Effects of mulching time on water use efficiency and yield of cotton in southern Xinjiang simulated by RZWQM2 model

Plastic film mulching plays an important role in Xinjiang cotton production. The main function of mulching is to maintain soil temperature and moisture. Plastic film mulching during the whole growth period may lead to soil temperature overheat and root respiration obstruction. In addition, the biodegradable film is expected to replace traditional plastic films. However, several studies have shown that the degradation period of biodegradable mulch film does not match the growth requirements of cotton. Therefore, balancing the relationship among mulching time, crop growth demand and the changes of farmland environmental factors is the key scientific issue to control the pollution of residual plastic film in Xinjiang. Field experiments were conducted in 2016 and 2017 to investigate the mechanisms behind the effects of mulching time on cotton (Gossypium hirsutum Linn.) growth. Six mulching time (40 d, 55 d, 70 d, 85 d, 100 d and whole growing season mulching) were designed. The cotton yield and Water Use Efficiency (WUE) were simulated by using Root Zone Water Quality Model-Version 2 (RZWQM2) model. The parameters of the RZWQM2 model were calibrated and verified by the measured data of 2016 and 2017. Then, the formation processes of biomass, yield and water use efficiency under the influence of different mulching times were simulated at intervals of 3 days, and the similar processes mentioned above were predicted in 2018 and 2019. The objective of this work was to provide a decision method for the model to simulate soil moisture changes and predict crop yields under the conditions of crop mulching, and it would be helpful to determine the reasonable mulching time and reduce the residual plastic film pollution in cotton field on the premise of ensuring high and stable yield. The model validation results showed that the Root Mean Square Error (RMSE) and Mean Square Error (MRE) between the simulated and measured soil water content in each layer of soil ranged from 0.02 to 0.04 cm3/cm3 and 5.90% to 28.66%. The simulation effect of the upper soil layer was not as good as that of the lower soil layer because of more complex temporal and spatial changes of soil surface organic matter content, macropores and other properties than deep soils. The aboveground biomass of two years increased with the extension of mulching time, and the seed cotton yield and WUE showed a trend of first increasing and then decreasing. The growth rate of seed cotton yield and water use efficiency was the highest when the mulching time was 73-81 d, and the yield was higher when the mulching time was mid-to-early August. At this time, the cotton canopy has been closed, and the surface receives less solar radiation, which maximally inhibits soil evaporation, thus reducing the non-productive water consumption and increasing the proportion of productive water consumption. At the same time, the soil ventilation and air permeability are improved after uncovering the film, and the soil microbial activity is enhanced. Therefore, it can be concluded that uncovering film in this period had little effect on cotton growth, and the tensile strength of mulching film at this time was still high, which was more conducive to mechanical recovery. The optimum mulching time of 91-99 d in southern Xinjiang was beneficial to reduce the risk of residual film contamination and maintain high seed cotton yield and water use efficiency. The mechanism of film mulching cycle on key physiological and ecological processes, such as water consumption structure transformation, salt distribution, nutrient migration and absorption in cotton field need further studies. Related work needs to be carried out in the future research, so as to improve the level of cognition and optimize relevant supporting measures.