Independent of soil saprotrophic microorganisms, ectomycorrhizal fungi facilitate N utilization from protein-condensed tannin complex by Red Oak (Quercus rubra)

Abstract:

Background/Question/Methods

The ability of ectomycorrhizal (ECM) fungi to obtain nitrogen (N) from organic forms such as amino acids and proteins is well established. However, no direct evidence has been gathered to demonstrate that ECM fungi could access N complexed in soil organic matter (SOM) such as polyphenol-protein complexes, which are dominated in forest ecosystems. Moreover, the interactions with saprotrophic microbes (SAP) adds extra uncertainties associated with the capacity of ECM fungi to obtain N from SOM. Here, we explored whether ECM fungi exhibit saprotrophic capabilities in utilizing complexed organic N without interaction with SAP.

We conducted a manipulated experiment using ¹⁵N-labeled protein-tannin complexes as the main N source for Quercus rubra seedlings inoculated with mixed ECM fungal species in the presence or absence of native soil SAP. Plant growth performance, mycorrhizal colonization, fine root morphological traits, and ¹⁵N enrichments within the root and leaf tissues were examined to test plant utilization of complexed organic N when associated with ECM fungi. Additionally, the activities of soil enzymes involved in depolymerization of tannin-protein complexes were determined to test the saprotrophic function of ECM fungi.

Main results/Conclusions

Both shoot and root system dry weights were significantly increased by ECM fungal inoculation irrespective of the presence of SAP, but were not influenced by the inoculation of SAP alone. Similarly, 2-fold higher ¹⁵N enrichment was detected in both leaves and roots in treatments with ECM fungi inoculated, compared with control and SAP treatments. Furthermore, the ECM fungal inoculation markedly stimulated the activities of soil enzymes with the functional potential to depolymerize polyphenol-protein complexes and proteins, even without SAP interactions. Collectively, our results provide a direct evidence that ECM fungi could independently obtain N from the protein-condensed tannin complex and enhance N supply of host plants, even without the interactions of saprotrophic microorganisms.