Intercropping enhances agroecosystem services and functioning: Current knowledge and perspectives

Intercropping is one of the traditional farming systems practiced by farmers in China for more than 2 000 years with some intriguing ecological principles. Previous studies have shown that intercropping enhanced not only crop productivity, but also the utilization efficiencies of resources, including above-ground (e.g., land, thermal, radiation and space) and below-ground (e.g., water and nutrients) resources. Recent efforts have made some progresses on intercropping research. Here, we reviewed the potential of intercropping to strength ecosystem services and functions at the agroecosystem level, prospective research directions and highlight practical uses in modern agriculture. Intercropping increased biodiversity, productivity and stability of agroecosystems. At the same time, intercropping enhanced water use by isolating the time for maximum water requirements of one species from the other, and spatial complementarity by hydraulic lift of water. The enhancement of nitrogen acquisition was attributed to niche differentiation of N resources in which cereals acquired more mineral N from the soil, while legumes fixed more N from air N2. This was because that cereals was more competitive than legumes and mineral N competition increased symbiotic N2 fixation of legumes. Some P mobilized species facilitated the conversion of soil unavailable P into available P, which benefited not only the species but also the neighboring immobilized other species. E.g., the roots of faba bean released carboxylates or proton to dissolve sparingly soluble P in soils. Also the roots of chickpea released phytase or phosphatase to decompose organic P in the soil, which increased available soil P. There were interspecific facilitations of iron (Fe) and zinc (Zn) nutrients in intercropping of dicotyledonous or non-graminaceous monocotyledonous (strategy I for Fe acquisition and non-Fe or Zn mobilization) species and graminaceous monocotyledonous (strategy Ⅱ for Fe acquisition and Fe or Zn mobilization) species, which benefited micronutrient availability in intercropped non-Fe-mobilizing or Zn-mobilizing species. In the paper, I also identified some important future directions of intercropping research and practical uses. The research directions include crop diversity and agricultural sustainability, signal-controlled interspecific interactions between intercropped species, linkage interactions to above-ground and below-ground diversities, functional, structural and empirical models for intercropping, etc. In application, intercropping can be used to develop ecologically intensive agriculture and organic farming, to enhance fertilizer recovery, and to enrich the contents of microelements in edible parts of crops. Finally, it will be useful to further develop suitable machinery and breed newer crop varieties for intercropping.