Print PageOrganic nitrogen use by mycorrhizal fungi and associated plants could close nitrogen budgets in nitrogen-limited systems and is commonly tested using isotopically labeled amino acids. However, artifacts accompanying these additions mean that such tracer experiments provide limited insight into natural systems. Here, we separated ectomycorrhizal fungi collected between 1951 and 1979 in the Pacific Northwest and in northern Finland into protein and structural carbon and used the 14C signal from thermonuclear testing in the 1950s and 1960s to directly demonstrate that fungi assimilated soil-derived organic nitrogen.
Results/Conclusions
In samples of the truffle Tuber oregonense collected between 1951 and 1979 in the Pacific Northwest, 10-15% of Tuber protein was derived from organic nitrogen up to 2000 years old. Structural carbon was exclusively derived from current-year photosynthate. Assuming fungi retain 65% of amino acid carbon during metabolic cycling, soil-derived organic nitrogen contributes 15-23% to fungal nitrogen budgets. Amino acids in the mineral horizons containing Tuber may remain biologically available to fungi for centuries. In other ectomycorrhizal species collected in 1971-1978 in Finland, two taxa adapted to taking up soluble nutrients had protein 14C higher than structural 14C, indicating uptake of young, post-bomb organic nitrogen. In contrast, two other taxa adapted to using insoluble, complex organic nutrients had protein 14C lower than structural 14C, indicating uptake of old, pre-bomb organic nitrogen. Since soil carbon age increases with soil depth and also increases in mineral horizons with increasing soil density, protein 14C reflects both the depth of fungal exploration and the assimilation of supposedly recalcitrant, mineral-associated carbon and nitrogen. Models of forest functioning should be revised to account for soil organic nitrogen as a potential source missed by current measurements.
