Film mulching affects root growth and function in dryland maize-soybean intercropping

• Maize roots morphological behaviors respond to soybean roots presence in dry and wet conditions. • Intercropped maize roots were shallower compared to sole maize. • More active root-diameter strategy for intercropped maize to adapt to varied water conditions. • Film mulching improved the overlap of interspecific roots. • Film mulching greatly increased maize grain yield with a lesser decline of soybean grain yield relative to sole soybean. Understanding the morphological response of maize roots in the presence of soybeans in fields under varied water conditions is vital for identifying maize-dominant intercropping systems suitable for climate change. In this study, two maize-soybean intercropping systems (maize with and without film mulching) were compared with maize and soybean sole crops in rainfed farmland on the Loess Plateau. Different features, including the maize root morphological traits, root vertical distribution, shoot traits, interspecific root overlap, and grain yield of intercrops, were determined after silking in 2017 (dry year) and 2018 (wet year). The intercropped maize (without film mulching) employed a thin root system under dry conditions and a profuse root system under wet conditions, with shallow roots and active root-diameter strategy to adapt to varied water conditions. These features resulted in higher grain yield in per unit area for the intercropped maize (16.97 % overyielding) than sole maize without film mulching. Film mulching of intercropped maize modified the roots with confined growth under dry conditions and inhibited the roots with excessive growth under wet conditions; as a consequence, the two morphologies of intercropped maize root system were reshaped into the optimal one (less profuse but not thin), with S-type vertical distribution (1 m soil profile) for root length and more beneficial root-diameter strategy. Besides, film mulching improved interspecific root interaction through an evident increase in root length or mass in the interspecific overlap regions. Finally, film mulching improved the maize overyielding dramatically (84 %) due to high leaf adaptability and efficient root-shoot synergy in maize plants. Importantly, the maize mulching did not decrease intercropped soybean yield significantly when compared with sole soybean due to the benefits from interspecific facilitation, while it enhanced total grain yield (10.67 %) for the intercropping system. Thus, film mulching improved maize root morphology and balanced soil resource exploitation and shoot development, consequently strengthening the attainable grain yield in the intercropping system. To conclude, the research elucidates the plant root mechanisms (root morphological changes) underlying overyielding of intercropped maize in varied water conditions. It identifies film mulching of maize as a strategy to optimize root growth and function for higher intercropping yield advantage in dryland.