Advances in the mechanism of improving crop mineral nutrients in intercropping and relay intercropping systems

【Objectives】Crop mineral nutrients can be improved in intercropping and relay intercropping systems. In recent years, productivity of crops has been investigated, and the mechanism of efficient utilization of mineral nutrients in intercropping and relay intercropping systems has been focused on. Dynamics of rhizosphere mineral nutrients contents and crop mineral nutrient acquisition in intercropping or relay intercropping systems were summarized in this paper. 【Major advances】1) The ammonium nitrogen and amino acid nitrogen in the root exudates of crops is the nitrogen sources for the adjacent crops, and the nitrogen fixation of leguminous crops is enhanced attributing to the induction of root exudates, so as to increasing nitrogen nutrient content in rhizosphere. 2) Organic acids in root exudates can mobilize insoluble phosphorus, iron and potassium in the rhizosphere, and enhance the availabilities of mineral nutrients for crops. 3) Root architecture and gene expression related to mineral nutrients uptakes are changed in intercropping or relay intercropping. The changes may be mediated by root exudates or above-ground interspecific interactions. The below-ground niche complementarity and enhanced gene expression may contribute to the efficient utilization of mineral nutrients. 4) The arbuscular mycorrhizal which would potentially create networks originating from crops by means of hyphal anastomoses is facilitative for the nutrition transfer among crops. 5) Soil biodiversity (soil fauna and microbes) which positively contributes to nutrients uptakes can be influenced by intercropping and relay intercropping. Some specific function microbes such as phosphorus-solubilizing bacteria are attracted and accumulated in the rhizosphere as a result of variation of microbes metabolism, thus increasing the mineral nutrients bioavailability. 6) The soil enzyme activities (such as urease, acid phosphatase and alkaline phosphatase) are increased in intercropping and relay intercropping, as a result of contributing to the conversion of soil organic nitrogen and phosphorus to inorganic nitrogen and phosphorus. In summary, root exudates, root architecture and niche complementarity, soil biodiversity as well as soil enzymatic activity play important role in efficient utilization of mineral nutrients in crops, and root exudates mediate the interaction between crop-crop, crop-soil and crop-soil microbials. 【Suggestions and expectations】However, knowledge of below-ground biological processes is limited because of the technical limitation and complexity of below-ground biological processes. The qualitative and quantitative analyses of root exudates component in situ, recognition and response between species, the effect of intercropping on soil biodiversity and feedback of soil biodiversity to crop growth, and as well as the identification and application of functional microbials are all deserved more attention.