Substantial nitrogen acquisition by arbuscular mycorrhizal fungi from organic material has implications for N cycling

TLDR

Arbuscular mycorrhizal fungi are obligate biotrophs that obtain carbon from host plants but can also proliferate hyphae in organic matter and acquire nitrogen from it. The study tested whether organic matter serves as a significant nitrogen source for the fungi themselves. Using microcosms that restricted 15N/13C‑labeled organic patches to the fungus, the authors showed that the fungus acquired nitrogen as decomposition products and that hyphae from both Glomus hoi and Glomus mosseae could colonize new hosts more effectively after exploiting the patch. The fungi extracted up to 31 % of their nitrogen from the patch, stimulated plant growth and nitrogen uptake, and their substantial biomass and nitrogen demand position them as a global nitrogen pool comparable to fine roots, underscoring an overlooked role in the nitrogen cycle.

Abstract

Arbuscular mycorrhizal (AM) fungi are obligate biotrophs that acquire carbon (C) solely from host plants. AM fungi can proliferate hyphae in, and acquire nitrogen (N) from, organic matter. Although they can transfer some of that N to plants, we tested the hypothesis that organic matter is an important N source for the AM fungi themselves. We grew pairs of plants with and without the AM fungus Glomus hoi in microcosms that allowed only the fungus access to a 15 N/ 13 C-labeled organic patch; in some cases, one plant was shaded to reduce C supply to the fungus. The fungal hyphae proliferated vigorously in the patch, irrespective of shading, and increased plant growth and N content; ∼3% of plant N came from the patch. The extraradical mycelium of the fungus was N-rich (3–5% N) and up to 31% of fungal N came from the patch, confirming the hypothesis. The fungus acquired N as decomposition products, because hyphae were not 13 C-enriched. In a second experiment, hyphae of both G. hoi and Glomus mosseae that exploited an organic material patch were also better able to colonize a new host plant, demonstrating a fungal growth response. These findings show that AM fungi can obtain substantial amounts of N from decomposing organic materials and can enhance their fitness as a result. The large biomass and high N demand of AM fungi means that they represent a global N pool equivalent in magnitude to fine roots and play a substantial and hitherto overlooked role in the nitrogen cycle.