Enhancement of rhizosphere citric acid and decrease of NO3−/NH4+ ratio by root interactions facilitate N fixation and transfer
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Cereal can stimulate legume N fixation through competition for soil mineral N. In addition, organic acids in the rhizosphere can mobilize external nutrients, which may help nodulation. However, how much NO3− and NH4+ competition and organic acids in the rhizosphere of intercropped crops contribute to N fixation remains unclear and requires study.
Field and greenhouse experiments were conducted to quantify effects of N competition and organic acids on N fixation in a maize/alfalfa intercropping system.
Intercropping increased nitrogen derived from the atmosphere (%Ndfa), nodules number, and leghemoglobin content by 43.66%, 85.53%, 141.05%, respectively, leading to significantly improved total N uptake and yield, compared to monoculture. The improved %Ndfa was not only due to the depletion of NO3−, but also significantly correlated with the decrease of NO3−/NH4+ ratio and the increase of citric acid in rhizosphere of alfalfa, which could mobilize soluble resources for N fixation under no N addition. Overall, 15.4%–21.5% of N fixed by alfalfa was transferred to associated maize, and the improved N fixation enhanced N transfer.
Our findings provide a mechanism for how root interactions facilitate N fixation, highlighting the importance of NO3−/NH4+ ratio and citric acid in nutrient mobilization for N fixation.
KeywordsCitric acid Intercropping N fixation NO3−/NH4+ ratio N transfer N uptake
This work was financially supported by the National Key Basic Research Program of China (2016YFC0500703), the National Natural Science Foundation of China (31670446, 31471945, 31870436, U1803110), and Jilin Special Program for Key Science and Technology Research (Y8D1161001).
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