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1. Wet-dry cycling influences the formation of mineral-associated organic matter and its sensitivity to simulated root exudates

   https://doi.org/10.1016/j.geoderma.2024.116869  

   Jilling, Andrea; Sokol, Noah W; Morán-Rivera, Karen; Grandy, A Stuart (May 2024, Geoderma)

   Mineral particles provide reactive sites for organic carbon (C) to bind in soil; this ‘mineral-associated organic matter’ (MAOM) may persist for centuries to millennia or cycle rapidly in minutes to days. The conditions and processes that influence short and long-term cycling of MAOM are poorly constrained. Soil moisture is one key control on organic matter cycling in soil, and projected shifts in moisture regimes towards more intense rainfall and prolonged drought under climate change may alter MAOM formation and cycling. Here, in a 3-week laboratory incubation study, we evaluated how two contrasting moisture regimes affected the formation and cycling of 13C-labeled MAOM from two mineralogically-distinct soil types. Repeated wet-dry cycling (between 3% and 60% of water-holding capacity) enhanced the formation of 13C-MAOM relative to constant moisture conditions. The two soil types differed in rates of MAOM formation and the sensitivity of newly-formed and pre-existing MAOM to subsequent priming in the presence of simulated exudates (glucose and/or oxalic acid). Wet-dry cycling enhanced the decomposition of newly-formed MAOM and it further accelerated the potential priming of pre-existing MAOM. Therefore, while repeated cycles between drought-like and “optimal” moisture conditions may promote the formation of MAOM, they may also undermine the stability of pre-existing MAOM and limit opportunities for new C inputs to enter more persistent forms. 

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2. Emergent temperature sensitivity of soil organic carbon driven by mineral associations

   https://doi.org/10.1038/s41561-024-01384-7  

   Georgiou, Katerina; Koven, Charles D; Wieder, William R; Hartman, Melannie D; Riley, William J; Pett-Ridge, Jennifer; Bouskill, Nicholas J; Abramoff, Rose Z; Slessarev, Eric W; Ahlström, Anders; et al (March 2024, Nature Geoscience)

   Soil organic matter decomposition and its interactions with climate depend on whether the organic matter is associated with soil minerals. However, data limitations have hindered global-scale analyses of mineral-associated and particulate soil organic carbon pools and their benchmarking in Earth system models used to estimate carbon cycle–climate feedbacks. Here we analyse observationally derived global estimates of soil carbon pools to quantify their relative proportions and compute their climatological temperature sensitivities as the decline in carbon with increasing temperature. We find that the climatological temperature sensitivity of particulate carbon is on average 28% higher than that of mineral-associated carbon, and up to 53% higher in cool climates. Moreover, the distribution of carbon between these underlying soil carbon pools drives the emergent climatological temperature sensitivity of bulk soil carbon stocks. However, global models vary widely in their predictions of soil carbon pool distributions. We show that the global proportion of model pools that are conceptually similar to mineral-protected carbon ranges from 16 to 85% across Earth system models from the Coupled Model Intercomparison Project Phase 6 and offline land models, with implications for bulk soil carbon ages and ecosystem responsiveness. To improve projections of carbon cycle–climate feedbacks, it is imperative to assess underlying soil carbon pools to accurately predict the distribution and vulnerability of soil carbon. 

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3. Moisture-driven divergence in mineral-associated soil carbon persistence

   https://doi.org/10.1073/pnas.2210044120  

   Heckman, Katherine A.; Possinger, Angela R.; Badgley, Brian D.; Bowman, Maggie M.; Gallo, Adrian C.; Hatten, Jeff A.; Nave, Lucas E.; SanClements, Michael D.; Swanston, Christopher W.; Weiglein, Tyler L.; et al (February 2023, Proceedings of the National Academy of Sciences)

   Mineral stabilization of soil organic matter is an important regulator of the global carbon (C) cycle. However, the vulnerability of mineral-stabilized organic matter (OM) to climate change is currently unknown. We examined soil profiles from 34 sites across the conterminous USA to investigate how the abundance and persistence of mineral-associated organic C varied with climate at the continental scale. Using a novel combination of radiocarbon and molecular composition measurements, we show that the relationship between the abundance and persistence of mineral-associated organic matter (MAOM) appears to be driven by moisture availability. In wetter climates where precipitation exceeds evapotranspiration, excess moisture leads to deeper and more prolonged periods of wetness, creating conditions which favor greater root abundance and also allow for greater diffusion and interaction of inputs with MAOM. In these humid soils, mineral-associated soil organic C concentration and persistence are strongly linked, whereas this relationship is absent in drier climates. In arid soils, root abundance is lower, and interaction of inputs with mineral surfaces is limited by shallower and briefer periods of moisture, resulting in a disconnect between concentration and persistence. Data suggest a tipping point in the cycling of mineral-associated C at a climate threshold where precipitation equals evaporation. As climate patterns shift, our findings emphasize that divergence in the mechanisms of OM persistence associated with historical climate legacies need to be considered in process-based models. 

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4. Warming and Reduced Rainfall Alter Fungal Necromass Decomposition Rates and Associated Microbial Community Composition and Functioning at a Temperate–Boreal Forest Ecotone

   https://doi.org/10.1111/gcb.70536  

   Cantoran, Anahi; Maillard, François; Bermudez, Raimundo; Stefanski, Artur; Reich, Peter B; Kennedy, Peter G (October 2025, Global Change Biology)

   ABSTRACT Changes in temperature and rainfall associated with altered climatic conditions are likely to significantly alter rates of soil organic matter decomposition. To determine how the combined effects of warming and drought impact the decomposition of fungal necromass, a large and fast‐cycling portion of the global soil organic carbon (C) pool, we incubatedHyaloscypha bicolornecromass under both ambient and altered conditions (+3.3°C air and soil warming and ~40% reduced rainfall) at the B4Warmed experiment in Minnesota, USA. We conducted two multi‐week incubations, one assessing mass loss and microbial community composition on decaying necromass after 1, 2, 7, and 14 weeks and the second characterizing the substrate utilization capacities of necromass‐associated microbial communities after Weeks 1 and 7. Warming and reduced rainfall accelerated the initial rate of necromass decay by ~20%, yet slowed overall mass loss by ~6% at the end of the 14‐week incubation. These different rates of decay over time paralleled shifting abiotic conditions, with altered plots experiencing warmer and relatively moist conditions early, but hotter and drier conditions later. The microbial community composition also varied by treatment and time, with warming and reduced rainfall stimulating fast‐growing fungi as well as fungal relative to bacterial growth overall. Additionally, the functional capacity of the microbial community also changed over time, having a higher metabolic capability to utilize C and N substrates in the altered plots early in decomposition but a lower capability later in decay. Collectively, our findings highlight a dynamic, stage‐dependent response of fungal necromass decomposition to altered climate regimes. By linking these decay dynamics to shifts in environmental conditions as well as microbial community composition and function, our study highlights the critical roles of both abiotic and biotic changes in mediating decomposition responses to climate change. 

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5. 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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