Does resource exchange in ectomycorrhizal symbiosis vary with competitive context and nitrogen addition?

Abstract

Ectomycorrhizal symbiosis is essential for the nutrition of most temperate forest trees and helps regulate the movement of carbon (C) and nitrogen (N) through forested ecosystems. The factors governing the exchange of plant C for fungal N, however, remain obscure.Because competition and soil resources may influence ectomycorrhizal resource movement, we performed a 10-month split-root microcosm study using Pinus muricata seedlings with Thelephora terrestris, Suillus pungens, or no ectomycorrhizal fungus, under two N concentrations in artificial soil. Fungi competed directly with roots and indirectly with each other. We used stable isotope enrichment to track plant photosynthate and fungal N.For T. terrestris, plants received N commensurate with the C given to their fungal partners. Thelephora terrestris was a superior mutualist under high-N conditions. For S. pungens, plant C and fungal N exchange were not coupled. However, in low-N conditions, plants preferentially allocated C to S. pungens rather than T. terrestris.Our results suggest that ectomycorrhizal resource transfer depends on competitive and nutritional context. Plants can exchange C for fungal N, but coupling of these resources can depend on the fungal species and soil N. Understanding the diversity of fungal strategies, and how they change with environmental context, reveals mechanisms driving this important symbiosis.

A synthesis on the role of mycorrhizal fungal traits for understanding forest tree functions

“…Despite pleas more than three decades ago (Read, 1991), the majority of current studies on mycorrhizal functions uses inoculation with a limited number of easily culturable mycorrhizal fungal species to identify the influence of fungal genetic identity, diversity, or community composition on tree functioning (e.g., Diagne et al ., 2013; Thoen et al ., 2020; Bogar et al ., 2022). Some studies stepped toward more natural conditions by growing host trees inoculated with a native pool of fungal species in the field (Siqueira et al ., 1998), or by growing trees with natural fungal assemblages under controlled experimental conditions (De Grandcourt et al ., 2004; Meier et al ., 2013; Köhler et al ., 2018).…”

“…While the former three are probably merely result of a Eurocentric and economic-targeted sampling bias, solving the latter is more complex since not all methods are applicable in the field. Despite pleas more than three decades ago (Read, 1991), the majority of current studies on mycorrhizal functions uses inoculation with a limited number of easily culturable mycorrhizal fungal species to identify the influence of fungal genetic identity, diversity, or community composition on tree functioning (e.g., Diagne et al, 2013;Thoen et al, 2020;Bogar et al, 2022). Some studies stepped toward more natural conditions by growing host trees inoculated with a native pool of fungal species in the field (Siqueira et al, 1998), or by growing trees with natural fungal assemblages under controlled experimental conditions (De Grandcourt et al, 2004;Meier et al, 2013;Köhler et al, 2018).…”

Development of split-root assays for loblolly pine (Pinus taeda L.) seedlings to study ectomycorrhizal symbioses

“…For example, a split-root system was used with 3- to 4-year-old beech trees ( Fagus sylvatica ) to determine spatial distribution of host photosynthates and fungal nitrogen in ectomycorrhizae [16] . Another experiment with Pinus muricata elaborated on a similar design by including different ECM fungal species in adjacent root compartments to study the impact of nitrogen access on indirect competition between fungi for host carbon resources [9] . Although the timeline of this setup was similar to ours, using a hydroponic approach in glass beakers might help in evaluating and controlling the formation of lateral roots.…”

“…Loblolly pine forms beneficial symbiotic associations with a variety of ectomycorrhizal (ECM) fungal species that play a major role in the tree hydromineral nutrition [4] , [5] , [6] . Split-root experiments have been used in other pine species to study interactions amongst ECM fungi, host trees, and soil conditions [7] , [8] , [9] . However, to this date, no method for the rapid establishment of a split-root system for studying ECM symbioses in loblolly pine has been described.…”

Soil nutrients and plant diversity affect ectomycorrhizal fungal community structure and functional traits across three subalpine coniferous forests

“…The photosynthetically derived carbon (C) from host plants is critical for EMF who in return, acquire nutrients for host plants by directly emanating their hyphae to scavenge inorganic nutrients, or through indirect provisioning of organic nutrients via enzymatic decomposition of the organic matter in soils ( Bogar et al, 2022 ). The EMF and host plant species are therefore highly specific and any changes in diversity of host plant species will greatly affect the foraging-related functional traits of the symbiotic EMF community ( Reis et al, 2018 ).…”