Complementary plant nutrient-acquisition strategies promote growth of neighbour species

1. Greater understanding of positive interspecific interactions in nutrient-poor soils is a priority, particularly in phosphorus (P)-limited ecosystems where plants with contrasting nutrient-acquiring strategies co-occur. It is also relevant to agro-ecosystems, since global P stocks are being depleted. In this study, we assess positive interactions between sympatric plants with contrasting nutrient-acquiring strategies from highly P-impoverished soils from the biodiversity hotspot of south-western Australia. 2. Four plant species (Banksia menziesii, Eucalyptus marginata, Verticordia nitens and Melaleuca preissiana) that are non-mycorrhizal (cluster-rooted), ectomycorrhizal (EM), arbuscular (AM) or dual AM/EM, respectively, were grown together in a specially designed 'common garden' microcosm with nutrient-poor or fertilized soil, with or without root intermingling and fungal hyphae contact. We measured growth, mycorrhizal colonization, root intermingling and nutrient uptake to determine positive or negative growth patterns amongst the various plant assemblies. 3. Growth of the AM/EM host was best when interacting with both the EM host and a nonmycorrhizal nutrient-mining plant with cluster roots (Banksia) in microcosms where root intermingling was not possible. Growth promotion was only seen in pots with nutrient-poor soils, where the better growth of Melaleuca coincided with higher shoot P, manganese, calcium, iron and boron content, whereas an increase in soil nutrient status through fertilizer addition resulted in a decrease in nutrient-sharing between co-occurring species. Furthermore, the dual AM/EM Melaleuca exhibited enhanced EM colonization and favoured EM over AM fungi when grown beside Eucalyptus and Banksia. We surmise that mycorrhizal networks were instrumental in the variation in both mycorrhizal type and colonization levels. 4. We conclude that complementarity of plant nutrient-acquisition strategies can promote growth of neighbour species. The results show a synergistic effect between EM hyphal scavenging and mobilization of limiting nutrients by cluster roots. The positive and negative interactions enable coexistence to go far beyond the traditional view that plants interact mainly through resource depletion. This study improves our understanding of how root interactions could shape plant communities and promote species diversity and packing in nutrient-impoverished habitats.