Underground interaction saves nitrogen input by driving nitrogen fixation and transfer in maize-soybean intercropping

Context

Legume-based intercropping have been shown to improve sustainability in cereal-dominated agricultural systems. However, how underground interactions affect intercropping advantages under different nitrogen (N) application rates remains unclear.

Objective or research question

To elucidate how underground interactions shape root niches and N distribution, influence N fixation and transfer, and thereby enhance productivity while reducing N input in maize-soybean intercropping.

Methods

From 2023–2024, field experiment was conducted with maize monoculture, soybean monoculture, and maize-soybean intercropping under varying N application rates (maize: 0, 120, 180, 240, and 300 kg N ha^–1; soybean: 0, 60 kg N ha^–1). In 2024, a micro-plot experiment was established within the field experiment plots, including treatments of underground interaction and separation, with ¹⁵N labeling of soybean used to quantify N transfer to maize.

Results

Maize-soybean intercropping demonstrated a productivity advantage (land equivalent ratio > 1) and improved N use efficiency, primarily driven by the competitive dominance of maize (aggressivity > 0). In maize-soybean intercropping, the N application rates that achieved maximum maize yield and N use efficiency were reduced to 183 and 149 kg ha^–1, respectively, compared to 241 and 313 kg ha^–1 in maize monoculture, respectively. In micro-plot experiment, underground interaction reduced soil mineral N accumulation by 9 – 28 % compared to underground separation, promoting a more uniform N distribution between maize and soybean sides under intercropping. In addition, under underground interaction, maize and soybean roots overlapped primarily on the soybean side, with the degree of overlap increasing with higher N application rates, despite the inhibition of soybean root growth compared to underground separation. Regardless of whether under underground separation or interaction, higher N application rates inhibited the percentage of N derived from the atmosphere of soybean but led to an increase in the total biological N fixation amount. Importantly, underground interactions drive N transfer from soybean to maize, with the percentage of N in the maize derived from transfer reaching 22 – 32 %. In summary, N application rates indirectly enhance N fixation and transfer by effecting soil N and root distribution in the underground interaction zone, enhancing benefits in maize-soybean intercropping.

Conclusions

An optimal N application rate of approximately 165 kg ha^–1 was identified for maize-soybean intercropping, achieving a balance productivity, N use efficiency, N fixation, and N transfer.