Carbon and nitrogen exchange between trees and mycorrhizal fungi at treeline ecotone

The formation of high elevation treelines is thought to result from direct low-temperature growth limitation, but the in-direct role of nitrogen (N) in modifying growth has rarely been evaluated. Slow N mineralization rates in cold soils may push trees to rely more on N supplied by symbiotic mycorrhizal fungi. Here, we investigated the carbon (C) and N exchange between trees and mycorrhizal fungi along an Alpine treeline ecotone using bomb ¹⁴C and natural abundance isotopes (¹³C and ¹⁵N). We collected fine roots, branches, and needles from two tree species (Larix decidua L. and Pinus mugo spp. uncinata Ramond) and sporocarps of mycorrhizal and free-living (saprotrophic) fungal genera. ¹⁴C measurements demonstrated that mycorrhizal fungi rely on new photo-assimilates derived from fine roots, while saprotrophic fungi feed on several years old C (4-10 yr). The C transfer root-fungi seems to have isotopic fractionation that enriches the fungi with ¹³C in 1-5‰. Mycorrhizal fungi had higher δ¹⁵N values and C:N ratio than saprotrophic fungi. Assuming that the two fungi types share the same N source in the soil, the ¹⁵N enrichment and the lower N concentration of the mycorrhizal fungi could be explained through preferential transfer of ¹⁴N to the hosting trees. The δ¹⁵N in the trees generally decreased with increasing elevation, suggesting a greater reliance on N supplied by mycorrhizal fungi in colder soils. However, abrupt increase in the Larix δ¹⁵N at the treeline suggests opening of the N cycle for this deciduous tree species, either by a decreasing N demand of slow-growing trees or a reduced competition for N with other plants. Overall, our results indicate that in cold treeline ecotones the sources and availabilities of soil N have a key influence on determing the N uptake pathways of trees and consequently plant growth.