Water use efficiency of maize-mung bean in intercropping systems under different water stress conditions

Introduction
The limitation of water resources, increase in water demand for food supply, land use changes, climate change, and reduction of soil fertility are the most important challenges facing the world's food security. Various approaches have been used to reduce water demand and increase production per unit area of ​​agricultural products, including modifying cropping patterns and planting methods, using scheduled deficit irrigation, and implementing new policies and laws in water resources management. Among these approaches, some researchers suggest that intercropping is the most sustainable method to improve the use of production resources. The intercropping system is the simultaneous growth of two or more crops in one part of the land. Intercropping has potential benefits such as higher crop yields, more efficient use of resources (sunlight, water, and fertilizers), reduced disease and pests, and improved environmental conditions. The intercropping farming can be an efficient solution to improve the productivity of water resources, especially in areas with limited water resources such as Iran. So far, no study has been conducted on the efficiency of water and land use in the maize-mung bean cropping system. So, the purpose of this study was to investigate the effect of maize-mung bean intercropping in different cropping patterns and different water stress conditions on water productivity.
 
Materials and Methods
The field experiment was a split plot experiment based on a randomized complete block design with three replications in the research farm, Faculty of Agriculture, Ilam University during the 2020-2021 growing season. Treatments included four levels of irrigation including 40, 60, 80, and 100 % of crop water requirement as main plots, and four planting pattern levels including additive intercropping series (100 % corn + 50 % mung bean), replacement intercropping series (50 % corn + 50 % mung bean) and monocultures of mung bean and corn, as subplots. The application of drought stress started after the establishment stage of the plant and continued until the harvest. The irrigation requirement was determined based on the TDR method and was applied using the drip irrigation method. At the end of the growth period, some parameters of both plants were measured, including plant height, thousand seed weight, grain yield, biological yield, and harvest index. Moreover, water use efficiency (WUE), land equality ratio (LER), and water equality ratio (WER) indicators were used to check the effectiveness of the intercropping system. Finally, analysis of variance (ANOVA) was performed using Minitab 17 software, and the means were compared by Duncan's test at a 5 % probability level (p≤0.05).
 
Results and Discussion
The results showed that the water use efficiency of mung bean was significantly lower than that of mung bean monoculture in two patterns of mixed and incremental cultivation. The highest mung bean water use efficiency occurred at the irrigation level of 80% of the water requirement and single crop (0.51 kg m^-3). Maize water productivity in all cropping patterns decreases with increasing irrigation water consumption. There was no significant difference between the water productivity of the additive intercropping and the monoculture of maize, but the water productivity of the replacement intercropping had a significant difference with the other two cropping patterns. The highest value of LER was obtained in the additive intercropping as 1.56 and 1.52 in 80 and 100% of water requirement, respectively, which was equivalent to a 56 and 52% increase in profitability compared to a monoculture of the two species. WER was greater than 1 in all water levels and two intercropping patterns, which indicated the superiority of intercropping with less water than monoculture. Also, the results showed that the highest WER value was 1.46 in the additive intercropping treatment with an 80% water consumption level. Additionally, the comparison of LER and WER indices showed that intercropping makes more efficient use of water and land resources. Also, the results indicated that the highest LER and WER occurred in the additive intercropping and the irrigation level was 80%. Therefore, additive intercropping and the use of an 80% irrigation level make the most efficient use of water and land resources.
 
Conclusion
The higher WER and LER indexes in this study suggest that an intercropping system may save 45%–56% of water and farmland, achieving similar yields in comparison to sole maize and sole mung bean. Thus, the intercropping system would be an advantageous cropping system for sustaining crop productivity and improving water and land use efficiency. However, additional research is required to comprehend the resource capture mechanism of intercrop species in intercropping systems under the changing climate. Especially, a series of comprehensive studies to develop the best management approaches regarding fertilization, irrigation, and pest control would support the sustainability of intercropping systems and farmer adoption. The successful adoption of the intercropping system will play an important role in meeting the food requirements of the increasing population, especially in arid and semiarid countries such as Iran which are facing challenges in resource limitation. The results of this study showed that by using intercropping system and optimal management of water consumption, it is possible to achieve performance similar to single crop cultivation with 45% less water and 56% less land use. However, more research is needed to understand the mechanism of how resources are absorbed by mixed species in intercropping systems, especially under different climatic conditions.