Mycorrhizas (fungal roots) play vital roles in plant nutrient acquisition, performance and productivity in terrestrial ecosystems. Arbuscular mycorrhizas (AM) and ectomycorrhizas (EM) are mostly important since soil nutrients, including NH₄⁺, NO₃⁻ and phosphorus, are translocated from mycorrhizal fungi to plants. Individual species, genera and even families of plants could be interconnected by mycorrhizal mycelia to form common mycorrhizal networks (CMNs). The function of CMNs is to provide pathways for movement or transfer of nutrients from one plant to another. In the past four decades, both ¹⁵N external labeling or enrichment (usually expressed as atom%) and ¹⁵N naturally occurring abundance (δ¹⁵N, ‰) techniques have been employed to trace the direction and magnitude of N transfer between plants, with their own advantages and limitations. The heavier stable isotope ¹⁵N is discriminated against ¹⁴N during biochemical, biogeochemical and physiological processes, due to a greater atomic mass. In general, non-N₂-fixing plants had greater δ¹⁵N values than N₂-fixing (∼0‰) ones. Foliar δ¹⁵N often varied by 5 to 10‰ in the order: non-mycorrhizas/AMs > EMs ≥ ericoid mycorrhizas. Differences in δ¹⁵N (‰) or ¹⁵N (atom%) values could thus provide N transfer information between plants. A range of between 0 to 80% of one-way N transfer had been observed from N₂-fixing mycorrhizal to non-N₂-fixing mycorrhizal plants, but generally less than or around 10% in the reverse direction. Plant-to-plant N transfer may provide practical implications for plant performance in N-limited habitats. Considering that N translocation or cycling is crucial, and the potential benefits of N transfer are great in both agricultural and natural ecosystems, more research is warranted on either one-way or two-way N transfers mediated by CMNs with different species and under field conditions.

• This study quantified global soil C research trends and the proportional focus on SOC and SIC pools. • Soil inorganic C is an overlooked pool with only 4% share in global soil C research. • Approximately 40% of the soil C research was dedicated to climate change. • Soil C research is concentrated in few countries with 70% research contributed by 9 countries. • It is imperative to expand research focus on SIC to gain a comprehensive understanding of global C cycle.