Abstract
Mycorrhizal fungi can affect soil organic matter cycling through several mechanisms including priming, nutrient competition, and direct enzyme production. Differences in nutrient foraging strategies between ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) fungi produce divergent belowground dynamics: where EcM can take up organic nitrogen and directly break down soil organic matter (SOM) by producing enzymes, AM fungi are limited to scavenging mineral N. EcM-associated tree species also have leaf litter with relatively higher ratios of carbon to nitrogen (C:N), and belowground saprotrophic communities more dominated by fungi. Consequently, free-living microbes in EcM-dominated soils should experience nitrogen limitation, with subsequent increases in enzyme production and decreased carbon use efficiency (CUE). However, the relative importance of the effects of substrate quality and fungal community composition on enzyme production and CUE are unclear. To assess this distinction, we sampled the organic horizon and 10 cm of the mineral horizon in northern temperate forest soils along a gradient of EcM dominance. We characterized fungal community composition by measuring EcM relative abundances from extracted fungal DNA and the fungal to bacterial (F:B) ratios from phospholipid fatty acid (PLFA) analysis. We assessed soil substrate quality as the soil C:N ratio. Soil microbial functions were measured as potential activities of five hydrolytic and two oxidative enzymes, and microbial CUE. We found that the fungal community, represented by either the F:B ratio, EcM relative abundance, or both, affected CUE and six measured enzyme activities, while the C:N ratio affected only oxidative and chitin-targeting extracellular enzyme activities. Our results highlight the use of EcM relative dominance as a predictor of soil microbial community composition and function independent of substrate quality.
Highlights
Global patterns show significant correlation between the mycorrhizal associations of canopy trees and soil carbon (C) and nitrogen (N) storage (Averill et al, 2014; Zhu et al, 2018; Lu and Hedin, 2019)
Litter with a low C:N degrades quickly and has been found to promote more efficient microbial growth or faster litter decomposition, but without a consistent increase in enzyme production (Sinsabaugh et al, 2002; Frey et al, 2013; Lashermes et al, 2016). These findings suggest that substrate quality, carbon use efficiency (CUE), and enzyme production are likely linked to interactions between mycorrhizal fungi and their free-living soil microbial community (Allison, 2012b)
The trends we observed along our gradient of EcM basal area support the conclusion that fungal community composition is a stronger driver of soil microbial functions than organic matter quality
Summary
Global patterns show significant correlation between the mycorrhizal associations of canopy trees and soil carbon (C) and nitrogen (N) storage (Averill et al, 2014; Zhu et al, 2018; Lu and Hedin, 2019). Given that mycorrhizal hyphal biomass can represent a third or more of the total microbial biomass in forest soils (Högberg and Högberg, 2002), the overall effect of EcM enzyme production should have a significant effect on organic matter decay (sensu Zak et al, 2019) and nutrient liberation even if their enzyme production rate is less than that of saprotrophic fungi (Talbot et al, 2008; Lindahl and Tunlid, 2015; Martin et al, 2016) Both EcM and AM fungi affect the surrounding soil microbial community by priming free-living saprotrophs with labile exudates to induce extracellular enzyme production and subsequent nutrient release (Hodge et al, 2001; Talbot et al, 2008; Kaiser et al, 2015; Gorka et al, 2019). Excess N, for example, has been shown to significantly decrease mycorrhizal-mediated exudation and EEA likely because plants decrease mycorrhizal access to photosynthate (Sinsabaugh et al, 2005; Högberg et al, 2010; Gorka et al, 2019)
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