Almost all higher land plants deliver recently assimilated carbon (C) to mycorrhizal fungi, and receive nutrients in exchange. The controls of this exchange, however, are still under debate. While some studies indicate reciprocal trading of C for nutrients between plants and arbuscular mycorrhizal fungi, not much is known about ‘terms of trade’ in the ectomycorrhizal symbiosis.
We investigated the exchange of C for N between beech (Fagus sylvatica) and ectomycorrhizal fungi at different scales. We exposed young beech trees associated with natural mycorrhizal fungal communities to a 13CO2 atmosphere and added 15N-labelled amino acids to a compartment exclusively accessible to mycorrhizal hyphae. Within two days after 15N and 13CO2 application, fine roots and mycorrhizal root-tips were harvested and analyzed by isotope-ratio mass spectrometry (EA-IRMS). Associated fungal communities were identified by ITS sequencing. Selected mycorrhizal root tips were prepared for nano-scale secondary ion mass spectrometry (NanoSIMS) to visualize the spatial distribution of 13C and 15N in cross-sections at the subcellular scale.
If the trading of C for N was reciprocal, we expect that 13C would be correlated to 15N (a) within the plant root system, (b) across mycorrhizal root tips, and/or (c) at the subcellular level within one mycorrhizal root tip.
Our results showed a significant, albeit weak correlation between 13C and 15N across collected fine roots and mycorrhizal root-tips. The variability of the latter was seemingly influenced by fungal colonization pattern. At the subcellular scale, however, NanoSIMS imaging of a cross-section of a mycorrhizal root tip revealed a strong spatial correlation between 13C and 15N in both plant and fungal cells (within the Hartig Net and the fungal mantle) as well as in fungal external hyphae (hyphae diverging away from the root tip). Intriguingly, individual ‘hotspots’ of external fungal hyphae that were highly enriched in 15N (delivering high amounts of the added 15N to the plant), were also always extraordinarily enriched in 13C (receiving more 13C in return).
Our results provide first evidence for a reciprocal exchange of C for N between plants and ectomycorrhizal fungi at the subcellular scale. This indicates that a mechanism exists at this scale, that (i) either allows plants to direct their C flow into N-delivering parts of the mycorrhizal hyphal network or (ii) allow the fungus to ‘draw’ more C from the plant (develop a higher sink strength) when it has access to N.