Plastic Film Mulching Affects Field Water Balance Components, Grain Yield, and Water Productivity of Rainfed Maize in the Loess Plateau, China: A Synthetic Analysis of Multi-Site Observations

Plastic film mulching (PM) has been widely practiced for rainfed maize (Zea mays L.) in the Loess Plateau, China. However, controversy remains regarding its impacts on field water balance components. There is also scant quantitative information on water-limited potential grain yield (GYw) and water productivity (WPw). A meta analysis and boundary line analysis were conducted using 1421 paired observations extracted from 83 peer reviewed publications to (1) assess PM-induced changes in soil water storage at planting (SWSP), evapotranspiration (ET), and soil water storage at harvest (SWSH), and (2) quantify GYw and WPw in PM fields. PM significantly increased SWSP by 4.0% on average due to an 83.2% average increase in precipitation storage during the fallow season. Overall, ET showed a significant positive response to PM (+2.9%), resulting in 56.4% more soil water depletion than no-mulching (NM) fields over the growing season. However, SWSH significantly increased by an average 1.8% after applying PM, indicating that increased storage of fallow season precipitation replenished increased depletion of growing season soil water. Averaged across all samples, PM had significant positive effects on GY (+56.1%) and WP (+47.5%), which were attributed to higher transpiration, ratio of transpiration to ET, and harvest index from the perspective of effective crop water use. Based on GY and WP boundary lines, GYw and WPw under NM were estimated to be 10,728 kg ha(-1) and 26.8 kg ha(-1) mm-1 on average, respectively, and the respective averages under PM were 13,702 kg ha(-1) and 34.8 kg ha(-1) mm(-1). The GY and WP gaps (i.e., differences between actual and potential values) averaged around 3300 kg ha(-1) and 7.0 kg ha(-1) mm(-1), respectively, in NM and PM fields. In the GY versus ET and WP versus ET scatter plots, the vertical distances between the measured data points on a specific field and the corresponding boundary line indicated field-specific gaps for GY and WP. This study increases our understanding of the effect sizes of PM on field water balance components and the benchmarks of GY and WP for rainfed maize in the Chinese Loess Plateau and other regions with similar conditions worldwide.