Maize shows intraspecific facilitation under phosphorus deficiency but competition under nitrogen deficiency when grown under increased plant densities in alkaline soil

Background and aims: Neighbouring plants compete for resources in intensive cropping systems when the plant density is high. Most studies on plant density have focused on yield responses, whereas only few studies have paid attention to belowground root-soil-interactions. Knowledge about belowground responses to different plant densities under nitrogen (N) or phosphorus (P) limitation remains scant. Methods: Two pot experiments were conducted in a glasshouse using a calcareous soil (pH 8.4). Five treatments were applied with different amounts of N or P and planted with different plant densities. Shoot and root biomass, and root morphological traits including total root length and proportions of root length in different diameter classes were examined in both the N and P experiment. Root physiological traits including rhizosheath pH, phosphatase activity and carboxylate concentration were measured in the P experiment. Results: Plant biomass, P content and total root length increased with increasing plant density in the P experiment, while plant biomass, N content and total root length decreased with increasing plant density in the N experiment. Maize with high plant density released carboxylates and phosphatases under P deficiency. Conclusion: Growing in calcareous soil, maize showed a competition effect at increasing plant density under N limitation, but an intraspecific facilitation effect at increasing plant density under P limitation. This study shows that maize (Zea mays L. cv. ZD958) released carboxylates and phosphatases in response to high soil pH under P-limiting conditions. The findings of this work are important towards the sustainability of intensive cropping systems.