Effect of nitrogen reduction on yield stability of sweet maize//soybean intercropping system in South China

The increasing demand for fresh sweet maize (Zea mays L. saccharata) in southern China has prioritized the need to find solutions to the environmental pollution caused by its continuous production and excessive use of chemical nitrogen fertilizers. A promising method for improving crop production and environmental conditions is to intercrop sweet maize with legumes and to reduce nitrogen fertilization. In this paper, a field experiment was conducted at the Experimental Center of South China Agriculture University for a total of 9 cropping season in five years (2013-2017) to investigate the dynamic changes of maize//soybean intercropping and system yields in sweet maize farmlands in South China under two nitrogen levelsreduced nitrogen dose of 300 kg·hm^-2 (N1) and conventional nitrogen dose of 360 kg·hm^-2 (N2) and four cropping patternssole sweet maize (SS), sweet maize//soybean intercropping with sweet maize to soybean line ratios of 2:3 (S2B3) and 2:4 (S2B4), sole soybean (SB). This study analyzed the dynamic change of land equivalent ratio and actual yield loss index, and evaluated the stability of system yield by W² (Wricke's ecovalence), CV (coefficient of variation) and SYI (sustainability index), and aimed to explore the effects of reduced nitrogen application on the time stability of sweet maize//soybean intercropping system in South China. Results showed that:1) the yields of sweet maize, soybean, and the total system under different treatments showed obvious seasonal dynamic changes and were significantly affected by cropping and planting patterns. Nitrogen application levels only significantly affected the yield of sweet maize. 2) The relative yield of sweet maize under all intercropping treatments was higher than that under monocropping, and the actual yield loss index (AYLs) of the intercropping system was greater than zero, indicating that sweet maize//soybean intercropping could maintain the intercropping advantage stably and significantly improving land use efficiency. 3) There were no significant differences in W², CV and SYI of sweet maize among different treatments, but the W²value of monocropping soybean was significantly higher than that of the intercropping patterns, and the yield stability of monocropping soybean was lower than that of intercropping soybean. Planting pattern had a significant effect on the stability of the total yield of the system, and intercropping soybean increased the stability of the total yield of the system. 4) Nitrogen fixation of intercropping soybean significantly increased the soil fertility contribution rate. The soil fertility contribution rate of S2B3 and S2B4 were 75.07% and 74.27%, respectively, which were 30.29 and 29.47 percentage points higher than that of SS, respectively. 5) Compared with monocropping sweet maize, sweet maize//soybean intercropping in 9 seasons significantly increased soil pH and alleviated the effect of soil acidification induced by a large amount of nitrogen application on soil fertility for a long time. The soil organic matter and total nutrient content in the sweet maize//soybean intercropping system were not affected by continuous reduced nitrogen application, and 300 kg·hm^-2 could meet the needs of sweet maize and soybean for nitrogen. Reducing nitrogen application and intercropping soybean are sustainable and green production models for efficient utilization of resources and stable system yield in sweet maize producing areas in South China.