Environmental dependency of ectomycorrhizal fungi as soil organic matter oxidizers

Introduction

Forests constitute a significant reservoir of carbon (C), the majority of which is stored belowground, primarily in the form of soil organic matter (SOM) (Pan et al., 2011; Schmidt et al., 2011). The decomposition of SOM in forests is integral to the global cycling of C and nitrogen (N), underpinning diverse and critical forest ecosystem services such as climate regulation, biomass production and habitat provision for forest species (Deluca & Boisvenue, 2012). Within temperate and boreal forests, evidence increasingly suggests that ectomycorrhizal (ECM) fungi are involved in the decomposition of SOM (Phillips et al., 2014; Lindahl et al., 2021) mainly to capture and immobilize N into their tissues, which they can then exchange with their plant hosts for photosynthetically derived C (Lindahl & Tunlid, 2015; Baldrian, 2017). However, our understanding of how SOM decomposition differs across ECM fungal species and environmental contexts is in its infancy. These fundamental gaps pose challenges to the refinement of strategies aimed at optimizing C sequestration within the context of climate change.

ECM fungi originate from multiple phylogenetic groups and their ability to decompose SOM exhibits considerable variation across evolutionary lineages (Kohler et al., 2015; Pellitier & Zak, 2018). For example, Amanita muscaria, which evolved within a clade of brown rot saprotrophs, has undergone a genetic loss resulting in a reduced capacity for decomposing SOM (Kohler et al., 2015). By contrast, Hebeloma cylindrosporum, descended from a white-rot ancestor that used class II fungal peroxidases to oxidize SOM, has retained three manganese peroxidase genes for SOM decomposition (Kohler et al., 2015). Furthermore, the genome of Cortinarius glaucopus contains 11 peroxidases, a number comparable to that observed in numerous white-rot wood decomposers, underscoring their likely significant contribution to the decomposition of SOM within forest ecosystems (Bödeker et al., 2009; Miyauchi et al., 2020). Given the inherent functional heterogeneity of ECM fungi, shifts in their community composition are likely to drive distinct and profound effects on C and N cycling within forest ecosystems (Sterkenburg et al., 2018; Lindahl et al., 2021).

An important driver of ECM fungal community composition is the availability of inorganic N (Zak et al., 2019), which can also act as a regulator of ECM-mediated SOM decomposition (Bogar et al., 2021; Argiroff et al., 2022). Recent findings demonstrated that ECM fungal communities thriving in environments characterized by limited inorganic N content manifest an elevated genomic capacity for SOM decomposition (Mayer et al., 2023). These communities are often characterized by the prevalence of genera such as Cortinarius and Hebeloma (Pellitier & Zak, 2021). By contrast, ECM communities in soils with high inorganic N concentrations are typically dominated by genera such as Scleroderma and Russula, which have a weaker capacity for SOM decay (van der Linde et al., 2018). Other studies have revealed significant positive correlations between lignin-derived SOM and soil C content with inorganic N availability (Argiroff et al., 2022). This association is attributed to the presence of ECM fungi equipped with peroxidase enzymes, which exhibit diminished occurrence with increasing inorganic N availability (Clemmensen et al., 2015; Argiroff et al., 2022). Interactions between soil N availability, ECM fungal community composition, and soil C sequestration have been demonstrated within natural forest ecosystems, but the intricate mechanisms underpinning these relationships remain unresolved.

Along with variations in soil chemistry, interspecific interactions among ECM fungi exert significant influence on the structure of entire ECM communities, consequently impacting SOM dynamics (Kennedy, 2010; Fernandez & Kennedy, 2016). Studies have demonstrated that competition for N resources between ECM fungi and free-living decomposers can slowdown overall soil C cycling and increase soil C storage (Averill & Hawkes, 2016; Fernandez et al., 2020). However, how interactions between ECM fungal species might also alter rates of C cycling remains unclear, even though interspecific competition for resources within this group has been widely demonstrated (Koide et al., 2005; Kennedy, 2010; Smith et al., 2023) and is recognized as a key determinant shaping their community composition (Kennedy, 2010) and structure (Pickles et al., 2012). Interspecific interactions between ECM species may lead to similar inhibition, or alternatively could enable facilitation, whereby those species possessing more powerful decomposition strategies free up nutrients from recalcitrant soil compounds, enabling poorer decomposers to persist, in turn accelerating soil C cycling (Tiunov & Scheu, 2005; Lindahl & Tunlid, 2015). The lack of studies employing natural composite SOM extracts in competition experiments involving ECM fungi contributes to the uncertainty surrounding the impact of species interactions on SOM decomposition.

Here, we conducted controlled pure culture experiments to address the knowledge gaps surrounding the mechanisms and context-dependency of SOM decomposition by ECM fungi, strengthening our understanding of C and N cycling in forest ecosystems. We identified and cultured ECM species that thrived under conditions of low inorganic N availability and that were expected to demonstrate enhanced capacity to decompose SOM, alongside ECM species typically from soils with high inorganic N availability. We used these isolates to explore the context-dependency of SOM decomposition, testing the hypothesis that an increase in inorganic N availability would lead to reduced ECM fungal decomposition of N compounds, while potentially enhancing their decomposition of C compounds as a regulatory mechanism to offset the C and N imbalance induced by inorganic N supplementation. Additionally, we tested the hypothesis that interspecific interactions might negatively affect ECM fungal growth, with the magnitude of this impact varying by fungal identity and environmental contexts, consequently shaping SOM decomposition patterns. Analyses of fungal growth and extracellular enzyme production were paired with pyrolysis gas chromatography mass spectrometry (Py-GC-MS) to examine the SOM dynamics at the molecular level.