Increased planting density of Chinese milk vetch weakens phosphorus uptake advantage by rapeseed in a mixed cropping system

Root interactions among plants play an important role in promoting nutrient uptake, enhancing the performance of neighbouring individuals and thus plant fitness in natural communities and agroecological systems. Plants can influence the growth of their interspecific neighbours by altering root morphological (root length and biomass) and physiological traits (exudation of organic and inorganic compounds). Intercropping agricultural practices – where two or more crop species are grown in close proximity – are largely built upon this principal, allowing for better use of soil resources than would be available to a single crop species.  For example, phosphorus-mobilizing (P-mobilizing) crops can improve P availability for themselves and neighbouring non-P-mobilizing species through exudation of carboxylates and phosphatases, as has been observed with maize intercropped with faba bean. However, exactly how root systems respond to neighbours with varied planting density is poorly understood.

In a recent study published in AoBP, Zhang et al. investigate how relative planting density of leguminous Chinese milk vetch influences uptake of nutrients and growth of neighbouring rapeseed plants. Using a pot-based experiment, rapeseed was grown alone as a single species (single rapeseed treatment), or mixed with 3, 6 or 15 plants of Chinese milk vetch (with corresponding milk vetch monocrop controls). The positive effects induced by the neighbouring legume on nutrient uptake depended on the legume planting density. At low planting densities, the Chinese milk vetch had a small root system but high rates of citrate and acid phosphatase exudation, thus facilitating rapeseed P uptake. As the Chinese milk vetch planting density increased, there was a decrease in rapeseed P uptake due to a reduction in root exudation and increased root-size in Chinese milk vetch. The results of Zhang et al. suggest that the root/rhizosphere could be better managed in intercropping systems through optimization of legume planting density, helping to improve crop productivity and nutrient-use efficiency.