Nitrogen level determines arbuscular mycorrhizal fungi nitrogen uptake rate of Stipa purpurea in alpine steppe

Roots and mycorrhizal fungi, especially the Root-pathway and Myc-pathway, represent two alternative strategies for plants in the process of soil nutrient foraging. However, the concurrent carbon (C) economy associated with resource acquisition through root and mycorrhizal pathways remains unclear, particularly across the nitrogen (N) enrichment gradient.

Using experimental microcosms with a dominant plant of alpine steppe, Stipa purpurea, and inoculated with native arbuscular mycorrhizal fungi (AMF) in combination with ¹⁵N and ¹³C dual-labeling, we quantified the direct N transfer to the host plant through both pathways, as well as the C allocated to the fungi and roots by the same plant, across three N addition treatments (control (0 kg·N ha⁻¹ yr⁻¹), low N (50 kg·N ha⁻¹ yr⁻¹) and high N (150 kg·N ha⁻¹ yr⁻¹)).

We found that, under the low N treatment, extraradical hyphae of AMF proliferated extensively, and the plant exhibited significantly positive mycorrhizal responses in terms of biomass and nutrient foraging. The contributions of N foraging through the Myc-pathway to S. purpurea were 13.85%、48.28%、30.59% across the N addition gradient, respectively. Especially under the low N addition, plants preferred the Myc-N pathway over the Root-N pathway. However, the C allocation to AMF by plants under different N levels showed no significant difference.

Upon comparing the ratios of N benefit to C cost, we believe that the low N addition can maximize the function of native AMF in N foraging for alpine plants.