How do nitrogen-limited alpine coniferous forests acquire nitrogen? A rhizosphere perspective

Alpine coniferous forest ecosystems dominated by ectomycorrhizal (ECM) tree species are generally characterized by low soil nitrogen (N) availability but stabilized plant productivity. Thus, elucidating potential mechanisms by which plants maintain efficient N acquisition is crucial for formulating optimized management practices in these ecosystems. We summarize empirical studies conducted at a long-term field monitoring station in the alpine coniferous forests on the eastern Tibetan Plateau, China. We propose a root-soil interaction-based framework encompassing key components including soil N supply, microbial N transformation, and root N uptake in the rhizosphere. We highlight that, (i) a considerable size of soil dissolved organic N pool mitigates plant dependence on inorganic N supply; (ii) ectomycorrhizal roots regulate soil N transformations through both rhizosphere and hyphosphere effects, providing a driving force for scavenging soil N; (iii) a complementary pattern of plant uptake of different soil N forms via root- and mycorrhizal mycelium-pathways enables efficient N acquisitions in response to changing soil N availability. Multiple rhizosphere processes abovementioned collaboratively contribute to efficient plant N acquisition in alpine coniferous forests. Finally, we identify several research outlooks and directions to improve the understanding and prediction of ecosystem functions in alpine coniferous forests under on-going global changes.