Maize–bean intercropping: N fertilization effects on yield, nitrate leaching and N fixation

Pure maize (Zea mays L.) cultivation is associated with excessive nitrogen (N) fertilization and high nitrate leaching risk. We assumed that maize intercropped with climbing beans (Phaseolus vulgaris L.) can reduce N fertilizer demand if the beans use biological nitrogen fixation (BNF) at reduced fertilization, thereby balancing yields across N fertilization levels. We further assumed that beans can reduce nitrate leaching through uptake of excess mineral N and increase the subsequent crop yield through additional crop residues. These hypotheses were tested in two experiments between 2019 and 2022 at two sites in Germany. In Experiment 1, intercropping and pure maize were tested with three N fertilization levels. Traits included yield, nitrate leaching, and subsequent crop yield. In Experiment 2, intercropping was additionally tested with rhizobia inoculation, and pure maize with full fertilization served as a reference. Traits included nodulation and BNF of bean as well as yield. Nodulation, BNF, and yield variables differed among the two sites. At the site with nodulation and BNF, intercropping yields were comparable across N fertilization levels, and the grain yield of the subsequent crop was significantly higher (+15% to +20%) after intercropping compared with pure maize at reduced N fertilization. Nitrate leaching was significantly lower (−40% to −48%) after reduced N fertilization compared with full fertilization, but did not differ significantly between intercropping and pure maize. Inoculation could neither induce nor enhance nodulation. Without N fertilization and with nodulation, BNF of beans reached 25 kg/ha. We recommend intercropping with reduced N fertilization on fields where rhizobia naturally occur.