Alternate intercropping of cotton and peanut increases productivity by increasing canopy photosynthesis and nutrient uptake under the influence of rhizobacteria

Alternate intercropping of cotton and peanut is an emerging cropping practice that involves the intercropping of two crops in a wide strip, with annual switching of planting positions. The productivity achieved through alternate intercropping, which combines intercropping and rotation, surpasses that of either mode used alone. However, the underlying mechanisms responsible for the observed increase in crop productivity remain elusive. We put forth the hypothesis that alternate intercropping enhances productivity by promoting canopy photosynthesis and nutrient uptake, facilitated by the influence of rhizosphere bacteria. A two-year field experiment was conducted employing a randomized block design to investigate the effects of monoculture, traditional intercropping, and alternate intercropping on yield, yield components, canopy photosynthesis, 13C photoassimilate partitioning, uptake of major nutrients, and diversity of rhizosphere bacterial communities. Alternate intercropping increased biological by 9.3% and seed cotton yield by 20.0% compared to monocultures without sacrificing peanut yields. In comparison to traditional intercropping, alternate intercropping also increased seed cotton yield by 3.9%, peanut pod yield by 6.7%, and total productivity by 10.6%. The cotton harvest index in alternate intercropping was similar to that in traditional intercropping but 9.9% higher than in monoculture. However, the peanut harvest index did not differ between alternate intercropping, traditional intercropping, and monoculture. Net ecosystem CO2 exchange (NEE) under alternate intercropping was higher in cotton than that under traditional intercropping and monoculture, and 13C partitioning to reproductive organs in cotton was higher in alternate intercropping than in monoculture, although there was no difference between alternate and traditional intercropping. In peanut, NEE was higher under alternate intercropping than under traditional intercropping at all stages. At peak pod setting, 13C partitioning to reproductive organs was higher under both alternate intercropping and monoculture than under traditional intercropping. Alternate intercropping resulted in a 7.2% increase in cotton nitrogen uptake compared to traditional intercropping, and a 8.8% increase in nitrogen uptake, 10.9% increase in phosphorus uptake, and 8.5% increase in potassium uptake compared to monoculture cotton. Peanut uptake of nitrogen, phosphorus, and potassium was greater in alternate intercropping than in traditional intercropping and was approximately the same as that in monoculture. Alternate intercropping increased the relative abundances of plant growth promoting rhizobacteria (PGPR). Significant correlations were detected between NEE and biological yield, 13C partitioning and harvest index, as well as the relative abundance of PGPR and uptake of major nutrients. The enhanced crop productivity observed in alternate intercropping can be attributed to improved shoot-root coordination, as evidenced by the increased canopy photosynthesis and nutrient uptake associated with altered rhizosphere bacterial communities.