Review of plastic film mulching system and its impact on soil ecological envi-ronment in China's rainfed drylands

Plastic film mulching, as an important and effective cultivation mode that improves grain yield, has been widely used in rainfed agricultural areas in Northwest China. This study reviewed crop yield, soil moisture content, soil temperature, soil nutrient transformation and transport, and microbial population and activity under plastic film mulching conditions. The review was designed to provide theoretical support for improving plastic film mulching system in China's rainfed dry regions. Researches results showed that maize, wheat and potato increased yields significantly under plastic film mulching compared to no mulching condition. Yield increase was influenced by plastic film cover mode, with an average increase rate of 26.2%, 37.1% and 29.8%, respectively. Full-film mulching most significantly increased crop yield. Average yields of maize, wheat and potato were respectively 30.0%, 5.1% and 26.4% higher than ridge and furrow system with ridge mulching. Water use efficiencies of maize, wheat and potato under film mulching increased respectively by 42.8%, 10.9% and 92.8% over no mulching. Film mulching influenced vertical distribution of nitrate in the soil profile, with nitrate accumulation in the topsoil layer under plastic film mulching. Also film mulching improved nitrogen fertilizer use efficiency, and reduced nitrogen leaching and ammonia volatilization. However, there were some different conclusions on denitrification under film mulching system, which required further studies. The influence of film mulching on organic carbon was related to climate, soil, crop and mulching time, which meant that the results were not consistent. In addition, film mulching increased farmland soil microbial biomass and changed soil physical properties. Although film mulching significantly improved crop yields, it certainly affected the ecological environment via such processes as "extravagant water deprivation", increased greenhouse gas emission, soil organic matter exhaustion, mulch film residues, etc. Thus further systematic research on the controlling mechanisms, improvement strategies of film mulching technology and comprehensive evaluation of the effect of film mulching system on ecological environment needed urgent attention for sustainable development of agricultural production in China's arid areas.