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1. Refining the role of nitrogen mineralization in mycorrhizal nutrient syndromes

   https://doi.org/10.1007/s10533-023-01038-7  

   Seyfried, Georgia S. ; Midgley, Meghan G. ; Phillips, Richard P. ; Yang, Wendy H. ( May 2023 , Biogeochemistry)

   Forest stands dominated by ectomycorrhizal (ECM) associated trees often have more closed nitrogen (N) cycling than stands dominated by arbuscular mycorrhizal (AM) associated trees, with slower N mineralization in ECM stands thought to suppress inorganic N cycling. However, most estimates of N mineralization come from measurements of net processes, which can lead to an incomplete view of ecosystem N retention and loss. To explore the mechanisms driving mycorrhizal N cycling syndromes, we measured gross N production and assimilation rates and net and potential N flux rates in paired N addition (from NH₄SO₄and NaNO₃) and control plots within ECM and AM-dominated stands. We observed greater gross N mineralization and microbial ammonium assimilation in ECM compared to AM stands, suggesting that increased microbial N demand drove lower net N mineralization rates in ECM stands. We found lower nitrification rates in ECM compared to AM stands and no effect of N addition on nitrification in ECM stands. Therefore, the low soil pH or high C:N ratios found in those stands, not limited ammonium supply, may have suppressed nitrification. Finally, potential denitrification rates and nitrous oxide fluxes were lower in ECM compared to AM stands with no effect of N addition, suggesting that denitrification is controlled by the endogenous supply of nitrate from nitrification, not exogenous nitrate inputs. Overall, we conclude that N mineralization may not play a central role in forming mycorrhizal nutrient syndromes, and that acidic conditions in ECM stands may ultimately control nitrification and the potential for ecosystem N loss.

    

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2. Distinct microbial communities alter litter decomposition rates in a fertilized coastal plain wetland

   https://doi.org/10.1002/ecs2.3619  

   Koceja, Megan_E ; Bledsoe, Regina_B ; Goodwillie, Carol ; Peralta, Ariane_L ( June 2021 , Ecosphere)

   Human activities have led to increased deposition of nitrogen (N) and phosphorus (P) into soils. Nutrient enrichment of soils is known to increase plant biomass and rates of microbial litter decomposition. However, interacting effects of hydrologic position and associated changes to soil moisture can constrain microbial activity and lead to unexpected nutrient feedbacks on microbial community structure–function relationships. Examining feedbacks of nutrient enrichment on decomposition rates is essential for predicting microbial contributions to carbon (C) cycling as atmospheric deposition of nutrients persists. This study explored how long‐term nutrient addition and contrasting litter chemical composition influenced soil bacterial community structure and function. We hypothesized that long‐term nutrient enrichment of low fertility soils alters bacterial community structure and leads to higher rates of litter decomposition especially for low C:N litter, but low‐nutrient and dry conditions limit microbial decomposition of high C:N ratio litter. We leveraged a long‐term fertilization experiment to test how nutrient enrichment and hydrologic manipulation (due to ditches) affected decomposition and soil bacterial community structure in a nutrient‐poor coastal plain wetland. We conducted a litter bag experiment and characterized litter‐associated and bulk soil microbiomes using 16S rRNA bacterial sequencing and quantified litter mass losses and soil physicochemical properties. Results revealed that distinct bacterial communities were involved in decomposing higher C:N ratio litter more quickly in fertilized compared to unfertilized soils especially under drier soil conditions, while decomposition rates of lower C:N ratio litter were similar between fertilized and unfertilized plots. Bacterial community structure in part explained litter decomposition rates, and long‐term fertilization and drier hydrologic status affected bacterial diversity and increased decomposition rates. However, community composition associated with high C:N litter was similar in wetter plots with available nitrate detected, regardless of fertilization treatment. This study provides insight into long‐term fertilization effects on soil bacterial diversity and composition, decomposition, and the increased potential for soil C loss as nutrient enrichment and hydrology interact to affect historically low‐nutrient ecosystems.

    

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3. Decelerated carbon cycling by ectomycorrhizal fungi is controlled by substrate quality and community composition

   https://doi.org/10.1111/nph.16269  

   Fernandez, Christopher W. ; See, Craig R. ; Kennedy, Peter G. ( November 2019 , New Phytologist)

   Interactions between symbiotic ectomycorrhizal (EM) and free‐living saprotrophs can result in significant deceleration of leaf litter decomposition. While this phenomenon is widely cited, its generality remains unclear, as both the direction and magnitude of EM fungal effects on leaf litter decomposition have been shown to vary among studies.

   Here we explicitly examine how contrasting leaf litter types and EM fungal communities may lead to differential effects on carbon (C) and nitrogen (N) cycling. Specifically, we measured the response of soil nutrient cycling, litter decay rates, litter chemistry and fungal community structure to the reduction of EM fungi (via trenching) with a reciprocal litter transplant experiment in adjacentPinus‐ orQuercus‐dominated sites.

