Water use of intercropped species: Maize-soybean, soybean-wheat and wheat-maize

Plant-plant interactions in the intercropping system can significantly affect crop productivity. However, it is poorly understood how the interactions affect the land equivalent ratios (LER) in the cereal-legumes and cereal-cereal intercropping systems in semi-arid agroecosystems. A two-year (2019–2020) field experiment was conducted in the Dryland Agricultural Experimental Station of Lanzhou University, a semiarid rainfed site of northwest China, to quantify the impact of crop diversification on land equivalent ratio, and its moisture-dependent mechanisms in three intercropping systems of maize-wheat, maize-soybean and wheat-soybean. The soybean-involved intercropping systems showed positive interactions, which substantially promoted crop productivity of maize and wheat by 18.1–20.9%. The soil water in soybean strips can be used by intercropped wheat or maize, which in turn promoted soil water storage (SWS) in maize or wheat strips by 0.6% and 11.0% respectively, during the co-growth period. This further improved the photosynthetic rate (Pn), instantaneous growth rate (IGR), and water use efficiency (WUE) for each species and thereafter elevated the land equivalence ratio (LER>1). However, in the wheat-maize intercropping system, interspecific competition was dominated with wheat as dominant species and maize as inferior one respectively. The competition plundered available water of maize strips (SWS decreased by 10.5%), and thus decreased its Pn and WUE by 12.7% and 20.0% (P < 0.05), respectively. Importantly, maize yield and LER were not improved via the compensation effect during the post-harvest period of wheat. While soil water was to some extent restored, the Pn and IGR of intercropped maize were still lower than those of monoculture maize. This trend resulted in maize yield loss during the reproductive period, and ultimately lower LER. For the first time, we found a rarely reported phenomenon, i.e. negative relationships between crop diversity and land equivalent ratios in semiarid agroecosystem, since the yield loss caused by competition was not compensated during the co-growth period. Therefore, a positive relationship between crop diversification and land equivalent ratio required rational crop species configuration, in terms of the tradeoff between crop diversity and its productivity at agricultural landscape scale.