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1. Dispersal alters soil microbial community response to drought

   https://doi.org/10.1111/1462-2920.14707  

   Evans, S_E; Bell‐Dereske, L_P; Dougherty, K_M; Kittredge, H_A (July 2019, Environmental Microbiology)

   Summary Microbial communities will experience novel climates in the future. Dispersal is now recognized as a driver of microbial diversity and function, but our understanding of how dispersal influences responses to novel climates is limited. We experimentally tested how the exclusion of aerially dispersed fungi and bacteria altered the compositional and functional response of soil microbial communities to drought. We manipulated dispersal and drought by collecting aerially deposited microbes after precipitation events and subjecting soil mesocosms to either filter‐sterilized rain (no dispersal) or unfiltered rain (dispersal) and to either drought (25% ambient) or ambient rainfall for 6 months. We characterized community composition by sequencing 16S and ITS rRNA regions and function using community‐level physiological profiles. Treatments without dispersal had lower soil microbial biomass and metabolic diversity but higher bacterial and fungal species richness. Dispersal also altered soil community response to drought; drought had a stronger effect on bacterial (but not fungal) community composition, and induced greater functional loss, when dispersal was present. Surprisingly, neither immigrants nor drought‐tolerant taxa had higher abundance in dispersal treatments. We show experimentally that natural aerial dispersal rate alters soil microbial responses to disturbance. Changes in dispersal rates should be considered when predicting microbial responses to climate change. 

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2. Rainfall Manipulation Study Vegetation Data from the Chihuahuan Desert Grassland and Creosote Shrubland at the Sevilleta National Wildlife Refuge, New Mexico (2003-2011)

   https://doi.org/10.6073/pasta/361fbce9ce7d4d9530e34b4a8ee3c02e  

   Pockman, William (January 2013, Environmental Data Initiative)

   The overall goal of the rainfall manipulation project is to understand the coupled ecological and hydrological responses of a grassland, shrubland and a mixed grass-shrub vegetation community to extended periods of increased or decreased rainfall. Rainfall manipulation plots have been established in each of these three vegetation communities in the Five Points area of Sevilleta National Wildlife Refuge. In each vegetation community, three control plots, three drought treatment plots, and three water addition plots have been installed, each approximately 10 x 15 m in size. In each plot, vertical profiles of soil moisture probes have been installed under each cover type (canopy and interspace in grassland and shrubland; grass canopy, shrub canopy and interspace at the ecotone (mixed grass-shrub) site). The probes measure differences in infiltration and soil water content and potential associations with these different cover types. In addition, TDR probes have been installed diagonally in each cover type to integrate the water content of the top 15 cm of soil. Each plot contains 18, 1m2 quads made up of 6, 1m2 quads along each of the 3 transects located across each plot. Each spring and fall, the following parameters are measured in every quad: live plant cover, height, and abundance by species; dead plant cover; soil cover; litter cover; and rock cover. Data collection began in the drought and control plots in the spring of 2002. Data collection began in the water addition plots in the spring of 2004.In the grassland and shrubland communities, all nine currently established plots are located together. The three drought plots were located under a single large roof with a 0.5 m path separating each plot (drought treatments ended in 2006). The control plots and water addition plots are similarly grouped, but without the shelter structure. In the ecotone community, the plots are in three groups; each group is comprised of one drought plot, one water addition plot, and one control plot. Control plots received no experimental treatment, while the sliding roofs over the drought plots were used to divert precipitation, producing a long-term drought. The roofs covering the drought plots were lowered when there was no precipitation so that the amount of sunlight received by the drought plots was minimally affected. Water addition was intended to impose a complementary increase in water supply on the water addition plots.  

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3. Microbial community response to a decade of simulated global changes depends on the plant community

   https://doi.org/10.1525/elementa.2021.00124  

   Finks, Sarai S.; Weihe, Claudia; Kimball, Sarah; Allison, Steven D.; Martiny, Adam C.; Treseder, Kathleen K.; Martiny, Jennifer B. (January 2021, Elementa: Science of the Anthropocene)

   Global changes such as increased drought and atmospheric nitrogen deposition perturb both the microbial and plant communities that mediate terrestrial ecosystem functioning. However, few studies consider how microbial responses to global changes may be influenced by interactions with plant communities. To begin to address the role of microbial–plant interactions, we tested the hypothesis that the response of microbial communities to global change depends on the plant community. We characterized bacterial and fungal communities from 395 plant litter samples taken from the Loma Ridge Global Change Experiment, a decade-long global change experiment in Southern California that manipulates rainfall and nitrogen levels across two adjacent ecosystems, a grassland and a coastal sage scrubland. The differences in bacterial and fungal composition between ecosystems paralleled distinctions in plant community composition. In addition to the direct main effects, the global change treatments altered microbial composition in an ecosystem-dependent manner, in support of our hypothesis. The interaction between the drought treatment and ecosystem explained nearly 5% of the variation in bacterial community composition, similar to the variation explained by the ecosystem-independent effects of drought. Unexpectedly, we found that the main effect of drought was approximately four times as strong on bacterial composition as that of nitrogen addition, which did not alter fungal or plant composition. Overall, the findings underscore the importance of considering plant–microbe interactions when considering the transferability of the results of global change experiments across ecosystems. 

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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. Climate legacies determine grassland responses to future rainfall regimes

   https://doi.org/10.1111/gcb.16084  

   Broderick, Caitlin M.; Wilkins, Kate; Smith, Melinda D.; Blair, John M. (January 2022, Global Change Biology)

   Abstract Climate variability and periodic droughts have complex effects on carbon (C) fluxes, with uncertain implications for ecosystem C balance under a changing climate. Responses to climate change can be modulated by persistent effects of climate history on plant communities, soil microbial activity, and nutrient cycling (i.e., legacies). To assess how legacies of past precipitation regimes influence tallgrass prairie C cycling under new precipitation regimes, we modified a long‐term irrigation experiment that simulated a wetter climate for >25 years. We reversed irrigated and control (ambient precipitation) treatments in some plots and imposed an experimental drought in plots with a history of irrigation or ambient precipitation to assess how climate legacies affect aboveground net primary productivity (ANPP), soil respiration, and selected soil C pools. Legacy effects of elevated precipitation (irrigation) included higher C fluxes and altered labile soil C pools, and in some cases altered sensitivity to new climate treatments. Indeed, decades of irrigation reduced the sensitivity of both ANPP and soil respiration to drought compared with controls. Positive legacy effects of irrigation on ANPP persisted for at least 3 years following treatment reversal, were apparent in both wet and dry years, and were associated with altered plant functional composition. In contrast, legacy effects on soil respiration were comparatively short‐lived and did not manifest under natural or experimentally‐imposed “wet years,” suggesting that legacy effects on CO₂efflux are contingent on current conditions. Although total soil C remained similar across treatments, long‐term irrigation increased labile soil C and the sensitivity of microbial biomass C to drought. Importantly, the magnitude of legacy effects for all response variables varied with topography, suggesting that landscape can modulate the strength and direction of climate legacies. Our results demonstrate the role of climate history as an important determinant of terrestrial C cycling responses to future climate changes. 

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