   We found clear evidence of EM fungal suppression of C and N cycling in thePinus‐dominated site, but no suppression in theQuercus‐dominated site. Additionally, in thePinus‐dominated site, only thePinuslitter decay rates were decelerated by EM fungi and were associated with decoupling of litter C and N cycling.

   Our results support the hypothesis that EM fungi can decelerate C cycling via N competition, but strongly suggest that the ‘Gadgil effect’ is dependent on both substrate quality and EM fungal community composition. We argue that understanding tree host traits as well as EM fungal functional diversity is critical to a more mechanistic understanding of how EM fungi mediate forest soil biogeochemical cycling.

    

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4. Arbuscular Mycorrhizal Tree Communities Have Greater Soil Fungal Diversity and Relative Abundances of Saprotrophs and Pathogens than Ectomycorrhizal Tree Communities

   https://doi.org/10.1128/AEM.01782-21  

   Eagar, Andrew C. ; Mushinski, Ryan M. ; Horning, Amber L. ; Smemo, Kurt A. ; Phillips, Richard P. ; Blackwood, Christopher B. ( January 2022 , Applied and Environmental Microbiology)

   Druzhinina, Irina S. (Ed.)

   ABSTRACT Trees associating with different mycorrhizas often differ in their effects on litter decomposition, nutrient cycling, soil organic matter (SOM) dynamics, and plant-soil interactions. For example, due to differences between arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree leaf and root traits, ECM-associated soil has lower rates of C and N cycling and lower N availability than AM-associated soil. These observations suggest that many groups of nonmycorrhizal fungi should be affected by the mycorrhizal associations of dominant trees through controls on nutrient availability. To test this overarching hypothesis, we explored the influence of predominant forest mycorrhizal type and mineral N availability on soil fungal communities using next-generation amplicon sequencing. Soils from four temperate hardwood forests in southern Indiana, United States, were studied; three forests formed a natural gradient of mycorrhizal dominance (100% AM tree basal area to 100% ECM basal area), while the fourth forest contained a factorial experiment testing long-term N addition in both dominant mycorrhizal types. We found that overall fungal diversity, as well as the diversity and relative abundance of plant pathogenic and saprotrophic fungi, increased with greater AM tree dominance. Additionally, tree community mycorrhizal associations explained more variation in fungal community composition than abiotic variables, including soil depth, SOM content, nitrification rate, and mineral N availability. Our findings suggest that tree mycorrhizal associations may be good predictors of the diversity, composition, and functional potential of soil fungal communities in temperate hardwood forests. These observations help explain differing biogeochemistry and community dynamics found in forest stands dominated by differing mycorrhizal types. IMPORTANCE Our work explores how differing mycorrhizal associations of temperate hardwood trees (i.e., arbuscular [AM] versus ectomycorrhizal [ECM] associations) affect soil fungal communities by altering the diversity and relative abundance of saprotrophic and plant-pathogenic fungi along natural gradients of mycorrhizal dominance. Because temperate hardwood forests are predicted to become more AM dominant with climate change, studies examining soil communities along mycorrhizal gradients are necessary to understand how these global changes may alter future soil fungal communities and their functional potential. Ours, along with other recent studies, identify possible global trends in the frequency of specific fungal functional groups responsible for nutrient cycling and plant-soil interactions as they relate to mycorrhizal associations. 

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5. Well-Aerated Southern Appalachian Forest Soils Demonstrate Significant Potential for Gaseous Nitrogen Loss

   https://doi.org/10.3390/f10121155  

   Baas, Peter ; Knoepp, Jennifer D. ; Mohan, Jacqueline E. ( December 2019 , Forests)

   null (Ed.)

   Understanding the dominant soil nitrogen (N) cycling processes in southern Appalachian forests is crucial for predicting ecosystem responses to changing N deposition and climate. The role of anaerobic nitrogen cycling processes in well-aerated soils has long been questioned, and recent N cycling research suggests it needs to be re-evaluated. We assessed gross and potential rates of soil N cycling processes, including mineralization, nitrification, denitrification, nitrifier denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) in sites representing a vegetation and elevation gradient in the U.S. Department of Agriculture (USDA) Forest Service Experimental Forest, Coweeta Hydrologic Laboratory in southwestern North Carolina, USA. N cycling processes varied among sites, with gross mineralization and nitrification being greatest in high-elevation northern hardwood forests. Gaseous N losses via nitrifier denitrification were common in all ecosystems but were greatest in northern hardwood. Ecosystem N retention via DNRA (nitrification-produced NO3 reduced to NH4) ranged from 2% to 20% of the total nitrification and was highest in the mixed-oak forest. Our results suggest the potential for gaseous N losses through anaerobic processes (nitrifier denitrification) are prevalent in well-aerated forest soils and may play a key role in ecosystem N cycling. 

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