Effects of the border on yield and water use in wheat/maize intercropping in rain-fed areas with different nitrogen levels

Intercropping has gained widespread adoption among farmers due to its ability to yield higher outputs. The primary reasons behind the yield advantage of intercropping systems are the border effects. Among various factors influencing the inter-species relationship, nitrogen (N) plays a crucial role. However, yield and border effects of wheat/maize intercropping at different N levels have been poorly studied, especially under rainfed conditions with different rainfall year types. The objective of this study was to compare the contribution of different crop strips to yield at different N levels and to explore the impact of different precipitation on crop physiological characteristics and water use of intercropping systems in rainfed areas. A field experiment was conducted between 2019 and 2021 in Yangling, Shaanxi Province, adopting a two-factor design, with the main treatment as planting pattern (sole wheat; sole maize; wheat/maize intercropping), and the secondary treatment as three N levels (0, 180, 300 kg ha−1 for wheat and 0, 235, 360 kg ha−1 for maize). Results showed that, intercropping increased total yield and land productivity, with a land equivalent ratio of 1.05–1.15. The intercropping system led to increased water consumption during crop growth, however, it also exhibited a notable improvement in water use efficiency, with a 5.2–16.9% increase compared to sole wheat cultivation. During the symbiotic period, intercropping wheat showed competitive advantages, resulting in yield increments of 16.4–31.2% over the two-year period. Border row wheat contributed 63.7% of the total production, with the increase in yield mainly attributed to an increase in the number of spikes and 1000-kernel weight. On the other hand, the intercropping maize showed a competitive disadvantage during the symbiotic period. The leaf area index, net photosynthetic rate, and yield of border row maize were comparatively lower than those of inner maize. However, no significant differences were observed between the yield of inner row maize and that of sole maize. After the wheat harvest, the intercropping maize resumed growth. The growth of intercropping maize was constrained by N and moisture conditions. In 2020 (Precipitation: 572.8 mm), the yield of intercropping maize increased by 3.2% compared to sole maize under N2 treatment, but in 2021 (Precipitation: 251.4 mm), the yield of intercropping maize decreased by 1.1%− 4.3%. Wheat/maize intercropping has clear advantages in land use in the Northwest China due to the availability of more resources for wheat/maize intercropping, especially in the border rows of wheat. The recovery of late-sown crops is limited by water and nitrogen, and increasing the water and nitrogen supply of late-sown crops can better play the advantages of intercropping. This study demonstrates that the contribution of intercropping system component crops to yield, and provides practical guidance for improving the productivity of wheat/maize intercropping systems in Northwest China.