Resolving the 'nitrogen paradox' of arbuscular mycorrhizas: fertilization with organic matter brings considerable benefits for plant nutrition and growth.

Tom J Thirkell,Angela Hodge,Duncan D Cameron

doi:10.1111/pce.12667

Tom J Thirkell, Angela Hodge + Show 1 more

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https://doi.org/10.1111/pce.12667

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Abstract

Arbuscular mycorrhizal fungi (AMF) can transfer nitrogen (N) to host plants, but the ecological relevance is debated, as total plant N and biomass do not generally increase. The extent to which the symbiosis is mutually beneficial is thought to rely on the stoichiometry of N, phosphorus (P) and carbon (C) availability. While inorganic N fertilization has been shown to elicit strong mutualism, characterized by improved plant and fungal growth and mineral nutrition, similar responses following organic N addition are lacking. Using a compartmented microcosm experiment, we determined the significance to a mycorrhizal plant of placing a 15N‐labelled, nitrogen‐rich patch of organic matter in a compartment to which only AMF hyphae had access. Control microcosms denied AMF hyphal access to the patch compartment. When permitted access to the patch compartment, the fungus proliferated extensively in the patch and transferred substantial quantities of N to the plant. Moreover, our data demonstrate that allowing hyphal access to an organic matter patch enhanced total plant N and P contents, with a simultaneous and substantial increase in plant biomass. Furthermore, we demonstrate that organic matter fertilization of arbuscular mycorrhizal plants can foster a mutually beneficial symbiosis based on nitrogen transfer, a phenomenon previously thought irrelevant.

Highlights

The arbuscular mycorrhizal (AM) association is the most common type of mycorrhizal symbiosis and forms between c. two-thirds of all land plant species and soil fungi in the phylum Glomeromycota
Total plant dry weight increased substantially when arbuscular mycorrhizal fungi (AMF) hyphae were allowed access to the patch compartment (3.44 ± 0.21 g with access versus 2.09 ± 0.23 g without access, T 1,37 = 4.33, P < 0.001). This increase in plant dry weight was driven by an increase in both the shoot and root mass, which increased by 62% and 73% respectively compared with those plants whose AMF partner was not permitted access to the organic patch (Fig. 2)
There was no significant difference in the root weight ratio (RWR; ratio of root dry weight to total plant dry weight) between any treatments, suggesting that allocation of biomass between roots and shoots did not change as a result of the plants’ AMF partner having access to the organic material substrate

Summary

Introduction

The arbuscular mycorrhizal (AM) association is the most common type of mycorrhizal symbiosis and forms between c. two-thirds of all land plant species and soil fungi in the phylum Glomeromycota. More realistic experiments, using whole plants and adding N as organic matter patches, have shown that AMF contribution to plant N uptake can be as high as 15-20% (Barrett et al 2014; Leigh et al 2009) This may suggest a significant nutritional contribution to the plant, the total plant N content (Hodge 2001, Hodge et al 2000a, Leigh et al 2009) and plant biomass (Herman et al 2012; Hodge et al 2001) is usually unaffected. The plant may even suffer a reduction in biomass (Reynolds et al 2005), implying providing N fertilization to N-limited symbioses may be deleterious

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