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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. 

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. 

Despite being present in many North American forest understories, the ectomycorrhizal (ECM) fungal communities associated with Corylus shrubs have received no prior study. To address this knowledge gap, we characterized the ECM fungal communities on roots of Corylus shrubs as well as co-occurring Quercus and Pinus trees in Minnesota, USA. ECM-colonized root tips from pairs of Corylus shrubs and four ECM tree species, Quercus macrocarpa, Quercus ellipsoidalis, Pinus strobus, and Pinus resinosa, growing in close proximity (<1 m), were sampled at the Cedar Creek Ecosystem Science Reserve. ECM fungal communities were assessed using high-throughput sequencing of the ITS2 region. ECM fungal operational taxonomic unit (OTU) richness was equivalent among the two Quercus species and their associated Corylus shrubs, but significantly higher on P. strobus–associated Corylus shrubs compared with P. strobus, P. resinosa, and P. resinosa–associated Corylus shrubs. ECM fungal community composition on Corylus shrubs largely mirrored that on each of the Quercus and Pinus species, although the two Pinus commu- nities were significantly different from each other. Further, the same ECM fungal OTUs were commonly encountered on paired Corylus–tree host samples, suggesting a high potential for co- colonization by the same fungal individuals. Collectively, these results support the growing consensus that woody understory plants often associate with similar ECM fungal communities as co-occurring tree hosts regardless of phylogenetic relatedness 

Despite the importance of fungi to forest carbon (C) cycling and increasing calls to include microbial interactions in ecosystem models, how shifting fungal guild abundances impact soil C stocks re- mains poorly quantified, particularly in mineral soils where most C is stored. Additionally, a greater understanding of how fungal interguild interac- tions affect belowground litter decomposition is needed to more fully characterize soil C dynamics. To address these knowledge gaps, we conducted a multi-year soil trenching experiment in two tem- perate Pinus strobus stands in Minnesota, USA. We found that after two years, trenching increased ectomycorrhizal fungal relative abundance while decreasing saprotrophic fungal relative abundance (decreased ectomycorrhizal/saprotrophic ratio) and concurrently decreased soil C stocks by 10%. The decreased C stocks were primarily due to changes in particulate organic matter and were largely constrained to the top 5 cm of the soil. Trenching also stimulated both root and fungal litter decom- position in surface soils. Together, these results support the often proposed but rarely tested hypothesis that shifting fungal guild abundances promote soil C accumulation. However, they also suggest this effect may be most relevant for short- term C storage in upper soil layers.