Search for: All records

Abstract Despite growing interest in fungal necromass decomposition due to its importance in soil carbon retention, whether a consistent group of microorganisms is associated with decomposing necromass remains unresolved. Here, we synthesize knowledge on the composition of the bacterial and fungal communities present on decomposing fungal necromass from a variety of fungal species, geographic locations, habitats, and incubation times. We found that there is a core group of both bacterial and fungal genera (i.e. a core fungal necrobiome), although the specific size of the core depended on definition. Based on a metric that included both microbial frequency and abundance, we demonstrate that the core is taxonomically and functionally diverse, including bacterial copiotrophs and oligotrophs as well as fungal saprotrophs, ectomycorrhizal fungi, and both fungal and animal parasites. We also show that the composition of the core necrobiome is notably dynamic over time, with many core bacterial and fungal genera having specific associations with the early, middle, or late stages of necromass decomposition. While this study establishes the existence of a core fungal necrobiome, we advocate that profiling the composition of fungal necromass decomposer communities in tropical environments and other terrestrial biomes beyond forests is needed to fill key knowledge gaps regarding the global nature of the fungal necrobiome. 

Abstract Ectomycorrhizal (ECM) fungi have long been thought to reduce litter decomposition in nitrogen (N)‐limited ecosystems by outcompeting saprotrophs for litter N (a phenomenon known as the ‘Gadgil effect’). However, recent research has called the generality of this effect into question, by demonstrating that ECM fungi can increase or decrease organic matter decomposition in different forests. The ecological factors driving such variation in the size and direction of ECM fungal effects on decomposition remain unclear.Here, we tested the hypothesis that ECM fungi would suppress decomposition of N‐poor, recalcitrant litter more in forests with lower N‐availability by exacerbating saprotrophic N limitation. We conducted an in situ ECM fungal and root reduction experiment (via soil trenching) in nine pine forests across three US states, which varied in soil and litter N content, climate and pine host (Pinus muricatain California,P. elliottiiin Florida and P.resinosain Minnesota). In each site, we decomposed needle litter from (1) a pine species native to that site and (2) a common litter,P. strobus, for 1 year.Contrary to our expectations, ECM fungi either stimulated (California) or had no effect on (Florida and Minnesota) pine needle decomposition. Across sites, ECM fungal stimulation of decomposition did increase with total soil N content, but was unrelated to inorganic N availability. Furthermore, despite previous work suggesting that competition for N structures ECM fungal–saprotroph interactions, trenching effects on decomposition did not differ between pine litter types, despite large differences in initial litter C:N ratios, recalcitrance and net litter N immobilization.Synthesis. Taken together, our results add to a growing body of evidence that the ‘Gadgil effect’ is not universal, even in the N‐poor litter of temperate pine forests where it was first described and is often invoked. Furthermore, the inconsistency of relationships between trenching effects with different metrics of decomposer N supply and demand calls into question the central role of N in structuring fungal interguild interactions. 

Abstract Mast seeding is a well‐documented phenomenon across diverse forest ecosystems. While its effect on aboveground food webs has been thoroughly studied, how it impacts the soil fungi that drive soil carbon and nutrient cycling has not yet been explored. To evaluate the relationship between mast seeding and fungal resource availability, we paired a Swiss 29‐year fungal sporocarp census with contemporaneous seed production for European beech (Fagus sylvaticaL.). On average, mast seeding was associated with a 55% reduction in sporocarp production and a compositional community shift towards drought‐tolerant taxa across both ectomycorrhizal and saprotrophic guilds. Among ectomycorrhizal fungi, traits associated with carbon cost did not explain species' sensitivity to seed production. Together, our results support a novel hypothesis that mast seeding limits annual resource availability and reproductive investment in soil fungi, creating an ecosystem ‘rhythm’ to forest processes that is synchronized above‐ and belowground. 

Summary Rising atmospheric carbon dioxide concentrations (CO₂) and atmospheric nitrogen (N) deposition have contrasting effects on ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) symbioses, potentially mediating forest responses to environmental change.In this study, we evaluated the cumulative effects of historical environmental change on N concentrations and δ¹⁵N values in AM plants, EM plants, EM fungi, and saprotrophic fungi using herbarium specimens collected in Minnesota, USA from 1871 to 2016. To better understand mycorrhizal mediation of foliar δ¹⁵N, we also analyzed a subset of previously published foliar δ¹⁵N values from across the United States to parse the effects of N deposition and CO₂rise.Over the last century in Minnesota, N concentrations declined among all groups except saprotrophic fungi. δ¹⁵N also declined among all groups of plants and fungi; however, foliar δ¹⁵N declined less in EM plants than in AM plants. In the analysis of previously published foliar δ¹⁵N values, this slope difference between EM and AM plants was better explained by nitrogen deposition than by CO₂rise.Mycorrhizal type did not explain trajectories of plant N concentrations. Instead, plants and EM fungi exhibited similar declines in N concentrations, consistent with declining forest N status despite moderate levels of N deposition. 

Abstract Microbial necromass is increasingly recognized as an important fast‐cycling component of the long‐term carbon present in soils. To better understand how fungi and bacteria individually contribute to the decomposition of fungal necromass, three particle sizes (>500, 250–500, and <250 μm) ofHyaloscypha bicolornecromass were incubated in laboratory microcosms inoculated with individual strains of two fungi and two bacteria. Decomposition was assessed after 15 and 28 days via necromass loss, microbial respiration, and changes in necromass pH, water content, and chemistry. To examine how fungal–bacterial interactions impact microbial growth on necromass, single and paired cultures of bacteria and fungi were grown in microplates containing necromass‐infused media. Microbial growth was measured after 5 days through quantitative PCR. Regardless of particle size, necromass colonized by fungi had higher mass loss and respiration than both bacteria and uninoculated controls. Fungal colonization increased necromass pH, water content, and altered chemistry, while necromass colonized by bacteria remained mostly unaltered. Bacteria grew significantly more when co‐cultured with a fungus, while fungal growth was not significantly affected by bacteria. Collectively, our results suggest that fungi act as key early decomposers of fungal necromass and that bacteria may require the presence of fungi to actively participate in necromass decomposition.