Radiocarbon evidence for the mining of organic nitrogen from soil by mycorrhizal fungi

Organic nitrogen use by mycorrhizal fungi and associated plants could fuel productivity in nitrogen-limited systems. To test whether fungi assimilated soil-derived organic nitrogen, we compared the ¹⁴C signal (expressed as Δ¹⁴C) from 1950s to 1960s thermonuclear testing in protein and structural carbon of ectomycorrhizal fungi. As expected, structural carbon had Δ¹⁴C similar to recent photosynthesis; however, protein Δ¹⁴C was either higher or lower than structural carbon depending on the fungal taxa. This suggests that some protein carbon derived from uptake of organic nitrogen with different Δ¹⁴C signals. Specimens from two taxa (Lactarius and Russula) adapted to taking up soluble nutrients had protein higher than structural carbon in Δ¹⁴C, indicating uptake of young, post-bomb organic nitrogen, whereas two taxa (Cortinarius and Leccinum) adapted to using insoluble, complex organic nutrients had protein lower than structural carbon in Δ¹⁴C, indicating uptake of old, pre-bomb organic nitrogen. Tuber, a genus common in mineral soil, was also consistently lower in Δ¹⁴C for protein than for structural carbon, with an estimated 10 % of protein carbon originating from old, deep organic nitrogen for this taxon. Our results indicate that radiocarbon can provide evidence of organic nitrogen use in ectomycorrhizal fungi and reflects the exploration depth of different taxa.