also considered the different diameter classes of roots. Specifically, the simulated values for very fine roots and fine roots of maize were highly consistent with the measured values, where the MRE of 14.91% and 14.96%, respectively, and the MRE of very coarse roots of 35.41%. Correspondingly, the relative root length density distribution of very fine roots (d≤0.5 mm) and fine roots (0.5 mm＜d≤2 mm) followed the quadratic function, whereas, the coarse roots followed the exponential function. A field experiment was also conducted to verify the RPF-NRLD distribution model. It was found that the root system maintained normal growth, especially the very fine root and fine root, when the amount of RPF in the farmland was controlled in the range of 0-100 kg/hm2. This finding can offer a scientific significance for the numerical simulation of crop growth and decision-making in the RPF-contaminated areas.