Overyielding and below-ground interspecific interactions in agricultural intercropping

Background/Question/Methods

Intercropping, i.e. growing individuals of at least two crop species in close proximity at (about) the same time, is associated with interspecific interactions, including above- and belowground competition and facilitation, which can lead to overyielding in tropical and temperate habitats. There were lots of research on interspecific interactions on above-ground parts, limited on below-ground parts. Recent decades, we have focused on below-ground interspecific interactions, especially on phosphorus or microelement facilitation between mobilizing and non-mobilizing species, and nitrogen complementary utilization between intercropped legumes and cereals, by rhizosphere research techniques in greenhouse and root barriers techniques under field conditions. This paper reviews the main results on the progress.

Results/Conclusions

There are significant productivity advantages of intercropping in various legumes/cereals and cereals/cereals intercropping. Overyielding of intercropping is almost completely dependent on below-ground interactions in faba bean/maize intercropping, and more than 1/3 of yield increases of intercropped wheat are derived from below-ground interactions in wheat/maize intercropping. The below-ground mechanisms underlying the overyielding are interspecific facilitation and complementary utilization for nutrients. The interspecific facilitation is that some crop species mobilize unavailable forms of one or more limiting soil nutrients (such as phosphorus, iron and zinc), and improve phosphorus, iron or zinc nutrition for themselves and neighboring non-mobilizing species by releasing acid phosphatases, protons and/or carboxylates, and phytosiderophores into the rhizosphere which increases the concentration of soluble inorganic phosphorus, iron, or Zn in soil. Acid phosphatase decomposed soil organic phosphorus that is not available to the majority of plant species, into soil soluble inorganic phosphorus that is available to non-P-mobilizing crop species. Protons and/or carboxylates are able to mobilize otherwise-unavailable inorganic phosphorus in soil, such as Ca₁₀-P, Al-P and Fe-P etc. Similarly, on calcareous soils with a very low availability of Fe and Zn, Fe- and Zn-mobilizing species, such as graminaceous monocotyledonous and cluster-rooted species, benefit themselves, and also reduce Fe or Zn-deficiency of neighboring species, by releasing chelating substances, such as phytosiderophores. When legumes involved, non-legumes, usually with more competitive to soil mineral nitrogen, acquire more soil mineral nitrogen, lead to reduction in soil nitrogen concentration, which facilitate to nodulation and biological N₂ fixation of associated legumes in the intercropping. Therefore, the mechnisms underlying overyielding derived from below-ground processes are interspecific facilitation on sparingly soluble nutrients in soil and complementary utilization of soil and atmosphere nitrogen in legumes-based intercropping.